NIJ Training Course for Forensic DNA Officers of the Court: Homozygous and Heterozygous Genotypes

Type at any particular locus on a pair  of chromosomes can be characterized as either homozygous or heterozygous. If that  locus displays two alleles that are the same, its genotype is referred to as homozygous;  if the locus displays two different alleles, that genotype is referred to as heterozygous. Note  that there are two possible homozygous genotypes:  in this case homozygous brown or homozygous  blonde. There is, however, only one heterozygous genotype possible at any particular locus,  although there are two ways to achieve it: the brown allele can be inherited from the father  and the blonde from the mother or vice versa, but the genetic outcome is the same either way.  The genotype displays one of each possible allele.