Basic Genetic Terminology

Locus: The specific physical location of a gene on a chromosome (plural loci)

Allele: One of two or more versions of a gene. A cat inherits two alleles for each gene, one from each parent. If the two alleles are the same, the cat is said to be homozygous. If the alleles are different, the cat is deemed heterozygous.

Genotype: The combination of alleles that a cat has at a particular locus

Phenotype: The physical appearance impacted by the genotype and/or environment

Recessive: Two copies of an allele are needed to express the phenotype (a recessive allele is hidden in heterozygous cats)

Dominant: Only one copy of an allele is needed to express the phenotype (a dominant allele will hide the other allele if the cat is a heterozygote)

Coat, Nose, Paw Pad, and Eye Color Pigment: All black or brown (“liver”) areas are caused by cells producing eumelanin (black/brown pigment). However, there are genes that change the structure or production of eumelanin, changing the phenotype to brown, blue, or isabella. Brown is caused by a change in the structure of eumelanin, blue is diluted black pigment, and isabella is diluted brown pigment.

Phaeomelanin is a reddish-yellow pigment produced in the coat. It does not occur in the eyes or the nose; only eumelanin occurs in those areas so only genes that affect eumelanin can affect their color.