Chromosome Abnormalities Fact Sheet

National Human Genome Research Institute

National Institutes of Health
U.S. Department of Health and Human Services


Chromosome Abnormalities

What are chromosomes?

Chromosomes are the structures that hold our genes. Genes are the individual instructions that tell our bodies how to develop and function; they govern our physical and medical characteristics, such as hair color, blood type and susceptability to disease.

Each chromosome has a p and q arm; p is the shorter arm and q is the longer arm. The arms are separated by a pinched region known as the centromere.

 

Top of page

Where are chromosomes found in the body?

The body is made up of individual units called cells. Your body has many different kinds of cells, such as skin cells, liver cells and blood cells. In the center of most cells is a structure called the nucleus. This is where chromosomes can be found.

 

Top of page

How many chromosomes do humans have?

The typical number of chromosomes in a human cell is 46 - two pairs of 23 - holding an estimated 25,000 genes. One set of 23 chromosomes is inherited from the biological mother (from the egg), and the other set is inherited from the biological father (from the sperm).

 

Top of page

How do scientists study chromosomes?

In order for chromosomes to be seen with a microscope, they need to be stained. Once stained, the chromosomes look like strings with light and dark "bands" and their picture can be taken. A picture, or chromosome map, of all 46 chromosomes is called a karyotype. The karyotype can help identify chromosome abnormalities that are evident in either the structure or the number of chromosomes.

To help identify chromosomes, the pairs have been numbered from 1 to 22, with the 23rd pair labeled "X" and "Y." In addition, each chromosome arm is defined further by numbering the bands that appear after staining; the higher the number, the further that area is from the centromere.

The first 22 pairs of chromosomes are called "autosomes" and the final pair is called the "sex chromosomes." The sex chromosomes an individual has determines that person's gender; females have two X chromosomes (XX), and males have an X and a Y chromosome (XY).

 

Top of page

What are chromosome abnormalities?

A chromosome abnormality reflects an abnormality of chromosome number or structure. There are many types of chromosome abnormalities. However, they can be organized into two basic groups:

Numerical Abnormalities:

When an individual is missing either a chromosome from a pair (monosomy) or has more than two chromosomes of a pair (trisomy). An example of a condition caused by numerical abnormalities is Down Syndrome, also known as Trisomy 21 (an individual with Down Syndrome has three copies of chromosome 21, rather than two). Turner Syndrome is an example of monosomy, where the individual - in this case a female - is born with only one sex chromosome, an X.

Structural Abnormalities:

When the chromosome's structure is altered. This can take several forms:

Most chromosome abnormalities occur as an accident in the egg or sperm. Therefore, the abnormality is present in every cell of the body. Some abnormalities, however, can happen after conception, resulting in mosaicism, where some cells have the abnormality and some do not.

Chromosome abnormalities can be inherited from a parent (such as a translocation) or be "de novo" (new to the individual). This is why chromosome studies are often performed on parents when a child is found to have an abnormality.

 

Top of page

How do chromosome abnormalities happen?

Chromosome abnormalities usually occur when there is an error in cell division. There are two kinds of cell division.

In both processes, the correct number of chromosomes is supposed to end up in the resulting cells. However, errors in cell division can result in cells with too few or too many copies of a chromosome. Errors can also occur when the chromosomes are being duplicated.

Other factors that can increase the risk of chromosome abnormalities are:

 

Top of page

Glossary of Terms

When reading about chromosomes and chromosome abnormalities, you may come across words that you are not familiar with. You can find the meanings of many of these words in the Talking Glossary of Genetic Terms. A few additional terms are listed in the glossary below.

Acrocentric chromosomes - those chromosomes, specifically numbers 13, 14, 15, 21 and 22, that are able to take part in Robertsonian translocations.

Chromatin - The complex of nucleic acids (DNA or RNA) and proteins that make up chromosomes; located in the cell nucleus.

de novo - a chromosome abnormality that occurred in the individual and was not inherited from the parents.

Inversions - a portion of the chromosome has broken off, turned upside down and reattached, therefore the genetic material is inverted.

Meiosis - cell division resulting in gametes (eggs and sperm cells), each containing half the number of chromosomes as normal cells.

Mitosis - cell division resulting in cells that have the normal amount of chromosomes.

Mosaicism - abnormal chromosome division resulting in two or more kinds of cells, each containing different numbers of chromosomes (chromosome mosaicism).

Reciprocal translocation - when segments from two different chromosomes have been exchanged.

Ring chromosome - a portion of a chromosome has broken off and formed a circle or ring. This can happen with or without loss of genetic material.

Robertsonian translocation - when two chromosomes fuse, usually at the centromere, creating a translocation. Only certain chromosomes, called acrocentric chromosomes, are capable of participating in this kind of translocation.

Telomere - specialized DNA sequences located at the end of chromosomes.

 

Top of page

Last Reviewed: February 12, 2014