Talking Glossary
of Genomic and Genetic Terms
The glossary features nearly 250 terms explained in an easy-to-understand way by leading scientists and professionals at the National Human Genome Research Institute.
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A
Adenine
Adenine (A) is one of the four nucleotide bases in DNA, with the other three being
cytosine (C), guanine (G) and thymine (T). Within a double-stranded DNA molecule, adenine bases on one
strand pair with thymine bases on the opposite strand. The sequence of the four nucleotide bases encodes
DNA’s information.
Allele
An allele is one of two or more versions of DNA sequence (a single base or a segment
of bases) at a given genomic location. An individual inherits two alleles, one from each parent, for any
given genomic location where such variation exists. If the two alleles are the same, the individual is
homozygous for that allele. If the alleles are different, the individual is heterozygous.
Amino Acid
An amino acid is the fundamental molecule that serves as the building block for
proteins. There are 20 different amino acids. A protein consists of one or more chains of amino acids
(called polypeptides) whose sequence is encoded in a gene. Some amino acids can be synthesized in the
body, but others (essential amino acids) cannot and must be obtained from a person’s diet.
Aneuploidy
Aneuploidy is an abnormality in the number of chromosomes in a cell due to loss or
duplication. In humans, aneuploidy would be any number of chromosomes other than the usual 46.
Animal Model
An animal model is a non-human species used in biomedical research because it can
mimic aspects of a biological process or disease found in humans. Animal models (e.g., mice, rats,
zebrafish and others) are sufficiently like humans in their anatomy, physiology or response to a
pathogen that researchers can extrapolate the results of animal model studies to better understand human
physiology and disease. By using animal models, researchers can perform experiments that would be
impractical or ethically prohibited with humans.
Anticodon
A codon is a DNA or RNA sequence of three nucleotides (a trinucleotide) that forms a
unit of genetic information encoding a particular amino acid. An anticodon is a trinucleotide sequence
located at one end of a transfer RNA (tRNA) molecule, which is complementary to a corresponding codon in
a messenger RNA (mRNA) sequence. Each time an amino acid is added to a growing polypeptide during
protein synthesis, a tRNA anticodon pairs with its complementary codon on the mRNA molecule, ensuring
that the appropriate amino acid is inserted into the polypeptide.
Antisense
Antisense is the non-coding DNA strand of a gene. In a cell, antisense DNA serves as
the template for producing messenger RNA (mRNA), which directs the synthesis of a protein.
Autism
Autism is a condition related to brain development that can cause significant social,
communication and behavioral challenges. Symptoms usually appear before the age of three. The exact
cause of autism is not entirely known, although genetics clearly plays an important role. Autism is one
of a group of related developmental conditions sometimes called the autism spectrum that affect people
differently and to varying degrees.
Autosomal Dominant Disorder
Autosomal dominant is a pattern of inheritance characteristic of some genetic
disorders. “Autosomal” means that the gene in question is located on one of the numbered, or non-sex,
chromosomes. “Dominant” means that a single copy of the mutated gene (from one parent) is enough to
cause the disorder. A child of a person affected by an autosomal dominant condition has a 50% chance of
being affected by that condition via inheritance of a dominant allele. By contrast, an autosomal
recessive disorder requires two copies of the mutated gene (one from each parent) to cause the disorder.
Huntington’s disease is an example of an autosomal dominant genetic disorder.
Autosomal Recessive Disorder
Autosomal recessive is a pattern of inheritance characteristic of some genetic
disorders. “Autosomal” means that the gene in question is located on one of the numbered, or non-sex,
chromosomes. “Recessive” means that two copies of the mutated gene (one from each parent) are required
to cause the disorder. In a family where both parents are carriers and do not have the disease, roughly
a quarter of their children will inherit two disease-causing alleles and have the disease. By contrast,
an autosomal dominant disorder requires only a single copy of the mutated gene from one parent to cause
the disorder. Sickle cell anemia is an example of an autosomal recessive genetic disorder.
Autosome
An autosome is one of the numbered chromosomes, as opposed to the sex chromosomes.
Humans have 22 pairs of autosomes and one pair of sex chromosomes (XX or XY). Autosomes are numbered
roughly in relation to their sizes. The largest autosome — chromosome 1 — has approximately 2,800 genes;
the smallest autosome — chromosome 22 — has approximately 750 genes.
B
Base Pair
A base pair consists of two complementary DNA nucleotide bases that pair together to
form a “rung of the DNA ladder.” DNA is made of two linked strands that wind around each other to
resemble a twisted ladder — a shape known as a double helix. Each strand has a backbone made of
alternating sugar (deoxyribose) and phosphate groups. Attached to each sugar is one of four bases:
adenine (A), cytosine (C), guanine (G) or thymine (T). The two strands are held together by hydrogen
bonds between pairs of bases: adenine pairs with thymine, and cytosine pairs with guanine.
Bioinformatics
Bioinformatics, as related to genetics and genomics, is a scientific subdiscipline
that involves using computer technology to collect, store, analyze and disseminate biological data and
information, such as DNA and amino acid sequences or annotations about those sequences. Scientists and
clinicians use databases that organize and index such biological information to increase our
understanding of health and disease and, in certain cases, as part of medical care.
Birth Defect
A birth defect (also called a congenital defect) is a physical or physiological
abnormality present in a baby at birth. Birth defects can be caused by genetic factors, prenatal events
during pregnancy or a combination of both. Some birth defects are easy to see (such as an extra or
missing finger), while others (such as an enzyme deficiency) are identified through special tests.
BRCA1/BRCA2
BRCA1 and BRCA2 are the first two genes found to be associated with
inherited forms
of breast cancer and ovarian cancer. People with mutations in either BRCA1 or BRCA2
have a much higher
risk for developing breast, ovarian or other types of cancer than those without mutations in the genes.
Both BRCA1 and BRCA2 normally act as tumor suppressors, meaning they help to regulate
cell division.
Most people have two active copies of these genes. When one of the two copies becomes inactive due to an
inherited mutation, a person’s cells are left with only one copy. If this remaining copy also becomes
inactivated, then uncontrolled cell growth results, which leads to breast, ovarian or other types of
cancer.
C
Cancer
Cancer is a disease in which some of the body’s cells grow uncontrollably. There are
many different types of cancer, and each begins when a single cell acquires a genomic change (or
mutation) that allows the cell to divide and multiply unchecked. Additional mutations can cause the
cancer to spread to other sites. Such mutations can be caused by errors during DNA replication or result
from DNA damage due to environmental exposures (such as tobacco smoke or the sun’s ultraviolet rays). In
certain cases, mutations in cancer genes are inherited, which increases a person’s risk of developing
cancer.
Cancer-Susceptibility Gene
A cancer-susceptibility gene is a gene that, when changed (or mutated), gives an
individual an increased risk for developing cancer. Individuals who have inherited mutations in certain
cancer-susceptibility genes have a lifetime risk of cancer that is significantly higher than the general
population (e.g., BRCA1/BRCA2). Individuals in the high-risk category may benefit from more
frequent cancer screens. There are also many gene variants associated with a small increase in risk. In
some cases, environmental factors may also play a role.
Candidate Gene
The term candidate gene refers to a gene that is believed to be related to a
particular trait, such as a disease or a physical attribute. Because of its genomic location or its
known function, the gene is suspected to play a role in that trait, thus making it a candidate for
additional study.
Carcinogen
A carcinogen is a substance, organism or agent capable of causing cancer. Carcinogens
may occur naturally in the environment (such as ultraviolet rays in sunlight and certain viruses) or may
be generated by humans (such as automobile exhaust fumes and cigarette smoke). Most carcinogens work by
interacting with a cell’s DNA to produce mutations.
Carrier
A carrier, as related to genetics, is an individual who “carries” and can pass on to
its offspring a genomic variant (allele) associated with a disease (or trait) that is inherited in an
autosomal recessive or sex-linked manner, and who does not show symptoms of that disease (or features of
that trait). The carrier has inherited the variant allele from one parent and a normal allele from the
other parent. Any offspring of carriers is at risk of inheriting a variant allele from their parents,
which would result in that child having the disease (or trait).
Carrier Screening
Carrier screening involves testing to see if a person “carries” a genetic variation
(allele) associated with a specific disease or trait. A carrier has inherited a normal and a variant
allele for a disease- or trait-associated gene, one from each parent. Most typically, carrier screening
is performed to look for recessively inherited diseases when the suspected carrier has no symptoms of
the disease, but that person’s offspring could have the disease if the other parent is a carrier of a
harmful variant in the same gene.
Copy DNA (cDNA)
cDNA (short for copy DNA; also called complementary DNA) is synthetic DNA that has
been transcribed from a specific mRNA through a reaction using the enzyme reverse transcriptase. While
DNA is composed of both coding and non-coding sequences, cDNA contains only coding sequences. Scientists
often synthesize and use cDNA as a tool in gene cloning and other research experiments.
Cell-Free DNA Testing
Cell-free DNA testing is a laboratory method that involves analyzing free (i.e.,
non-cellular) DNA contained within a biological sample, most often to look for genomic variants
associated with a hereditary or genetic disorder. For example, prenatal cell-free DNA testing is a
non-invasive method used during pregnancy that examines the fetal DNA that is naturally present in the
maternal bloodstream. Cell-free DNA testing is also used for the detection and characterization of some
cancers and to monitor cancer therapy.
Centimorgan (cM)
A centimorgan (abbreviated cM) is a unit of measure for the frequency of genetic
recombination. One centimorgan is equal to a 1% chance that two markers on a chromosome will become
separated from one another due to a recombination event during meiosis (which occurs during the
formation of egg and sperm cells). On average, one centimorgan corresponds to roughly 1 million base
pairs in the human genome.
Central Dogma
The central dogma of molecular biology is a theory first proposed by Francis Crick in
1958. It states that genetic information flows only in one direction, from DNA to RNA to protein.
Scientists have since discovered several exceptions to the theory.
Centromere
The centromere appears as a constricted region of a chromosome and plays a key role
in helping the cell divide up its DNA during division (mitosis and meiosis). Specifically, it is the
region where the cell’s spindle fibers attach. Following attachment of the spindle fibers to the
centromere, the two identical sister chromatids that make up the replicated chromosome are pulled to
opposite sides of the dividing cell, such that the two resulting daughter cells end up with identical
DNA.
Chromatid
A chromatid is one of the two identical halves of a chromosome that has been
replicated in preparation for cell division. The two “sister” chromatids are joined at a constricted
region of the chromosome called the centromere. During cell division, spindle fibers attach to the
centromere and pull each of the sister chromatids to opposite sides of the cell. Soon after, the cell
divides in two, resulting in daughter cells with identical DNA.
Chromatin
Chromatin refers to a mixture of DNA and proteins that form the chromosomes found in
the cells of humans and other higher organisms. Many of the proteins — namely, histones — package the
massive amount of DNA in a genome into a highly compact form that can fit in the cell nucleus.
Chromosome
Chromosomes are threadlike structures made of protein and a single molecule of DNA
that serve to carry the genomic information from cell to cell. In plants and animals (including humans),
chromosomes reside in the nucleus of cells. Humans have 22 pairs of numbered chromosomes (autosomes) and
one pair of sex chromosomes (XX or XY), for a total of 46. Each pair contains two chromosomes, one
coming from each parent, which means that children inherit half of their chromosomes from their mother
and half from their father. Chromosomes can be seen through a microscope when the nucleus dissolves
during cell division.
Cloning
Cloning, as it relates to genetics and genomics, involves using scientific methods to
make identical, or virtually identical, copies of an organism, cell or DNA sequence. The phrase
“molecular cloning” typically refers to isolating and copying a particular DNA segment of interest for
further study.
Codominance
Codominance, as it relates to genetics, refers to a type of inheritance in which two
versions (alleles) of the same gene are expressed separately to yield different traits in an individual.
That is, instead of one trait being dominant over the other, both traits appear, such as in a plant or
animal that has more than one pigment color.
Codon
A codon is a DNA or RNA sequence of three nucleotides (a trinucleotide) that forms a
unit of genomic information encoding a particular amino acid or signaling the termination of protein
synthesis (stop signals). There are 64 different codons: 61 specify amino acids and 3 are used as stop
signals.
Complex Disease
A complex disease (or condition), when discussed in the context of genetics, reflects
a disorder that results from the contributions of multiple genomic variants and genes in conjunction
with significant influences of the physical and social environment. For this reason, complex diseases
are also called multifactorial diseases. This stands in contrast to a “simple” genetic disease that is
more directly caused by mutations in a single gene. Common examples of complex genetic diseases include
heart disease, diabetes, and cancer.
Congenital
Congenital refers to a condition or trait that exists at birth. Congenital conditions
or traits may be hereditary or result from an action or exposure occurring during pregnancy or at birth,
or they may be due to a combination of these factors.
Contig
A contig (as related to genomic studies; derived from the word “contiguous”) is a set
of DNA segments or sequences that overlap in a way that provides a contiguous representation of a
genomic region. For example, a clone contig provides a physical map of a set of cloned segments of DNA
across a genomic region, while a sequence contig provides the actual DNA sequence of a genomic region.
Copy Number Variation (CNV)
Copy number variation (abbreviated CNV) refers to a circumstance in which the number
of copies of a specific segment of DNA varies among different individuals’ genomes. The individual
variants may be short or include thousands of bases. These structural differences may have come about
through duplications, deletions or other changes and can affect long stretches of DNA. Such regions may
or may not contain a gene(s).
CRISPR
CRISPR (short for “clustered regularly interspaced short palindromic repeats”) is a
technology that research scientists use to selectively modify the DNA of living organisms. CRISPR was
adapted for use in the laboratory from naturally occurring genome editing systems found in bacteria.
Crossing Over
Crossing over, as related to genetics and genomics, refers to the exchange of DNA
between paired homologous chromosomes (one from each parent) that occurs during the development of egg
and sperm cells (meiosis). This process results in new combinations of alleles in the gametes (egg or
sperm) formed, which ensures genomic variation in any offspring produced.
Cystic Fibrosis (CF)
Cystic fibrosis (abbreviated CF) is a genetic disorder that causes mucus to build up
in certain organs of the body, particularly the lungs and pancreas, resulting in breathing problems,
respiratory infections and faulty digestion. Caused by a mutation in a single gene (called CFTR),
the disorder is inherited as an autosomal recessive trait, meaning that an affected individual inherits
two mutated copies of the gene, one from each parent. In the past, CF was almost always fatal in
childhood. Today, however, with improvements in screening and treatments, individuals with CF may live
into their 30s or 40s, or even longer.
Cytogenetics
Cytogenetics is a branch of biology focused on the study of chromosomes and their
inheritance, especially as applied to medical genetics. Chromosomes are microscopic structures
containing DNA that reside within the nucleus of a cell. During cell division, these structures become
condensed and are visible with a microscope. Special staining techniques can be used to assess the
number and structure of a person’s chromosomes as part of diagnostic testing. The number and/or
structure of chromosomes is known to be altered in certain genetic diseases.
Cytosine
Cytosine (C) is one of the four nucleotide bases in DNA, with the other three being
adenine (A), guanine (G) and thymine (T). Within a double-stranded DNA molecule, cytosine bases on one
strand pair with guanine bases on the opposite strand. The sequence of the four nucleotide bases encodes
DNA’s information.
D
Data Science
Data science involves the study of large, complex data sets that arise from various
types of research projects. With respect to genomic studies, such work requires expertise in
quantitative scientific disciplines such as bioinformatics, computational biology and biostatistics.
Deletion
A deletion, as related to genomics, is a type of mutation that involves the loss of
one or more nucleotides from a segment of DNA. A deletion can involve the loss of any number of
nucleotides, from a single nucleotide to an entire piece of a chromosome.
Deoxyribonucleic Acid (DNA)
Deoxyribonucleic acid (abbreviated DNA) is the molecule that carries genetic
information for the development and functioning of an organism. DNA is made of two linked strands that
wind around each other to resemble a twisted ladder — a shape known as a double helix. Each strand has a
backbone made of alternating sugar (deoxyribose) and phosphate groups. Attached to each sugar is one of
four bases: adenine (A), cytosine (C), guanine (G) or thymine (T). The two strands are connected by
chemical bonds between the bases: adenine bonds with thymine, and cytosine bonds with guanine. The
sequence of the bases along DNA’s backbone encodes biological information, such as the instructions for
making a protein or RNA molecule.
Diploid
Diploid is a term that refers to the presence of two complete sets of chromosomes in
an organism’s cells, with each parent contributing a chromosome to each pair. Humans are diploid, and
most of the body’s cells contain 23 chromosomes pairs. Human gametes (egg and sperm cells), however,
contain a single set of chromosomes and are said to be haploid.
DNA Fingerprinting
DNA fingerprinting is a laboratory technique used to determine the probable identity
of a person based on the nucleotide sequences of certain regions of human DNA that are unique to
individuals. DNA fingerprinting is used in a variety of situations, such as criminal investigations,
other forensic purposes and paternity testing. In these situations, one aims to “match” two DNA
fingerprints with one another, such as a DNA sample from a known person and one from an unknown person.
DNA Replication
DNA replication is the process by which the genome’s DNA is copied in cells. Before a
cell divides, it must first copy (or replicate) its entire genome so that each resulting daughter cell
ends up with its own complete genome.
DNA Sequencing
DNA sequencing refers to the general laboratory technique for determining the exact
sequence of nucleotides, or bases, in a DNA molecule. The sequence of the bases (often referred to by
the first letters of their chemical names: A, T, C, and G) encodes the biological information that cells
use to develop and operate. Establishing the sequence of DNA is key to understanding the function of
genes and other parts of the genome. There are now several different methods available for DNA
sequencing, each with its own characteristics, and the development of additional methods represents an
active area of genomics research.
Dominant Traits and Alleles
Dominant, as related to genetics, refers to the relationship between an observed
trait and the two inherited versions of a gene related to that trait. Individuals inherit two versions
of each gene, known as alleles, from each parent. In the case of a dominant trait, only one copy of the
dominant allele is required to express the trait. The effect of the other allele (the recessive allele)
is masked by the dominant allele. Typically, an individual who carries two copies of a dominant allele
exhibits the same trait as those who carry only one copy. This contrasts to a recessive trait, which
requires that both alleles be present to express the trait.
Double Helix
Double helix, as related to genomics, is a term used to describe the physical
structure of DNA. A DNA molecule is made up of two linked strands that wind around each other to
resemble a twisted ladder in a helix-like shape. Each strand has a backbone made of alternating sugar
(deoxyribose) and phosphate groups. Attached to each sugar is one of four bases: adenine (A), cytosine
(C), guanine (G) or thymine (T). The two strands are connected by chemical bonds between the bases:
adenine bonds with thymine, and cytosine bonds with guanine.
Down Syndrome (Trisomy 21)
Down syndrome (also called Trisomy 21) is a genetic condition caused by an error in
the process that replicates and then divides up the pairs of chromosomes during cell division, resulting
in the inheritance of an extra full or partial copy of chromosome 21 from a parent. This extra
chromosomal DNA causes the intellectual disabilities and physical features characteristic of Down
syndrome, which vary among individuals.
Duplication
Duplication, as related to genomics, refers to a type of mutation in which one or
more copies of a DNA segment (which can be as small as a few bases or as large as a major chromosomal
region) is produced. Duplications occur in all organisms. For example, they are especially prominent in
plants, although they can also cause genetic diseases in humans. Duplications have been an important
mechanism in the evolution of the genomes of humans and other organisms.
E
Electrophoresis
Electrophoresis is a laboratory technique used to separate DNA, RNA or protein
molecules based on their size and electrical charge. An electric current is used to move the molecules
through a gel or other matrix. Pores in the gel or matrix work like a sieve, allowing smaller molecules
to move faster than larger molecules. To determine the size of the molecules in a sample, standards of
known sizes are separated on the same gel and then compared to the sample.
Environmental Factors
Environmental factors, as related to genetics, refers to exposures to substances
(such as pesticides or industrial waste) where we live or work, behaviors (such as smoking or poor diet)
that can increase an individual’s risk of disease or stressful situations (such as racism). Genetic
studies often take environmental factors into consideration, as these exposures can increase an
individual’s risk of genetic damage or disease.
Epigenetics
Epigenetics (also sometimes called epigenomics) is a field of study focused on
changes in DNA that do not involve alterations to the underlying sequence. The DNA letters and the
proteins that interact with DNA can have chemical modifications that change the degrees to which genes
are turned on and off. Certain epigenetic modifications may be passed on from parent cell to daughter
cell during cell division or from one generation to the next. The collection of all epigenetic changes
in a genome is called an epigenome.
Epistasis
Epistasis is a circumstance where the expression of one gene is modified (e.g.,
masked, inhibited or suppressed) by the expression of one or more other genes.
Eugenics
Eugenics is a discredited belief that selective breeding for certain inherited human
traits can improve the “fitness” of future generations. For eugenicists, “fitness” corresponded to a
narrow view of humanity and society that developed directly from the ideologies and practices of
scientific racism, colonialism, ableism and imperialism.
Evolution
Evolution, as related to genomics, refers to the process by which living organisms
change over time through changes in the genome. Such evolutionary changes result from mutations that
produce genomic variation, giving rise to individuals whose biological functions or physical traits are
altered. Those individuals who are best at adapting to their surroundings leave behind more offspring
than less well-adapted individuals. Thus, over successive generations (in some cases spanning millions
of years), one species may evolve to take on divergent functions or physical characteristics or may even
evolve into a different species.
Exome
An exome is the sequence of all the exons in a genome, reflecting the protein-coding
portion of a genome. In humans, the exome is about 1.5% of the genome.
Exon
An exon is a region of the genome that ends up within an mRNA molecule. Some exons
are coding, in that they contain information for making a protein, whereas others are non-coding. Genes
in the genome consist of exons and introns.
F
Family History
A family history, as related to medicine, is a record of the diseases and health
conditions of an individual and that person’s biological family members, both living and deceased. A
family history can help determine whether someone has an increased genetic risk of having or developing
certain diseases, disorders or conditions. It is often recorded by drawing a pedigree (a family tree)
that illustrates the relationships among individuals.
Fibroblast
A fibroblast is a type of cell that contributes to the formation of connective
tissue, a fibrous cellular material that supports and connects other tissues or organs in the body.
Fibroblasts secrete collagen proteins that help maintain the structural framework of tissues. They also
play an important role in healing wounds. Obtained from a person through a simple skin biopsy,
fibroblasts can be grown in the laboratory for use in genetic and other scientific studies of that
individual.
First-Degree Relative
A first-degree relative is a family member who shares about half of their genetic
information with specific other individuals in their family. First-degree relatives include an
individual’s parents, siblings and offspring.
Fluorescence In Situ Hybridization (FISH)
Fluorescence in situ hybridization (abbreviated FISH) is a laboratory technique used
to detect and locate a specific DNA sequence on a chromosome. In this technique, the full set of
chromosomes from an individual is affixed to a glass slide and then exposed to a “probe”—a small piece
of purified DNA tagged with a fluorescent dye. The fluorescently labeled probe finds and then binds to
its matching sequence within the set of chromosomes. With the use of a special microscope, the
chromosome and sub-chromosomal location where the fluorescent probe bound can be seen.
Founder Effect
A founder effect, as related to genetics, refers to the reduction in genomic
variability that occurs when a small group of individuals becomes separated from a larger population.
Over time, the resulting new subpopulation will have genotypes and physical traits resembling the
initial small, separated group, and these may be very different from the original larger population. A
founder effect can also explain why certain inherited diseases are found more frequently in some limited
population groups. In some cases, a founder effect can play a role in the emergence of new species.
Fragile X Syndrome
Fragile X syndrome is a genetic condition that affects a person’s development, in
particular their ability to learn and their social behavior. The syndrome results from mutations in a
gene on the X chromosome. Because males have only one copy of the X chromosome, they are more likely to
show severe symptoms if they inherit the mutated gene compared to females (who have two copies of the X
chromosome).
Frameshift Mutation
A frameshift mutation in a gene refers to the insertion or deletion of nucleotide
bases in numbers that are not multiples of three. This is important because a cell reads a gene’s code
in groups of three bases when making a protein. Each of these “triplet codons” corresponds to one of 20
different amino acids used to build a protein. If a mutation disrupts this normal reading frame, then
the entire gene sequence following the mutation will be incorrectly read. This can result in the
addition of the wrong amino acids to the protein and/or the creation of a codon that stops the protein
from growing longer.
Fraternal Twins
Fraternal twins (also called dizygotic twins) result from the fertilization of two
separate eggs with two different sperm during the same pregnancy. Fraternal twins may not have the same
sex or appearance. They share half their genomes, just like any other siblings. In contrast, identical
twins (or monozygotic twins) result from the fertilization of a single egg by a single sperm, with the
fertilized egg then splitting into two. As a result, identical twins share the same genomes and are
always the same sex.
G
Gamete
A gamete is a reproductive cell of an animal or plant. In animals, female gametes are
called ova or egg cells, and male gametes are called sperm. Ova and sperm are haploid cells, with each
cell carrying only one copy of each chromosome. During fertilization, a sperm and ovum unite to form a
new diploid organism.
Gender
Gender is an evolving term, and there are different definitions. In a general sense,
gender involves how a person identifies along a broad, fluid spectrum, rather than strictly by the sex
assignment they received at birth. Sex refers to the physical differences (e.g., chromosome composition,
anatomy, and physiology) among people who are born male, female, or intersex. The term gender may also
be used to refer to the social or cultural constructs of “roles” or “norms” typically associated with
being masculine or feminine. Gender does not always directly relate to sex assigned at birth.
Gene
The gene is considered the basic unit of inheritance. Genes are passed from parents
to offspring and contain the information needed to specify physical and biological traits. Most genes
code for specific proteins, or segments of proteins, which have differing functions within the body.
Humans have approximately 20,000 protein-coding genes.
Gene Amplification
Gene amplification refers to an increase in the number of copies of a gene in a
genome. Cancer cells, for example, sometimes produce multiple copies of a gene(s) in response to signals
from other cells or the environment.
Gene Expression
Gene expression is the process by which the information encoded in a gene is used to
either make RNA molecules that code for proteins or to make non-coding RNA molecules that serve other
functions. Gene expression acts as an “on/off switch” to control when and where RNA molecules and
proteins are made and as a “volume control” to determine how much of those products are made. The
process of gene expression is carefully regulated, changing substantially under different conditions.
The RNA and protein products of many genes serve to regulate the expression of other genes.
Gene Mapping
Gene mapping refers to the process of determining the location of genes on
chromosomes. Today, the most efficient approach for gene mapping involves sequencing a genome and then
using computer programs to analyze the sequence to identify the location of genes.
Gene Pool
A gene pool refers to the combination of all the genes (including alleles) present in
a reproducing population or species. A large gene pool has extensive genomic diversity and is better
able to withstand environmental challenges. Inbreeding contributes to a smaller gene pool, making
populations or species less able to adapt and survive when faced with environmental challenges.
Gene Regulation
Gene regulation is the process used to control the timing, location and amount in
which genes are expressed. The process can be complicated and is carried out by a variety of mechanisms,
including through regulatory proteins and chemical modification of DNA. Gene regulation is key to the
ability of an organism to respond to environmental changes.
Gene Therapy
Gene therapy is a technique that uses a gene(s) to treat, prevent or cure a disease
or medical disorder. Often, gene therapy works by adding new copies of a gene that is broken, or by
replacing a defective or missing gene in a patient’s cells with a healthy version of that gene. Both
inherited genetic diseases (e.g., hemophilia and sickle cell disease) and acquired disorders (e.g.,
leukemia) have been treated with gene therapy.
Gene–Environment Interaction
Gene–environment interaction refers to the interplay of genes (and, more broadly,
genome function) and the physical and social environment. These interactions influence the expression of
phenotypes. For example, most human traits and diseases are influenced by how one or more genes interact
in complex ways with environmental factors, such as chemicals in the air or water, nutrition,
ultraviolet radiation from the sun and social context.
Genetic Ancestry
Genetic ancestry refers to information about the people that an individual is
biologically descended from, including their genetic relationships. Genetic information can be combined
with historical information to infer where an individual’s distant ancestors lived.
Genetic Code
Genetic code refers to the instructions contained in a gene that tell a cell how to
make a specific protein. Each gene’s code uses the four nucleotide bases of DNA: adenine (A), cytosine
(C), guanine (G) and thymine (T) — in various ways to spell out three-letter “codons” that specify which
amino acid is needed at each position within a protein.
Genetic Counseling
Genetic counseling refers to guidance relating to genetic disorders that a
specialized healthcare professional (genetic counselor) provides to an individual or family. A genetic
counselor might provide information about how a genetic condition could affect an individual or family
and/or interpret genetic tests designed to help estimate the risk of a disease. The genetic counselor
conveys information to address the concerns of the individual or family, helps them make an informed
decision about their medical situation and provides psychological counseling to help them adapt to their
condition or risk.
Genetic Discrimination
Genetic discrimination. Genetic discrimination refers to the unequal treatment of
individuals based on an aspect of their genetic code or genome, such as the risk for genetic disorder.
Genetic discrimination can involve such genomic information being used against individuals in a variety
of circumstances, such as employment, health or disability, insurance status, or education, or health
care.
Genetic Drift
Genetic drift is a mechanism of evolution characterized by random fluctuations in the
frequency of a particular version of a gene (allele) in a population. Though it primarily affects small,
isolated populations, the effects of genetic drift can be strong, sometimes causing traits to become
overwhelmingly frequent or to disappear from a population.
Genetic Engineering
Genetic engineering (also called genetic modification) is a process that uses
laboratory-based technologies to alter the DNA makeup of an organism. This may involve changing a single
base pair (A-T or C-G), deleting a region of DNA or adding a new segment of DNA. For example, genetic
engineering may involve adding a gene from one species to an organism from a different species to
produce a desired trait. Used in research and industry, genetic engineering has been applied to the
production of cancer therapies, brewing yeasts, genetically modified plants and livestock, and more.
Genetic Epidemiology
Genetic epidemiology is a field of science focused on the study of how genetic
factors influence human traits, such as human health and disease. In many cases, the interaction of
genes with the environment is also measured. Genetic epidemiologists seek to understand the causes,
distribution and control of inherited disease in groups and the multifactorial causes of genetic
diseases in populations.
Genetic Imprinting
Genomic imprinting is the process by which only one copy of a gene in an individual
(either from their mother or their father) is expressed, while the other copy is suppressed. Unlike
genomic mutations that can affect the ability of inherited genes to be expressed, genomic imprinting
does not affect the DNA sequence itself. Instead, gene expression is silenced by the epigenetic addition
of chemical tags to the DNA during egg or sperm formation. Epigenetic tags on imprinted genes usually
stay in place for the life of the individual.
Genetic Information Nondiscrimination Act (GINA)
The Genetic Information Nondiscrimination Act (abbreviated GINA) is federal
legislation in the United States that protects individuals against discrimination based on their
personal genetic information, as it applies to health insurance and employment. These protections are
intended to encourage Americans to take advantage of genetic testing as part of their medical care. GINA
was signed into law on May 22, 2008.
Genetic Map
A genetic map (also called a linkage map) shows the relative location of genetic
markers (reflecting sites of genomic variants) on a chromosome. A genetic map is based on the concept of
genetic linkage: the closer two markers are to each other on a chromosome, the greater the probability
that they will be inherited together. By studying inheritance patterns, the relative order and location
of genetic markers along a chromosome can be established.
Genetic Testing
Genetic testing is the use of a laboratory test to examine an individual’s DNA for
variations, typically performed in the context of medical care, ancestry studies or forensics. In a
medical setting, the results of a genetic test can be used to confirm or rule out a suspected genetic
disease. Results may also be used to determine the likelihood of parents passing on a genetic mutation
to their offspring. Genetic testing may be performed prenatally or after birth. Genetic testing is also
used to study the genomes of tumors in cancer cases.
Genetics
Genetics is the branch of biology concerned with the study of inheritance, including
the interplay of genes, DNA variation and their interactions with environmental factors.
Genome
The genome is the entire set of DNA instructions found in a cell. In humans, the
genome consists of 23 pairs of chromosomes located in the cell’s nucleus, as well as a small chromosome
in the cell’s mitochondria. A genome contains all the information needed for an individual to develop
and function.
Genome-Wide Association Study (GWAS)
A genome-wide association study (abbreviated GWAS) is a research approach used to
identify genomic variants that are statistically associated with a risk for a disease or a particular
trait. The method involves surveying the genomes of many people, looking for genomic variants that occur
more frequently in those with a specific disease or trait compared to those without the disease or
trait. Once such genomic variants are identified, they are typically used to search for nearby variants
that contribute directly to the disease or trait.
Genomic Medicine
Genomic medicine is a medical discipline that involves using a person’s genomic
information as part of their clinical care. Other similar terms include individualized medicine,
personalized medicine and precision medicine. For some conditions, genomic information can be used to
help diagnose disease, predict outcomes and guide treatment.
Genomic Variation
Genomic variation refers to DNA sequence differences among individuals or
populations. Some variants influence biological function (such as a mutation that causes a genetic
disease), while others have no biological effects.
Genomics
Genomics is a field of biology focused on studying all the DNA of an organism — that
is, its genome. Such work includes identifying and characterizing all the genes and functional elements
in an organism’s genome as well as how they interact.
Genotype
A genotype is a scoring of the type of variant present at a given location (i.e., a
locus) in the genome. It can be represented by symbols. For example, BB, Bb, bb could be used to
represent a given variant in a gene. Genotypes can also be represented by the actual DNA sequence at a
specific location, such as CC, CT, TT. DNA sequencing and other methods can be used to determine the
genotypes at millions of locations in a genome in a single experiment. Some genotypes contribute to an
individual’s observable traits, called the phenotype.
Germ Line
Germ line refers to the sex cells (eggs and sperm) that sexually reproducing
organisms use to pass on their genomes from one generation to the next (parents to offspring). Egg and
sperm cells are called germ cells, in contrast to the other cells of the body, which are called somatic
cells.
Gigabase (Gb)
A gigabase (abbreviated Gb) is a unit of measurement used to help designate the
length of DNA. One gigabase is equal to 1 billion bases.
GMO (Genetically Modified Organism)
GMO (short for “genetically modified organism”) is a plant, animal or microbe in
which one or more changes have been made to the genome, typically using high-tech genetic engineering,
in an attempt to alter the characteristics of an organism. Genes can be introduced, enhanced or deleted
within a species, across species or even across kingdoms. GMOs may be used for a variety of purposes,
such as making human insulin, producing fermented beverages and developing pesticide resistance in crop
plants.
Guanine
Guanine (G) is one of the four nucleotide bases in DNA, with the other three being
adenine (A), cytosine (C) and thymine (T). Within a double-stranded DNA molecule, guanine bases on one
strand pair with cytosine bases on the opposite strand. The sequence of the four nucleotide bases
encodes DNA’s information.
H
Haploid
Haploid refers to the presence of a single set of chromosomes in an organism’s cells.
Sexually reproducing organisms are diploid (having two sets of chromosomes, one from each parent). In
humans, only the egg and sperm cells are haploid.
Haplotype
A haplotype is a physical grouping of genomic variants (or polymorphisms) that tend
to be inherited together. A specific haplotype typically reflects a unique combination of variants that
reside near each other on a chromosome.
Heterozygous
Heterozygous, as related to genetics, refers to having inherited different versions
(alleles) of a genomic marker from each biological parent. Thus, an individual who is heterozygous for a
genomic marker has two different versions of that marker. By contrast, an individual who is homozygous
for a marker has identical versions of that marker.
Histone
A histone is a protein that provides structural support for a chromosome. Each
chromosome contains a long molecule of DNA, which must fit into the cell nucleus. To do that, the DNA
wraps around complexes of histone proteins, giving the chromosome a more compact shape. Histones also
play a role in the regulation of gene expression.
Homologous Recombination
Homologous recombination is a type of genetic recombination in which nucleotide
sequences are exchanged between two similar or identical molecules of DNA. During the formation of egg
and sperm cells (meiosis), paired chromosomes from the male and female parents align so that similar DNA
sequences can cross over, or be exchanged, from one chromosome to the other. This exchanging of DNA is
an important source of the genomic variation seen among offspring.
Homozygous
Homozygous, as related to genetics, refers to having inherited the same versions
(alleles) of a genomic marker from each biological parent. Thus, an individual who is homozygous for a
genomic marker has two identical versions of that marker. By contrast, an individual who is heterozygous
for a marker has two different versions of that marker.
Human Genome Project
The Human Genome Project was a large international, collaborative effort that mapped
and sequenced the human genome for the first time. Conducted from 1990 to 2003, the project was historic
in its scope and scale as well as its groundbreaking approach for the free release of genomic data well
ahead of publication, leading to a new ethos for data sharing in biomedical research.
Human Genome Reference Sequence
A human genome reference sequence is an accepted representation of the human genome
sequence that is used by researchers as a standard for comparison to DNA sequences generated in their
studies. The scientists responsible for assembling and updating such reference sequences aim to provide
the highest-quality, best possible consensus representations of the sequence and structural diversity
found in the human genome among populations. The genome reference sequence provides a general framework
and is not the DNA sequence of a single person.
Huntington’s Disease
Huntington’s disease is a rare inherited disorder associated with the progressive
loss of brain and muscle function. Symptoms usually develop during middle age and may include
uncontrolled movements, loss of intellectual abilities and various emotional and psychiatric symptoms.
Huntington’s disease is inherited as an autosomal dominant trait, meaning that a single mutated copy of
the responsible gene (called HTT) is sufficient to cause the disease.
Hybridization
Hybridization, as related to genomics, is the process in which two complementary
single-stranded DNA and/or RNA molecules bond together to form a double-stranded molecule. The bonding
is dependent on the appropriate base-pairing across the two single-stranded molecules. Hybridization is
an important process in various research and clinical laboratory techniques.
I
Identical Twins
Identical twins (also called monozygotic twins) result from the fertilization of a
single egg by a single sperm, with the fertilized egg then splitting into two. Identical twins share the
same genomes and are always of the same sex. In contrast, fraternal (dizygotic) twins result from the
fertilization of two separate eggs with two different sperm during the same pregnancy. They share half
of their genomes, just like any other siblings. Fraternal twins may not be of the same sex or have
similar appearances.
In Situ Hybridization
In situ hybridization is a laboratory technique used to localize a sequence of DNA or
RNA in a biological sample. In this technique, a biological sample consisting of tissue sections, cells
or chromosomes from an individual is affixed to a glass slide and then exposed to a “probe”—a small
piece of single-stranded DNA tagged with a chemical or fluorescent dye. The labeled probe finds and then
binds to its matching sequence within the biological sample. The location of the bound probe can then be
seen with the use of a microscope.
Inherited
Inherited, as related to genetics, refers to a trait or variants encoded in DNA and
passed from parent to offspring during reproduction. Inheritance is determined by the rules of Mendelian
genetics.
Insertion
An insertion, as related to genomics, is a type of mutation that involves the
addition of one or more nucleotides into a segment of DNA. An insertion can involve the addition of any
number of nucleotides, from a single nucleotide to an entire piece of a chromosome.
Intergenic Regions
Intergenic regions are the stretches of DNA located between genes. In humans,
intergenic regions are non-protein-coding and comprise a large majority of the genome. Some intergenic
DNA is known to regulate the expression of nearby genes.
Intron
An intron is a region that resides within a gene but does not remain in the final
mature mRNA molecule following transcription of that gene and does not code for amino acids that make up
the protein encoded by that gene. Most protein-coding genes in the human genome consist of exons and
introns.
Inversion
An inversion in a chromosome occurs when a segment breaks off and reattaches within
the same chromosome, but in reverse orientation. DNA may or may not be lost in the process.
K
Karyotype
A karyotype is an individual’s complete set of chromosomes. The term also refers to a
laboratory-produced image of a person’s chromosomes isolated from an individual cell and arranged in
numerical order. A karyotype may be used to look for abnormalities in chromosome number or structure.
Kilobase (kb)
A kilobase (abbreviated kb) is a unit of measurement used to help designate the
length of DNA or RNA. One kilobase is equal to 1,000 bases.
Knockout
A knockout, as related to genomics, refers to the use of genetic engineering to
inactivate or remove one or more specific genes from an organism. Scientists create knockout organisms
to study the impact of removing a gene from an organism, which often allows them to then learn something
about that gene’s function.
L
Linkage
Linkage, as related to genetics and genomics, refers to the closeness of genes or
other DNA sequences to one another on the same chromosome. The closer two genes or sequences are to each
other on a chromosome, the greater the probability that they will be inherited together.
Locus
A locus, as related to genomics, is a physical site or location within a genome (such
as a gene or another DNA segment of interest), somewhat like a street address. The plural of locus is
loci.
LOD Score
An LOD (short for “logarithm of the odds”) score is a statistical estimate of the
relative probability that two loci (e.g., a disease-associated gene and another sequence of interest,
such as a variant or another gene) are located near each other on a chromosome and are therefore likely
to be inherited together.
LOH (Loss of Heterozygosity)
LOH (short for “ loss of heterozygosity ”) refers to a type of mutation that results in the loss of one
copy of a segment of DNA (typically containing a gene or group of genes). For most parts of the genome,
human cells have two copies of any genomic segment—one from each parent—so in the case of LOH, only one
copy would still be present.
Lyonization
Lyonization (also called X-inactivation) refers to the normal phenomenon in which one
of the two X chromosomes in every cell of a female individual is inactivated during embryonic
development. This inactivation prevents females from having twice as many X chromosome gene products as
males, who possess only a single copy of the X chromosome. Lyonization is named after Mary F. Lyon, the
British geneticist who discovered the phenomenon.
M
Mapping
Mapping refers to the process of determining the relative locations of landmarks or
markers (such as genes, variants and other DNA sequences of interest) within a chromosome or genome.
Historically, there have been two approaches for mapping: physical mapping, which established maps based
on physical distances between landmarks, and genetic mapping, which established maps based on the
frequency with which two landmarks are inherited together. Today, the most efficient approach for
mapping involves sequencing a genome and then using computer programs to analyze the sequence to
identify the locations of landmarks.
Marker
A marker (largely synonymous with the word “landmark” and often referred to as a
genomic marker or a genetic marker) is a DNA sequence, typically with a known location in a genome.
Markers can reflect random sequences, genomic variants or genes. Markers are used as signposts (or
landmarks) in the construction of DNA and genome maps. Markers can also be used to track inheritance of
traits or disease risk in families.
Megabase (Mb)
A megabase (abbreviated Mb) is a unit of measurement used to help designate the
length of DNA. One megabase is equal to 1 million bases.
Meiosis
Meiosis is a type of cell division in sexually reproducing organisms that reduces the
number of chromosomes in gametes (the sex cells, or egg and sperm). In humans, body (or somatic) cells
are diploid, containing two sets of chromosomes (one from each parent). To maintain this state, the egg
and sperm that unite during fertilization must be haploid, with a single set of chromosomes. During
meiosis, each diploid cell undergoes two rounds of division to yield four haploid daughter cells — the
gametes.""Mendel, Johann (Gregor)""Gregor Mendel was an Austrian monk in the 19th century who worked out
the basic laws of inheritance through experiments with pea plants. In his monastery garden, Mendel
performed thousands of crosses with pea plants, discovering how characteristics are passed down from one
generation to the next — namely, dominant and recessive traits. Mendel’s early experiments provided the
basis of modern genetics.
Mendel, Johann (Gregor)
Gregor Mendel was an Austrian monk in the 19th century who worked out the basic laws
of inheritance through experiments with pea plants. In his monastery garden, Mendel performed thousands
of crosses with pea plants, discovering how characteristics are passed down from one generation to the
next — namely, dominant and recessive traits. Mendel’s early experiments provided the basis of modern
genetics.
Mendelian Inheritance
Mendelian inheritance refers to certain patterns of how traits are passed from
parents to offspring. These general patterns were established by the Austrian monk Gregor Mendel, who
performed thousands of experiments with pea plants in the 19th century. Mendel’s discoveries of how
traits (such as color and shape) are passed down from one generation to the next introduced the concept
of dominant and recessive modes of inheritance.
Messenger RNA (mRNA)
Messenger RNA (abbreviated mRNA) is a type of single-stranded RNA involved in protein
synthesis. mRNA is made from a DNA template during the process of transcription. The role of mRNA is to
carry protein information from the DNA in a cell’s nucleus to the cell’s cytoplasm (watery interior),
where the protein-making machinery reads the mRNA sequence and translates each three-base codon into its
corresponding amino acid in a growing protein chain.
Metagenomics
Metagenomics is the study of the structure and function of entire nucleotide
sequences isolated and analyzed from all the organisms (typically microbes) in a bulk sample.
Metagenomics is often used to study a specific community of microorganisms, such as those residing on
human skin, in the soil or in a water sample.
Metaphase
Metaphase is a stage during the process of cell division (mitosis or meiosis).
Normally, individual chromosomes are spread out in the cell nucleus. During metaphase, the nucleus
dissolves and the cell’s chromosomes condense and move together, aligning in the center of the dividing
cell. At this stage, the chromosomes are distinguishable when viewed through a microscope. Metaphase
chromosomes are used in karyotyping, a laboratory technique for identifying chromosomal abnormalities.
Methylation
Methylation is a chemical modification of DNA and other molecules that may be
retained as cells divide to make more cells. When found in DNA, methylation can alter gene expression.
In this process, chemical tags called methyl groups attach to a particular location within DNA where
they turn a gene on or off, thereby regulating the production of proteins that the gene encodes.
Microarray Technology
Microarray technology is a general laboratory approach that involves binding an array
of thousands to millions of known nucleic acid fragments to a solid surface, referred to as a “chip.”
The chip is then bathed with DNA or RNA isolated from a study sample (such as cells or tissue).
Complementary base pairing between the sample and the chip-immobilized fragments produces light through
fluorescence that can be detected using a specialized machine. Microarray technology can be used for a
variety of purposes in research and clinical studies, such as measuring gene expression and detecting
specific DNA sequences (e.g., single-nucleotide polymorphisms, or SNPs ).
Microbiome
The microbiome is the community of microorganisms (such as fungi, bacteria and
viruses) that exists in a particular environment. In humans, the term is often used to describe the
microorganisms that live in or on a particular part of the body, such as the skin or gastrointestinal
tract. These groups of microorganisms are dynamic and change in response to a host of environmental
factors, such as exercise, diet, medication and other exposures.
Microsatellite
Microsatellite, as related to genomics, refers to a short segment of DNA, usually one
to six or more base pairs in length, that is repeated multiple times in succession at a particular
genomic location. These DNA sequences are typically non-coding. The number of repeated segments within a
microsatellite sequence often varies among people, which makes them useful as polymorphic markers for
studying inheritance patterns in families or for creating a DNA fingerprint from crime scene samples.
Missense Mutation
A missense mutation is a DNA change that results in different amino acids being
encoded at a particular position in the resulting protein. Some missense mutations alter the function of
the resulting protein.
Mitochondrial DNA
Mitochondrial DNA is the circular chromosome found inside the cellular organelles
called mitochondria. Located in the cytoplasm, mitochondria are the site of the cell’s energy production
and other metabolic functions. Offspring inherit mitochondria — and as a result mitochondrial DNA — from
their mother.
Mitosis
Mitosis is the process by which a cell replicates its chromosomes and then segregates
them, producing two identical nuclei in preparation for cell division. Mitosis is generally followed by
equal division of the cell’s content into two daughter cells that have identical genomes.
Monosomy
Monosomy refers to the condition in which only one chromosome from a pair is present
in cells rather than the two copies usually found in diploid cells. When cells have one chromosome from
a pair plus a portion of the second chromosome, this is referred to as partial monosomy. Monosomy, or
partial monosomy, causes certain human diseases such as Turner syndrome and Cri du chat syndrome.
Mosaicism
Mosaicism refers to the presence of cells in a person that have a different genome
from the body’s other cells. This difference could be due to a specific genomic variant, for example, or
the addition or loss of a chromosome. The condition can stem from a genetic error that occurs after
fertilization of an egg, during very early embryo development, or it could occur later in development.
Mosaicism can affect any type of cell and does not always cause disease.
Mutagen
A mutagen is a chemical or physical agent capable of inducing changes in DNA called
mutations. Examples of mutagens include tobacco products, radioactive substances, x-rays, ultraviolet
radiation and a wide variety of chemicals. Exposure to a mutagen can produce DNA mutations that cause or
contribute to certain diseases.
Mutation
A mutation is a change in the DNA sequence of an organism. Mutations can result from
errors in DNA replication during cell division, exposure to mutagens or a viral infection. Germline
mutations (that occur in eggs and sperm) can be passed on to offspring, while somatic mutations (that
occur in body cells) are not passed on.
N
Nanopore DNA Sequencing
Nanopore DNA sequencing is a laboratory technique for determining the exact sequence
of nucleotides, or bases, in a DNA molecule. The sequence of the bases (often referred to by the first
letters of their chemical names: A, T, C and G) encodes the biological information that cells use to
develop and operate. Nanopore DNA sequencing involves reading the code of single DNA strands as they are
threaded through extremely tiny pores (nanopores) embedded within a membrane. As the DNA moves through
the pore, it creates signals that can be converted to read each base. This approach offers a low-cost,
rapid process for studying long stretches of DNA.
Nanotechnology
Nanotechnology (often shortened to nanotech) is the understanding and use of matter
on an atomic and molecular scale for industrial purposes. Manipulating matter at nanoscale — between
approximately 1 and 100 nanometers — holds potential for novel applications in many fields, including
genomics, engineering, computer science and medicine.
Newborn Genetic Screening
Newborn screening is a set of laboratory tests performed on newborn babies to detect
a set of known genetic diseases. Typically, this testing is performed on a blood sample obtained from a
heel prick when the baby is two or three days old. In the United States, newborn screening is mandatory
for a defined set of genetic diseases, although the exact set differs from state to state. Newborn
screening tests focus on conditions for which early diagnosis is important to treating or preventing
disease.
Next-Generation DNA Sequencing
DNA sequencing establishes the order of the bases that make up DNA. Next-generation
DNA sequencing (abbreviated NGS) refers to the use of technologies for sequencing DNA that became
available shortly after the completion of the Human Genome Project (which relied on the first-generation
method of Sanger sequencing). Faster and cheaper than their predecessors, NGS technologies can sequence
an entire human genome in a single day and for less than 1,000.
Non-Coding DNA
Non-coding DNA corresponds to the portions of an organism’s genome that do not code
for amino acids, the building blocks of proteins. Some non-coding DNA sequences are known to serve
functional roles, such as in the regulation of gene expression, while other areas of non-coding DNA have
no known function.
Nonsense Mutation
A nonsense mutation occurs in DNA when a sequence change gives rise to a stop codon
rather than a codon specifying an amino acid. The presence of the new stop codon results in the
production of a shortened protein that is likely non-functional.
Northern Blot
Northern blot is a laboratory analysis method used to study RNA. Specifically,
purified RNA fragments from a biological sample (such as blood or tissue) are separated by using an
electric current to move them through a sieve-like gel or matrix, which allows smaller fragments to move
faster than larger fragments. The RNA fragments are transferred out of the gel or matrix onto a solid
membrane, which is then exposed to a DNA probe labeled with a radioactive, fluorescent or chemical tag.
The tag allows any RNA fragments containing complementary sequences with the DNA probe sequence to be
visualized within the Northern blot.
Nuclear Membrane
The nuclear membrane is a double layer that encloses the cell’s nucleus, where the
chromosomes reside. The nuclear membrane serves to separate the chromosomes from the cell’s cytoplasm
and other contents. An array of small holes or pores in the nuclear membrane permits the selective
passage of certain materials, such as nucleic acids and proteins, between the nucleus and cytoplasm.
Nucleic Acids
Nucleic acids are large biomolecules that play essential roles in all cells and
viruses. A major function of nucleic acids involves the storage and expression of genomic information.
Deoxyribonucleic acid, or DNA, encodes the information cells need to make proteins. A related type of
nucleic acid, called ribonucleic acid (RNA), comes in different molecular forms that play multiple
cellular roles, including protein synthesis.
Nucleolus
The nucleolus is a spherical structure found in the cell’s nucleus whose primary
function is to produce and assemble the cell’s ribosomes. The nucleolus is also where ribosomal RNA
genes are transcribed. Once assembled, ribosomes are transported to the cell cytoplasm, where they serve
as the sites for protein synthesis.
Nucleopore
A nucleopore is one of a series of openings found in the cell’s nuclear membrane.
Nucleopores serve as channels for the selective transport of nucleic acids and proteins into and out of
the cell nucleus.
Nucleosome
A nucleosome is the basic repeating subunit of chromatin packaged inside the cell’s
nucleus. In humans, about six feet of DNA must be packaged into a nucleus with a diameter less than a
human hair, and nucleosomes play a key role in that process. A single nucleosome consists of about 150
base pairs of DNA sequence wrapped around a core of histone proteins. In forming a chromosome, the
nucleosomes repeatedly fold in on themselves to tighten and condense the packaged DNA.
Nucleotide
A nucleotide is the basic building block of nucleic acids (RNA and DNA). A nucleotide
consists of a sugar molecule (either ribose in RNA or deoxyribose in DNA) attached to a phosphate group
and a nitrogen-containing base. The bases used in DNA are adenine (A), cytosine (C), guanine (G) and
thymine (T). In RNA, the base uracil (U) takes the place of thymine. DNA and RNA molecules are polymers
made up of long chains of nucleotides.
Nucleus
A nucleus, as related to genomics, is the membrane-enclosed organelle within a cell
that contains the chromosomes. An array of holes, or pores, in the nuclear membrane allows for the
selective passage of certain molecules (such as proteins and nucleic acids) into and out of the nucleus.
O
Oncogene
An oncogene is a mutated gene that has the potential to cause cancer. Before an
oncogene becomes mutated, it is called a proto-oncogene, and it plays a role in regulating normal cell
division. Cancer can arise when a proto-oncogene is mutated, changing it into an oncogene and causing
the cell to divide and multiply uncontrollably. Some oncogenes work like an accelerator pedal in a car,
pushing a cell to divide again and again. Others work like a faulty brake in a car parked on a hill,
also causing the cell to divide unchecked.
Open Reading Frame
An open reading frame, as related to genomics, is a portion of a DNA sequence that
does not include a stop codon (which functions as a stop signal). A codon is a DNA or RNA sequence of
three nucleotides (a trinucleotide) that forms a unit of genomic information encoding a particular amino
acid or signaling the termination of protein synthesis (stop codon). There are 64 different codons: 61
specify amino acids and 3 are used as stop codons. A long open reading frame is often part of a gene
(that is, a sequence directly coding for a protein).
P
Pedigree
A pedigree, as related to genetics, is a chart that diagrams the inheritance of a
trait or health condition through generations of a family. The pedigree particularly shows the
relationships among family members and, when the information is available, indicates which individuals
have a trait(s) of interest.
Peptide
A peptide is a short chain of amino acids (typically 2 to 50) linked by chemical
bonds (called peptide bonds). A longer chain of linked amino acids (51 or more) is a polypeptide. The
proteins manufactured inside cells are made from one or more polypeptides.
Pharmacogenomics
Pharmacogenomics (also called pharmacogenetics) is a component of genomic medicine
that involves using a patient’s genomic information to tailor the selection of drugs used in their
medical management. In this way, pharmacogenomics aims to provide a more individualized (or precise)
approach to the use of available medication in treating patients.
Phenotype
Phenotype refers to an individual’s observable traits, such as height, eye color and
blood type. A person’s phenotype is determined by both their genomic makeup (genotype) and environmental
factors.
Physical Map
A physical map, as related to genomics, is a graphical representation of physical
locations of landmarks or markers (such as genes, variants and other DNA sequences of interest) within a
chromosome or genome. A complete genome sequence is one type of physical map. Physical maps are used to
identify genes or other sequences believed to play a role in health conditions or diseases. They are
also valuable in providing an organizational framework for generating complete sequences of genomes.
Plasmid
A plasmid is a small circular DNA molecule found in bacteria and some other
microscopic organisms. Plasmids are physically separate from chromosomal DNA and replicate
independently. They typically have a small number of genes — notably, some associated with antibiotic
resistance — and can be passed from one cell to another. Scientists use recombinant DNA methods to
splice genes that they want to study into a plasmid. When the plasmid copies itself, it also makes
copies of the inserted gene.
Point Mutation
A point mutation occurs in a genome when a single base pair is added, deleted or
changed. While most point mutations are benign, they can also have various functional consequences,
including changes in gene expression or alterations in encoded proteins.
Polydactyly
Polydactyly is a condition in which a person has more than the normal number of
fingers or toes. It can occur in association with other physical anomalies or intellectual impairment,
or it may occur as an isolated birth defect. Polydactyly can either be inherited or it can arise
sporadically in an individual.
Polygenic Risk Score (PRS)
A polygenic risk score (abbreviated PRS) uses genomic information alone to assess a
person’s chances of having or developing a particular medical condition. A person’s PRS is a statistical
calculation based on the presence or absence of multiple genomic variants, without taking environmental
or other factors into account.
Polygenic Trait
A polygenic trait is a characteristic, such as height or skin color, that is
influenced by two or more genes. Because multiple genes are involved, polygenic traits do not follow the
patterns of Mendelian inheritance. Many polygenic traits are also influenced by the environment and are
called multifactorial.
Polymerase Chain Reaction (PCR)
Polymerase chain reaction (abbreviated PCR) is a laboratory technique for rapidly
producing (amplifying) millions to billions of copies of a specific segment of DNA, which can then be
studied in greater detail. PCR involves using short synthetic DNA fragments called primers to select a
segment of the genome to be amplified, and then multiple rounds of DNA synthesis to amplify that
segment.
Polymorphism
Polymorphism, as related to genomics, refers to the presence of two or more variant
forms of a specific DNA sequence that can occur among different individuals or populations. The most
common type of polymorphism involves variation at a single nucleotide (also called a single-nucleotide
polymorphism, or SNP). Other polymorphisms can be much larger, involving longer stretches of DNA.
Population Genomics
Population genomics is the large-scale application of genomic technologies to study
populations of individuals. For example, population genomics research can be used to study human
ancestry, migrations and health.
Positional Cloning
Positional cloning is a laboratory approach used to locate the position of a
disease-associated gene on a chromosome. Such a strategy can succeed even when nothing is known about
the role of the gene’s encoded protein in the disease. The technique typically relies on the use of
known polymorphic markers whose inheritance can be traced through various members of families affected
by the disease.
Precision Medicine
Precision medicine (generally considered analogous to personalized medicine or
individualized medicine) is an innovative approach that uses information about an individual’s genomic,
environmental and lifestyle information to guide decisions related to their medical management. The goal
of precision medicine is to provide more a precise approach for the prevention, diagnosis and treatment
of disease.
Primer
A primer, as related to genomics, is a short single-stranded DNA fragment used in
certain laboratory techniques, such as the polymerase chain reaction (PCR). In the PCR method, a pair of
primers hybridizes with the sample DNA and defines the region that will be amplified, resulting in
millions and millions of copies in a very short timeframe. Primers are also used in DNA sequencing and
other experimental processes.
Proband
A proband is an individual who is affected by a genetic condition or who is concerned
they are at risk. Usually, the proband is the first person in a family who brings the concern of a
genetic disorder to the attention of healthcare professionals.
Promoter
A promoter, as related to genomics, is a region of DNA upstream of a gene where
relevant proteins (such as RNA polymerase and transcription factors) bind to initiate transcription of
that gene. The resulting transcription produces an RNA molecule (such as mRNA).
Protein
Proteins are large, complex molecules that play many important roles in the body.
They are critical to most of the work done by cells and are required for the structure, function and
regulation of the body’s tissues and organs. A protein is made up of one or more long, folded chains of
amino acids (each called a polypeptide), whose sequences are determined by the DNA sequence of the
protein-encoding gene.
Pseudogene
A pseudogene is a segment of DNA that structurally resembles a gene but is not
capable of coding for a protein. Pseudogenes are most often derived from genes that have lost their
protein-coding ability due to accumulated mutations that have occurred over the course of evolution.
R
Race
Race is a social construct used to group people. Race was constructed as a hierarchal
human-grouping system, generating racial classifications to identify, distinguish and marginalize some
groups across nations, regions and the world. Race divides human populations into groups often based on
physical appearance, social factors and cultural backgrounds.
Recessive Traits and Alleles
Recessive, as related to genetics, refers to the relationship between an observed
trait and the two inherited versions of a gene related to that trait. Individuals inherit two versions
of each gene, known as alleles, from each parent. In the case of a recessive trait, the alleles of the
trait-causing gene are the same, and both (recessive) alleles must be present to express the trait. A
recessive allele does not produce a trait at all when only one copy is present. This contrasts to a
dominant trait, which requires that only one of the two alleles be present to express the trait.
Recombinant DNA Technology
Recombinant DNA technology involves using enzymes and various laboratory techniques
to manipulate and isolate DNA segments of interest. This method can be used to combine (or splice) DNA
from different species or to create genes with new functions. The resulting copies are often referred to
as recombinant DNA. Such work typically involves propagating the recombinant DNA in a bacterial or yeast
cell, whose cellular machinery copies the engineered DNA along with its own.
Repressor
A repressor, as related to genomics, is a protein that inhibits the expression of one
or more genes. The repressor protein works by binding to the promoter region of the gene(s), which
prevents the production of messenger RNA (mRNA). Repressor proteins are essential for the regulation of
gene expression in cells.
Restriction Enzyme
A restriction enzyme is a protein isolated from bacteria that cleaves DNA sequences
at sequence-specific sites, producing DNA fragments with a known sequence at each end. The use of
restriction enzymes is critical to certain laboratory methods, including recombinant DNA technology and
genetic engineering.
Restriction Fragment Length Polymorphism (RFLP)
Restriction fragment length polymorphism (abbreviated RFLP) refers to differences (or
variations) among people in their DNA sequences at sites recognized by restriction enzymes. Such
variation results in different sized (or length) DNA fragments produced by digesting the DNA with a
restriction enzyme. RFLPs can be used as genetic markers, which are often used to follow the inheritance
of DNA through families.
Retrovirus
A retrovirus is a virus that uses RNA as its genomic material. Upon infection with a
retrovirus, a cell converts the retroviral RNA into DNA, which in turn is inserted into the DNA of the
host cell. The cell then produces more retroviruses, which infect other cells. Many retroviruses are
associated with diseases, including AIDS and some forms of cancer.
Ribonucleic Acid (RNA)
Ribonucleic acid (abbreviated RNA) is a nucleic acid present in all living cells that
has structural similarities to DNA. Unlike DNA, however, RNA is most often single-stranded. An RNA
molecule has a backbone made of alternating phosphate groups and the sugar ribose, rather than the
deoxyribose found in DNA. Attached to each sugar is one of four bases: adenine (A), uracil (U), cytosine
(C) or guanine (G). Different types of RNA exist in cells: messenger RNA (mRNA), ribosomal RNA (rRNA)
and transfer RNA (tRNA). In addition, some RNAs are involved in regulating gene expression. Certain
viruses use RNA as their genomic material.
Ribosome
A ribosome is an intercellular structure made of both RNA and protein, and it is the
site of protein synthesis in the cell. The ribosome reads the messenger RNA (mRNA) sequence and
translates that genetic code into a specified string of amino acids, which grow into long chains that
fold to form proteins.
Risk
Risk, as related to genetics, refers to the probability that an individual will be
affected by a particular heritable or genetic disorder. Both a person’s genome and environmental
exposures can influence risk. An individual’s risk may be higher because they inherit a genetic variant
(or allele) in one gene or a combination of many variants in different genes that increases
susceptibility to or overtly causes a disorder. Other individuals may be at higher risk because they
have been exposed to one or more environmental factors that promote the development of a certain
disorder.
S
Scientific Racism
Scientific racism is a historical pattern of ideologies that generate and perpetuate
pseudoscientific racist beliefs that lead to racial bias and discrimination in science and research.
Leading scientists across scientific institutions in the 19th and early 20th centuries were proponents
of such ideologies. By the mid-20th century, pseudoscientific racist beliefs were widely disproven.
However, evidence shows that scientific racism persists in science and research.
Secondary Genomic Finding
A secondary genomic finding refers to a genomic variant, found through the analysis
of a person’s genome, that is of potential medical value yet is unrelated to the initial reason for
examining the person’s genome. In certain cases, a secondary genomic finding might offer clinicians the
chance to identify a previously unrecognized risk for disease that could change the medical management
of that patient and potentially prevent or more effectively treat the disease.
Sex Chromosome
A sex chromosome is a type of chromosome involved in sex determination. Humans and
most other mammals have two sex chromosomes, X and Y, that in combination determine the sex of an
individual. Females have two X chromosomes in their cells, while males have one X and one Y.
Sex-Linked
Sex-linked, as related to genetics, refers to characteristics (or traits) that are
influenced by genes carried on the sex chromosomes. In humans, the term often refers to traits or
disorders influenced by genes on the X chromosome, as it contains many more genes than the smaller Y
chromosome. Males, who have only a single copy of the X chromosome, are more likely to be affected by a
sex-linked disorder than females, who have two copies. In females, the presence of a second, non-mutated
copy may cause different, milder, or no symptoms of a sex-linked disorder.
Shotgun Sequencing
Shotgun sequencing is a laboratory technique for determining the DNA sequence of an
organism’s genome. The method involves randomly breaking up the genome into small DNA fragments that are
sequenced individually. A computer program looks for overlaps in the DNA sequences, using them to
reassemble the fragments in their correct order to reconstitute the genome.
Single Nucleotide Polymorphism (SNP)
A single nucleotide polymorphism (abbreviated SNP, pronounced snip) is a genomic
variant at a single base position in the DNA. Scientists study if and how SNPs in a genome influence
health, disease, drug response and other traits.
Somatic Cells
Somatic cells are the cells in the body other than sperm and egg cells (which are
called germ cells). In humans, somatic cells are diploid, meaning they contain two sets of chromosomes,
one inherited from each parent. DNA mutations in somatic cells can affect an individual, but they cannot
be passed on to their offspring.
Southern Blot
Southern blot analysis is a laboratory method used to study DNA. Specifically,
purified DNA from a biological sample (such as blood or tissue) is digested with a restriction
enzyme(s), and the resulting DNA fragments are separated by using an electric current to move them
through a sieve-like gel or matrix, which allows smaller fragments move faster than larger fragments.
The DNA fragments are transferred out of the gel or matrix onto a solid membrane, which is then exposed
to a DNA probe labeled with a radioactive, fluorescent or chemical tag. The tag allows any DNA fragments
containing complementary sequences with the DNA probe sequence to be visualized within the Southern
blot. The method is named for its creator, British molecular biologist Edwin Southern.
Stop Codon
A stop codon is a sequence of three nucleotides (a trinucleotide) in DNA or messenger
RNA (mRNA) that signals a halt to protein synthesis in the cell. There are 64 different trinucleotide
codons: 61 specify amino acids and 3 are stop codons (i.e., UAA, UAG and UGA).
Substitution
Substitution, as related to genomics, is a type of mutation in which one nucleotide
is replaced by a different nucleotide. The term can also refer to the replacement of one amino acid in a
protein with a different amino acid.
Susceptibility
Susceptibility, as related to genetics, refers to the state of being predisposed to,
or sensitive to, developing a certain disease. An individual’s disease susceptibility is influenced by a
combination of genetic and environmental factors.
Syndrome
A syndrome, as related to genetics, is a group of traits or conditions that tend to
occur together and characterize a recognizable disease. Some syndromes have a genetic cause.
T
Tandem Repeat
A tandem repeat is a sequence of two or more DNA bases that is repeated numerous
times in a head-to-tail manner on a chromosome. Tandem repeats are generally present in non-coding DNA.
In some cases, tandem repeats can serve as genetic markers to track inheritance in families. They can
also be useful for DNA fingerprinting in forensic studies.
Telomere
A telomere is a region of repetitive DNA sequences at the end of a chromosome.
Telomeres protect the ends of chromosomes from becoming frayed or tangled. Each time a cell divides, the
telomeres become slightly shorter. Eventually, they become so short that the cell can no longer divide
successfully, and the cell dies.
Thymine
Thymine (T) is one of the four nucleotide bases in DNA, with the other three being
adenine (A), cytosine (C) and guanine (G). Within a double-stranded DNA molecule, thymine bases on one
strand pair with adenine bases on the opposite strand. The sequence of the four nucleotide bases encodes
DNA’s information.
Trait
A trait, as related to genetics, is a specific characteristic of an individual.
Traits can be determined by genes, environmental factors or by a combination of both. Traits can be
qualitative (such as eye color) or quantitative (such as height or blood pressure). A given trait is
part of an individual’s overall phenotype.
Transcription
Transcription, as related to genomics, is the process of making an RNA copy of a
gene’s DNA sequence. This copy, called messenger RNA (mRNA), carries the gene’s protein information
encoded in DNA. In humans and other complex organisms, mRNA moves from the cell nucleus to the cell
cytoplasm (watery interior), where it is used for synthesizing the encoded protein.
Transfer RNA (tRNA)
Transfer RNA (abbreviated tRNA) is a small RNA molecule that plays a key role in
protein synthesis. Transfer RNA serves as a link (or adaptor) between the messenger RNA (mRNA) molecule
and the growing chain of amino acids that make up a protein. Each time an amino acid is added to the
chain, a specific tRNA pairs with its complementary sequence on the mRNA molecule, ensuring that the
appropriate amino acid is inserted into the protein being synthesized.
Transgenic
Transgenic refers to an organism or cell whose genome has been altered by the
introduction of one or more foreign DNA sequences from another species by artificial means. Transgenic
organisms are generated in the laboratory for research purposes.
Translation
Translation, as related to genomics, is the process through which information encoded
in messenger RNA (mRNA) directs the addition of amino acids during protein synthesis. Translation takes
place on ribosomes in the cell cytoplasm, where mRNA is read and translated into the string of amino
acid chains that make up the synthesized protein.
Translocation
A translocation, as related to genetics, occurs when a chromosome breaks and the
(typically two) fragmented pieces re-attach to different chromosomes. The detection of chromosomal
translocations can be important for the diagnosis of certain genetic diseases and disorders.
Tumor Suppressor Gene
A tumor suppressor gene encodes a protein that acts to regulate cell division,
keeping it in check. When a tumor suppressor gene is inactivated by a mutation, the protein it encodes
is not produced or does not function properly, and as a result, uncontrolled cell division may occur.
Such mutations may contribute to the development of a cancer.
U
Uracil
Uracil (U) is one of the four nucleotide bases in RNA, with the other three being
adenine (A), cytosine (C) and guanine (G). In RNA, uracil pairs with adenine. In a DNA molecule, the
nucleotide thymine (T) is used in place of uracil.
V
Variant of Uncertain Significance (VUS)
When analysis of a patient’s genome identifies a variant, but it is unclear whether
that variant is actually connected to a health condition, the finding is called a variant of uncertain
significance (abbreviated VUS). In many cases, these variants are so rare in the population that little
information is available about them. Typically, more information is required to determine if the variant
is disease related. Such information may include more extensive population data, functional studies, and
tracing the variant in other family members who have or do not have the same health condition.
Vector
A vector, as related to molecular biology, is a DNA molecule (often plasmid or virus)
that is used as a vehicle to carry a particular DNA segment into a host cell as part of a cloning or
recombinant DNA technique. The vector typically assists in replicating and/or expressing the inserted
DNA sequence inside the host cell.
Virus
A virus is an infectious microbe consisting of a segment of nucleic acid (either DNA
or RNA) surrounded by a protein coat. A virus cannot replicate alone; instead, it must infect cells and
use components of the host cell to make copies of itself. Often, a virus ends up killing the host cell
in the process, causing damage to the host organism. Well-known examples of viruses causing human
disease include AIDS, COVID-19, measles and smallpox.
X
X Chromosome
The X chromosome is one of the two sex chromosomes that are involved in sex
determination. Humans and most other mammals have two sex chromosomes (X and Y) that in combination
determine the sex of an individual. Females have two X chromosomes in their cells, while males have one
X and one Y.
X-Linked
X-linked, as related to genetics, refers to characteristics or traits that are
influenced by genes on the X chromosome. Humans and most other mammals have two sex chromosomes, X and
Y. Females have two X chromosomes in their cells, while males have one X and one Y. In the case of an
X-linked disease, it is usually males that are affected because they have a single copy of the X
chromosome that carries the disease-causing mutation. In females, the presence of a second, non-mutated
copy may cause different, milder, or no symptoms of a sex-linked disorder.
Y
Y Chromosome
The Y chromosome is one of the two sex chromosomes that are involved in sex
determination. Humans and most other mammals have two sex chromosomes (X and Y) that in combination
determine the sex of an individual. Females have two X chromosomes in their cells, while males have one
X and one Y.
