6.5: Inevitable inbreeding

Inbreeding due to genetic drift cannot be fully avoided because drift always occurs in a population. To get a feeling for why this is, consider the fact that everybody has two parents, which makes four grandparents, 16 great-grandparents, etc. The number of ancestral parents n generations ago thus becomes 2ⁿ. This figure becomes very large in only a limited number of generations. In other words, your parents must be related, so you are inbred. Now it is easier to understand that drift occurs in all populations, but especially in smaller ones. The larger the population the smaller the chance that related individuals will mate by coincidence. Genetic diversity is at its largest when all animals would be heterozygous. Increased homozygosity means reduction in genetic diversity. Mating related individuals increases homozygosity, and thus decreases genetic diversity. Mating related individuals because you can’t avoid it results in loss of alleles because of genetic drift. Mating related individuals on purpose also creates homozygous animals not necessarily results in loss of alleles because families are no longer mixed. Alleles do get fixed, but different alleles may get fixed in different families. At population level this does not have an influence on allele frequencies.