genotype

Your genotype is the particular set of genetic instructions you carry, the exact spelling of your genes. If the genome is a thick book of recipes, your genotype is your personal copy, with all of its individual wordings, that no one else carries in exactly the same way.

More formally, a genotype is the combination of alleles an organism has, either across its whole genome or at one gene of interest. Because most cells carry two copies of each gene, a genotype at a single gene is usually written as a pair, such as two identical alleles or two different ones. This pairing decides how the gene's instruction is read.

The genotype is inherited and, apart from rare mutations, stays the same in every cell throughout life. It sets the menu of possibilities for an organism, but it does not by itself determine the final outcome. The same genotype can lead to somewhat different results depending on environment and chance.

Genotype is best understood as one half of a pair, the other being the phenotype, the observable result. Geneticists constantly move between the two: predicting a phenotype from a known genotype, or inferring a hidden genotype from an observed phenotype and a family history.