Genetics and Breeding

We can easily identify homozygous recessive genotypes and partially-dominant traits simply by looking at the phenotypes of the progeny. Identifying carriers of recessive alleles isn’t as simple, as recessive alleles are hidden — the phenotype of an ‘AA’ animal is indistinguishable from that of an ‘Aa’ animal. The purpose of test matings is to identify carriers of recessive alleles by forcing any such alleles that may be present to appear in progeny. It takes just one such progeny to be born to show without doubt that the tested parent is indeed a carrier. But as there is no guarantee of such a birth, it is more a matter of knowing how many offspring must be born to be sure that the tested animal is definitively not a carrier. more »

Consider any species that typically gives birth to one offspring per mating, and that within that species is any simply-inherited trait with a dominant ‘A’ allele and a recessive ‘a’ allele. Now consider four individuals of that species, all with known genotypes: a heterozygous dominant male (’Aa’), a homozygous dominant female (’AA’), a heterozygous dominant female (’Aa’), and a homozygous recessive female (’aa’). Assume a genetic test doesn’t exist. The mating outcomes of the male over each of these females can be summarised as: more »