The phenotype of a diploid cell may depend upon the interaction of two alleles of a gene because a recessive gene can express itself only in the absence of its dominant allele (note, however, that some genes are neither dominant or recessive). This brings us to two very important words in the language of genetics, homozygote and heterozygote that describe the general genetic constitution of diploid cells.
A homozygote is a zygote, or other diploid cell, derived from the union of two gametes of identical genotype. In practice, however, homozygosity requires evidence from genetic crosses that a particular pair of genes is identical, and it is obviously impossible to prove that all the thousands of pairs of genes in what appears to be a homozygous individual are in fact identical. So the words homozygote and heterozygote have a narrower connotation and denote the relationship between specific homologous chromosome regions or genes.
We say that a diploid cell is homozygous for gene A, which means that it has the genetic constitution either AA or aa, or, as it is usually written, A/A and a/a, where A and a are two distinguishable alleles. A cell may be homozygous for gene A but heterozygous for gene B (AB/Ab).
