6.14: Predict the rate of inbreeding (2024)

Thus far we have evaluated the rate of inbreeding in retrospect. However, as it provides an indication of the expected increase in inbreeding depression, it would be nice to be able to predict the rate of inbreeding in future generations. Predicting the exact future is not possible, but you can make an approximation. A simple formula to get some idea about the effect of selection decisions with respect to the number of breeding animals is:

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In other words, if you know the number of males and females that are used for breeding, you can predict what the rate of inbreeding will be. Of course the exact rate of inbreeding will depend on the genetic relationships between the animals, and that is not taken into account in this formula. And as we have seen, the rate of inbreeding depends on your population size, more than on mating strategy. This formula will provide you with an approximation, assuming that these numbers represent your population parameters, and assuming absence of selection and no extremely small or large family sizes, relative to the sizes of the other families.

So 20 males and 300 females, that is 320 breeding animals, results in a rate of inbreeding of 0.67%. Would it matter how these 320 animals were divided across males and females? Try it yourself. What if you would use 160 males and 160 females? And what if you would use 2 males and 318 females? You will find out that the more skewed the proportion of breeding males to females is, the higher the rate of inbreeding.

How about population size? Would that matter? What if you would use only one male and one female for breeding? And how would the rate on inbreeding change if you would increase that number to 10 males and 10 females? Or 100 males and 100 females? You will find out that in very small breeding populations the rate of inbreeding can’t be controlled by using equal numbers of males and females for breeding.

So far we have assumed that family size, so number of offspring in males and females, is equal for all families. In real life this is not the case. The rate of inbreeding is most influenced by the largest family, because they will have the largest proportion in the next generation(s). We also have assumed that the population size remains constant across generations. In real life this may not always be the case. Populations may decrease in size because of, for example, decrease in popularity or a disease outbreak. They may increase in size because of, for example, an increase in popularity, or a smaller mortality than anticipated.

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