Genetics and Breeding

Outbreeding (also called crossbreeding) is the opposite of inbreeding, in that unrelated animals are mated with the effect of increasing heterozygosity. Let’s step through an example by starting with two unrelated populations, 1 and 2. Population 1 has gene frequencies at the A locus of p1 = 0.8 and q1 = 0.2. Population 2 has gene frequencies at the A locus of p2 = 0.1 and q2 = 0.9. Right away we can tell the two are unrelated as the gene frequencies differ so much. Now cross them to create an F1 generation. (Back in Explaining Mendel’s Results Visually we defined an F1 generation as one resulting from the crossing of two purebred populations. Here we are using it more broadly to include two unrelated, but not necessarily purebred, populations.) more »

Once animals have been selected for breeding, the next step is to decide which are mated to which via a mating system. Some examples of mating systems, each with different intentions, were briefly covered here. Last week we saw how selecting for a particular allelic expression (ie selecting for ‘best’ phenotype) can change the gene frequency significantly in a population. Genotypic frequencies change indirectly as a result — they ‘tag along’. Mating systems do however change genotypic frequencies directly, with gene frequencies less affected. These systems fall into two general categories: inbreeding systems and outbreeding systems. Inbreeding is a mating system which increases homozygosity. Outbreeding is a mating system which increases heterozygosity. more »