7.5: Gene Expression Patterns

Dominant alleles are expressed to the exclusion of recessive alleles in heterozygous individuals. A recessive allele is only expressed when the dominant allele is absent from the animal's genome (i.e., in homozygous recessive individuals).

However, not all inheritance follows simple dominant-recessive patterns. In codominance, neither allele masks the other, and both are expressed simultaneously in the phenotype (e.g., roan coat color in cattle, where red and white hairs appear together). Incomplete dominance occurs when no dominance exists, and the heterozygous individual expresses an intermediate phenotype between the two homozygous forms.

Epistasis describes the interaction among genes at different loci, where the expression of genes at one locus depends on the presence of specific alleles at another locus. When a gene has more than two alleles present within a population—termed multiple alleles—an individual can still only possess two alleles at that locus, but population-wide variation increases.

A testcross is a practical breeding tool in which an animal expressing the dominant phenotype (homozygous dominant or heterozygous) is mated with a homozygous recessive individual to determine the unknown genotype based on the offspring phenotypes.

Sex-linked inheritance involves traits carried on the X or Y chromosomes. Genes on the X chromosome can be passed to either sex, but Y-linked genes are only inherited by males. Sex-influenced traits are carried on autosomes but expressed differently in males and females due to hormonal differences (e.g., horn development in sheep). Sex-limited traits are unique to only one sex, even though both sexes carry the genes (e.g., milk production, litter size).

Image credit: A. G. Rosales from Pexels: https://www.pexels.com/photo/majesti...ield-28410824/

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