Why Inbreeding Leads to Increased Homozygosity

These feral dogs on Sri Lanka appear to be coming from a close-breeding population, which would explain their highly similar phenotypes. Image is under a Creative Commons license on Flickr.com.

In my previous post, I mentioned that inbreeding leads to an increase in homozygosity in the population. I thought it would be best if I explained why this is true. Again, it goes back to a basic understanding of population genetics.

Let's say that there is a population of random-bred dogs that has a genotypic ratio of 1:2:1 for the gene for a long coat. That means 25% of the population is homozygous dominant, 50% of the population is heterozygous, and 25% of the population is homozygous recessive. If mating were to continue to be random, then the genotypic proportions for the next generation would continue to be 1:2:1. It is one of the basics of Hardy-Weinberg equilibrium.The Hardy-Weinberg principle (which states conditions under which evolution, i.e. changes in a population, cannot occur) and equilibrium are simple ways to calculate whether a population is changing over time and gauging the rate of selection. As such is a basic principle for determining the health of populations, and can be applied to incidences of both natural and artificial selection.

Even feral population can undergo heavy inbreeding, but the selection that leads to that inbreeding is natural rather than artificial and can thus produce a far healthier population. Image is copyright free from Wikimedia Commons.

The rub comes when the mating becomes nonrandom. This is most easily represented with selfing (mating with oneself, which is only possible in monoecious individuals), which is the most extreme form of inbreeding possible. So, if the individuals with the different genotypes only bred with their own genotype, the following would occur: the homozygotes would only produce other homozygous individuals, but the heterozygotes would continue to produce the 1:2:1 ratio. So, there would be a net increase in the number of homozygotes in the next generation. Generation after generation, more and more homozygotes would result, until the percentage of heterozygous individuals approaches or even reaches zero.

This same principle applies when inbreeding in less extreme ways, whether it is brother-sister, grandfather-granddaughter, or even if it is somewhat less close of a relationship. Individuals who are relatives are more likely to share the same genotype, and thus to cause a net increase in homozygosity. Many breeders see this as a positive things, as it helps fix "type," but it can also easily lead to some deleterious recessives cropping up at much higher frequencies than if mating were more random. This is why so many health problems have become serious issue in purebred populations. Rampant hip dysplasia, patellar luxation, depressed immune systems, epilepsy, bladder stones and all of the other strange issues that plague breeds are all a fault of high levels of homozygosity.

With modern medicine, many of these bad traits are ignored and continue to be bred from. Even if efforts are occurring to breed for healthy individuals, in a closed population, inbreeding is still inbreeding. Even if a breed that has a high incidence of hip dysplasia can breed it out, it is quite possible that some other serious health issue will pop up due to the continued increase in homozygosity. The newly occurring issue may be even worse than the one that was initially bred out.

If your would like to read more on inbreeding, Christopher over at Border Wars wrote a post that has a lot more in-depth information on inbreeding, particularly the coefficients thereof. Also, there is a look at inbreeding in dioecious organisms.