Definition
What is a genotype?
A genotype is the set of alleles an organism carries. The word can refer to a single locus, one specific position in the genome, or to the organism's entire genetic makeup. At a locus with two alleles, the possible genotypes are written with letters, such as AA, Aa and aa, or with the actual DNA bases present, such as CC, CT and TT. The genotype includes alleles that stay hidden, such as a recessive allele masked by a dominant one in a heterozygote.
The genotype contributes to the phenotype, the traits that can be observed, but the two do not match one to one. Different genotypes can give the same phenotype, as when AA and Aa pea plants both show the dominant trait, and the environment also shapes how genes show their effects. Geneticists infer hidden genotypes from crosses, such as a test cross with a homozygous recessive partner, and from family pedigrees.
Today, genotypes can also be read directly. Genotyping by DNA sequencing or by SNP arrays scores millions of positions at once, which is the basis of genome-wide association studies.
Word parts
Where does the word “genotype” come from?
- gen-
- birth, descentGreek genos
- -type
- mark, modelGreek typos
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Terms that go with it
- PhenotypeA phenotype is the set of observable traits of an organism, such as its height or blood group, produced by its genotype acting with its environment.
- AlleleAn allele is any of the alternative forms of a gene or DNA sequence that can occupy the same position, or locus, on a chromosome.
- HomozygousHomozygous describes an organism that carries two identical alleles of a gene at a given locus, one inherited from each parent.
- HeterozygousHeterozygous describes an organism that carries two different alleles of a gene at a given locus, such as one dominant and one recessive allele.
- Single nucleotide polymorphismA single nucleotide polymorphism (SNP) is a position in the genome where people differ by a single DNA base, the most common kind of genetic variation.
- Genome-wide association studyA genome-wide association study (GWAS) scans the genomes of many people to find genetic variants that are more common in those with a trait or disease.
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