Inbreeding is Screwing Yourself

Inbreeding is like screwing yourself over and over again.

And it produces about the same results.  It starts alone, in the dark, with good intentions but you’re settling for self-soothing  because you couldn’t get what you wanted from someone, anyone, else.  And in the end you end up a shell of a being only half as fulfilled as you could have been.

It feels good at that moment because it is instant gratification.  You don’t have to worry about being “approved” by the other party, no “stud fees” required.  Results in minutes. You don’t have to study, seduce, or even try to understand the other players in the game.  Just know your own “lines,” no scary new experiences required.

But it’s lazy and it makes you complacent.  Why put the effort in finding satisfaction elsewhere when that might require effort, time, money and risk rejection and disappointment?

Sure, you might venture out from the basement once in a blue moon and pay for some “strange” but it’s so much easier if you lower your standards and keep full control; after all, who knows your lines like you do? Love yourself!  If you don’t mingle naughty bits you’ll never get a new disease!

While it’s more of a male thing, females get involved too, although rarely with the obsessiveness and fervor of the male strain.  Rampant mocking and endless jokes about it haven’t done much to make anyone stop doing it.

Although we don’t enjoy admitting it, almost everyone did it in their adolescence to prepare for the bigger challenge of swapping genes with another being in the dating pool, and sure, it still has its place in a healthy and circumspect sex life, but if you do it too much you probably will go blind or grow hair in places you don’t want or make it fall out of places you do.

In fact, self pollination (“selfing”) is the most extreme form of inbreeding, and it’s thus the most powerful tool to accomplish the goal of any form of inbreeding or line-breeding: homozygosity.  Homozygosity is Greek for “one and the same yoke” and is the state of having two identical copies (alleles) of a given gene.

The reason breeders select for homozygosity is because having two identical copies of a specific gene means that the animal will be guaranteed to display the trait associated with that gene and is guaranteed to pass that allele along to its offspring.  If that gene is dominant, it will again be guaranteed to be expressed in the offspring, if it is recessive, being homozygous in both parents will guarantee expression in the offspring.

It’s WYSIWYG for selective breeding.  Breeders call this quality “prepotency.”

A stud animal of any species is judged on his pre-potency, which means that not only must he exhibit the best phonetic traits himself, but he should also demonstrate his ability to pass along these traits to his progeny over a diverse female population.

The greater theory of Prepotency is bunk: that superman sires should be able to overcome any problems in the dam, ideally the female being but a vessel to produce perfect copies of the sire.  What isn’t bunk is the very real effect of rising homozygosity: the more homozygous the parent, the less variation in genes passed along to the offspring.

Likewise, line breeding and inbreeding are really no different in their effect or results, good and bad, they only differ in what percent of alleles are likely to become matched each attempt.

There is no way to double up on desired genes and not on undesired genes, and the truth is that people inbreed for the purpose of doubling up on a very small number of genes while they have no interest in the millions of other genes becoming homozygous.  What is good for locking in a harmless coat color gene could be disastrous for all of the deleterious recessive genes that are also getting doubled down on unintentionally.

And the ratio of unintended doubles vs. intended doubles in any given breeding is easily many-thousands to 1.  The canine genome has ~3 billion base pairs and the most focused on genes like coat color and structure make up a small percent.  Harmful mutations start out rare, but with odds like these, it’s not surprising that they rear their ugly head in every breed.

Inbreeding is like burning down the forest to fell one tree.

To demonstrate how dramatically inbreeding can transform a heterozygous (maximum genetic diversity) individual into a homozygous one, I’ve run a simulation on self-pollination.  The act of “selfing” is twice as potent a force as a father x daughter mating or a sibling x sibling mating in its ability to pair genes: selfing is expected to double up 50% of the alleles in any generation, whereas a father x daughter mating is expected to double up 25% of the alleles.  Both are potent and dangerous.

In the upper left we have a representation of a fully heterozygous individual. All of its genes are paired Aa, two different alleles at every location.  It’s a universal carrier.  Each different row can represent a single gene or a group of genes, and to easily track how these alleles can jump around from left and right and recombine, I’ve painted all the alleles on the right blue, all the alleles on the left red.

Although the intervening steps look more chaotic, they actually represent a loss of information.  Remember that matching colors up and down the chromosome isn’t what matters, it’s matching the colors in the same row left and right.  The chart below shows the alleles that are lost forever by continuing to breed on this line at each stage.

As we move to the right, the resulting chromosomes are the product of “breeding” the previous chromosome with itself.  The results were randomly generated.  Thus we are both inbreeding and selecting as we move right just as we would in a breeding program.

This random progression is a good representation of the average scenario of continued selfing.  The chart on the left shows what the expected degree of doubling up would be each generation.  You can see how fast this converges on 99%, so even if we lucked out in the first generation and got only 15% or 25% doubling, successive inbreeding would still converge quickly to the extreme.

Inbreeding alone doesn’t cause loss, it doesn’t even change the frequency of alleles in the offspring taken as a group. It simply changes the ratio of those alleles from being mostly mixed Aa into mostly paired, AA and aa.  What causes the loss of information is that breeders don’t inbreed blindly and randomly select offspring.  They select very few offspring each generation to breed, thus large amounts of genes are lost.  We inbreed and then select so few offspring to continue the line.  Since these offspring carry less individual information, the loss of information from culling is magnified greatly compared to the same selection process working on heterozygous offspring.

The logical and definitive result of continued inbreeding is to create what is essentially a half-being.  And these two halves don’t make a proper whole.  Even with a less extreme form of inbreeding, one available to dog breeders like father to daughter (25%) or sibling to sibling (25%), the ability to quickly drive up the Coefficient of Inbreeding (the percent of genes that are expected to be homozygous resulting from doubling a single allele from the same ancestor) abbreviated COI or F is still dramatic.

COI/F is both the probability that two alleles in an individual are identical by descent and also the proportional decrease in heterozygosity under inbreeding.


What’s disconcerting is that you don’t have to get to extreme levels of inbreeding/homozygosity before the offspring are not viable.  A Border Collie breeder tried to recreate Wiston Cap by inbreeding very highly on him and his offspring, reaching a COR level of over 80% and a COI of over 60%.  No sustained offspring.

That’s not a surprise as we see inbreeding depression on offspring from the plant kingdom all the way up the food chain.  Inbreeding tries to take care of itself.  But in a dog culture where most breeders covet their copy of “Puppy Intensive Care” where you can triage a puppy that nature would cull long enough to drop a fortune on it until it’s breeding age, we not only don’t allow nature to correct itself, we push as hard as we can using the most devastating and blunt tool we have available to emphasize the extreme and leave fallow the middle ground.

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About Christopher

Christopher Landauer is a fifth generation Colorado native and second generation Border Collie enthusiast. Border Collies have been the Landauer family dogs since the 1960s and Christopher got his first one as a toddler. He began his own modest breeding program with the purchase of Dublin and Celeste in 2006 and currently shares his home with their children Mercury and Gemma as well. His interest in genetics began in AP Chemistry and AP Biology and was honed at Stanford University.