Traits that are linked to alleles can be either homozygous or heterozygous, according to the Genetics Home Reference at the National Institutes of Health. These are also known as recessive and dominant traits, respectively.
A trait is homozygous, or recessive, if it requires expression on both alleles to be shown. A trait is heterozygous, or dominant, if it requires expression on only one allele to be shown. For instance, the agouti fur color in guinea pigs and hamsters is dominant, and is expressed as AA, while the black fur color is recessive and expressed as aa. An individual inherits one allele from each parent for each gene.