The Argument for Assortative Mating
by John Armstrong

Breeders often talk about inbreeding and outcrossing as though they were the only possibilities - and generally with negative comments about the latter. There are others, and I have long been a proponent of assortative mating. It is not a theoretical concept that doesn't work in practice; I know several breeders who do it an achieve good results. This essay will attempt to explain why it is a good idea -- but first I need to define some other terms.

Random mating. Though not a common breeding practice, understanding what this implies is important. Random mating is exactly what the name implies. Mates are chosen with no regard for similarity or relatedness (if the population is inbred to some extent, mates may be related).

Random mating is one of the assumptions behind the Hardy-Weinberg formula that allows one to calculate the frequency of heterozygous carriers from the frequency of individuals expressing some recessive trait in a population. Because inbreeding among purebred dogs and in other small populations decreases the frequency of heterozygotes, these estimates may be higher than the actual incidence.

Inbreeding is the mating of two individuals who share a common ancestor. (Sometimes it is defined as the mating of two individuals who share a greater common ancestry than the average for the population. However, I regard that as unfair.)

Because the number of ancestors potentially doubles every generation you go back in a pedigree, you eventually get to a point, even in a very large population, where there are simply not enough ancestors. Thus, all populations are inbred to some degree. A maximum number of ancestors - the "effective population size" - is reached in some past generation. This number will be governed by various factors such as the total population size, how far individuals travel during their lifetime and whether there are inbreeding taboos or other mechanisms that reduce the likelihood of close relatives mating.

Inbreeding, by itself, does not lead to a change in allele frequencies, but increases the proportion of homozygotes. Thus, in the absence of other practices, it will lead to a higher proportion of individuals homozygous for deleterious genes and these are likely to be lost by natural selection (they do not survive to reproductive age) or their removal from the breeding pool by man.

Studies in many species suggest that the resulting population is not healthier as a result and suggests that a certain amount of heterozygosity may be desirable.

Line-breeding is merely a term used for a particular type of inbreeding. An example of inbreeding would be the mating of full sibs or double first cousins. One is doubling up on two ancestors equally. More often there is one ancestor who is considered exceptional and, particularly if it is a male, he may end up as father, grandfather and great-grandfather in the same pedigree. Father-daughter, mother-son and some other combinations result in a disproportionate number of genes coming from one ancestor.

Assortative mating is the mating of individuals that are phenotypically similar. It is a normal practice, to some degree, for humans and various other species. Though phenotype is a product of both genotype and environment, such individuals are more likely to carry the same alleles for genes determining morphology. If we are talking about a conformation that is basically sound from the structural point of view, the genes involved will have been subjected to natural selection for thousands of years, and will most likely be dominant. The major characteristics that set one breed apart from another will likely have been fixed early in the breed's history. ("Fixed" means that there is only one allele of present in the population. If there is only one allele, the question of dominance does not apply unless you mix breeds.)

Consequently, when you look at a dog, you are looking at his genes. If the conformation (or, for that matter, the temperament, intelligence or whatever) is not good, then you are very likely looking at a dog or a breed that is homozygous for one or more recessive alleles that you would probably like to get rid of. If it is the dog and not the breed, you may elect not to breed him, or you may look for a mate that covers the problem. If it is the breed, the only solution would be to introduce some genes from another breed. (That would be an outcross!)

Breeding together animals that share dominant good alleles for most of their genes will produce mainly puppies that also carry these genes. Even it be parents are not homozygous for all these good alleles, you should still get many that are suitable. More important, if animals heterozygous for certain genes are more fit, assortative mating will preserve more heterozygosity than inbreeding. However, unlike inbreeding, assortative mating should not result in an increased risk of the parents sharing hidden recessive mutations. Though we might like to eliminate delererious recessives, everyone carries a few. Trying to find the "perfect dog", without either visible or hidden flaws, is like betting on the lottary. There may conceivably be a big winner out there, but thery are certainly not common.

The risks involved.

Some trait that breeders consider desirable could be the result of homozygosity for a recessive allele for gene A or gene B. Obviously, crossing an AAbb with an aaBB will produce AaBb progeny that will not express this trait. (Aside from some of the genes affecting coat color, I can think of no examples.)

If care is not taken to go back far enough in the pedigrees, you may have two animals with similar phenotypes resulting from common ancestry. Whether you are inbreeding unintentionally or intentionally, the consequences are the same. The solution is simple: check the heritage.

Because assortative mating involves selection (you are hopefully mating the best together, and not the worst), you are denying some dogs the opportunity to pass their genes on to the next generation. This is, perhaps, the subtlest of risks as it does not seem to involve doing anything "wrong". Most would argue that it is merely doing what nature does - eliminating the least fit. But what if some of these "less-than-best" happen to be the only ones to carry the best allele for some gene? Out goes the good with the bad!

This is primarily a "low-numbers" risk. The larger the population, the less likely we are to find that important alleles are carried by only a few individuals. However, it pays to know where the diversity lies. Do any of you know which, among the current dogs, are most likely to carry the genes of any given founder?


The more you try to cover the deficiencies in one dog with good qualities in another, the less the dogs will have in common. If, then, the results are unsatisfactory, they should not be blamed on assortative mating as that is no longer what you are doing.

John B. Armstrong, 1997

This article was originally published on The Canine Diversity Project website.
Published here by permission.

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