Inbreeding is the mating of closely related animals to increase homozygosity within a population. Common alleles become more concentrated — the gene frequency increases in other words — and animals become more and more closely related with each generation. The reliability of high performing animals producing more high performing animals becomes very predictable. It sounds like the only breeding approach you’ll ever need, but there can be consequences.
Outbreeding on the other hand increases heterozygosity by mating unrelated animals. New alleles are introduced and the gene pool widens. From this and the Hardy-Weinberg Equilibrium, it would appear that a breeding programme would go nowhere fast were it to rely solely on outbreeding. Yet there can be benefits.
Inbreeding is the time-honoured way of ‘fixing’ genes within a population. In some populations, a particular allele may be the only one at a particular locus, ensuring that that allele, and only that allele, is ever passed on to progeny. A breeding advantage has been obtained if the phenotype it codes for is particularly desirable and superior to other phenotypes outside that population. The breeding value of that population, and the animals within it, has increased.
Inbreeding leads to uniformity — animals homozygous for several alleles can only ever pass those alleles on, and populations of animals of similar homozygosity ensures their offspring will also be much like them. Some inbred animals are prepotent — so exceptional in their ability to produce offspring much like themselves that those offspring have a distinct and recognisable ‘look’.
Fixing genes within a population isn’t always all good and one way though. Inbreeding can fix the undesirable genes too. Many genetic diseases are rare and recessive in ‘natural’ populations — carriers would be heterozygous and unaffected, and affected animals (those with two copies of the recessive allele) would normally be expected to appear only every other generation, if at all.
Before genetics was much known about, it wasn’t unusual for people to mate, say, a prize bull with his daughters as well as his granddaughters. This was a very effective way of increasing the frequency of ‘him’ in subsequent generations. However, it also wasn’t unusual for abnormalities to suddenly appear in the new generations. That prize bull would have been heterozygous for some fault masked by his heterozygosity, but the ‘bad’ allele increased in frequency along with his desirable alleles, and expressed itself more and more as his descendants became more and more homozygous at that locus.
Thus the gene frequency of usually rare diseases can actually increase in an inbred population, and made worse in small populations through random genetic drift.
Inbreeding can also lead to inbreeding depression, which is where a population becomes less reproductively sound over time because of the increased homozygosity within it. Too much homozygosity is linked to reduced fertility, increased stillbirths, and higher mortality of those that are born. There are not enough ‘other’ alleles available to counter the effects of too many doubled-up alleles in the genome. There is not enough genetic diversity, in other words. Read the sad, sorry tale of King Charles II of Spain, more inbred than if his parents were brother and sister!
Removal or disappearance of certain genes via inbreeding and genetic drift can indirectly remove other genes if linked on the same chromosome. A genome may decrease significantly as a result, and a lack of genetic variation in a population can make it less adaptive to diseases or other threats. Genetic drift may remove a chance desirable gene or lead to fixation of an undesirable one.
On the surface, increasing heterozygosity would appear to take a breeding programme backwards, and make it harder to fix uniformity within a population. Yet increasing heterozygosity is what makes outbreeding an effective tool to offset the cons of too much inbreeding.
Outbreeding — whether between two different breeds or between two unrelated populations of the same breed — is the only way to introduce new alleles or reintroduce old ones. The increased genetic diversity creates the opposite of inbreeding depression: heterosis, also known as hybrid vigour or outbreeding enhancement. Increased fertility, fewer stillbirths, and higher survival rates result. The larger gene pool offers more resilience to disease as well.
Undesirable alleles which became fixed can be bred out, as there are more, different, alleles at those loci to work with. And outcrossing is the only way to introduce completely new genes from one breed to another, whether the intent is to use those hybrids commercially (eg as with black baldies) or to fix those genes into the second breed with careful selections and matings of that F1 generation.
Increased heterozygosity can be undesirable in some situations. Too much outbreeding can restore the Hardy-Weinberg Equilibrium and undo years of effort. Outbreeding a small population may, in extreme cases, lead to a loss of desirable alleles through random genetic drift.
A breeder’s role is to shift gene and genotypic frequencies in their animal population so as to produce the ‘best’ they possibly can. A breeder must take into account both the good and the bad of inbreeding and outbreeding, as well as consider the consequences of both the Hardy-Weinberg Equilibrium and random genetic drift.
A good breeder will know that too much inbreeding comes with a large health cost, and that while outbreeding maintains a genetically healthy population, it cannot drive that population to ‘best’. It would seem that a programme that utilises some inbreeding for allele fixation with some outbreeding for the benefits that brings is a good compromise.
Genetic advancement will be slower, but commitment, patience and perseverance are just some of the requirements of a successful breeder. If you’ve read everything on this blog up until now, you’re also accumulating a fair bit of knowledge and information as well. There is plenty more of that to come still, but we will also soon be getting into the other item on that list, the application of all that theory!
This concludes this introduction to Population Genetics. The next few posts will examine simple and polygenic traits — their differences, their commonalities, and how to work with each in a breeding programme.