Any one gene is located at a particular place on a chromosome, called its genetic locus

Because each individual possesses a double set of chromosomes, it also possesses a double set of each of its genes. an individual is therefore said to have two genes at each genetic locus in its DNA.

One gene comes from its father and the other from its mother. The two genes at a locus are called a genotype.

The two copies of a gene in an individual may be the same, or slightly different. If they are the same the genotype is a homozygote; if they differ it is called a heterozygote.

The different forms of the gene that can be present at a locus are called alleles.

The genotype at a locus should be distinguished from the phenotype it produces: the complete physical characteristics of an organism.

Biologists often disagree about the exact definition of the gene. Here, the geneticist Linda Partidge offers her own definition.