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updated: September 28, 2022


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.



Genotype. Imagine that your friend is reading some text out of a book and you're writing it down. The text is, "It was a dark and stormy night." You might ask, what is the second word? Your friend reads the text and says, "was." In this example, your friend is reading the text and telling you what is present at a certain position in the sentence. In a different sentence, the word might be blue or windy. For DNA, the genotype is simply the specific information encoded at a given position in the genome. Like the sentence example, there can be misspellings. For the example sentence, perhaps the word "was" is spelled "waz" in the book. For DNA, a genotype misspelling is sometimes called a mutation and may have consequences for health and well-being.

David Adams
David Adams, M.D., Ph.D.

Senior Clinician

Office of the Clinical Director, NHGRI