A genotype is the genetic makeup of an individual cell or organism that determines or contributes to its phenotype. In a broad sense, the term "genotype" refers to the genetic makeup of an organism; in other words, it describes an organisms complete set of genes. In a more narrow sense, the term can be used to refer to the alleles, or variant forms of a gene, that are carried by an organism. For example, humans are diploid organisms, which means that they have two alleles at each genetic position, or locus, with one allele inherited from each parent. Each pair of alleles represents the genotype of a specific gene.

Genotypes can be represented by symbols, such as BB, Bb, bb, or 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.

A particular genotype is described as homozygous if it features two identical alleles and as heterozygous if the two alleles differ. The process of determining a genotype is called genotyping.

In summary, a genotype refers to the genetic makeup of an organism, including the alleles or variant forms of a gene that are carried by an organism. Genotypes can be represented by symbols or by the actual DNA sequence at a specific location. Some genotypes contribute to an individual’s observable traits, called the phenotype.