The Genetic Tradeoff

There is a trade-off between achieving maximum genetic gain and building up inbreeding too quickly. State-of-the-art scientific methods help to find the right balance between these two when selecting and mating your breeding animals.

Optimal Contributions Theory

Maximising genetic gain without considering the impact on future inbreeding will result in higher levels of coancestry (relatedness of individuals – see the article on Coancestry) and this will lead to undesirable levels of inbreeding (see the article on Inbreeding). There is an optimal balance between high genetic gain and low coancestry, achieved by using individuals in an optimal way. This is known as Optimal Contributions Theory.

Achieving the optimal contributions of individuals’ genes to the breeding program results in genetic gain that is more sustainable over time because it will not cause an excessive build-up of inbreeding in future.

You could achieve sustainable genetic gain by taking a conservative approach that keeps coancestry at bay, but this level of genetic gain is often less than desirable.

If you calculate the highest genetic gain that is possible and the corresponding level of coancestry, then the lowest possible coancestry with the corresponding level of genetic gain, as well as a number of points in-between the two, you can connect up the points and you have the “frontier of genetic gain and coancestry”.

This represents the best that your breeding population can do this year, given the constraints you have put on sire uses and the number of females being mated. Each point on the frontier is a mating list that you could implement, not a theoretical position. This, however, is not a trivial calculation to do by hand.

In X'Aim, we have chosen 3 points on this frontier to provide a range of possibilities for Index, coancestry, and inbreeding. We refer to each point as a "mating strategy" which you can implement when requesting a new mating analysis. We've also retained and named the original balanced mating strategy that doesn't require a genetic selection Index:

• Balanced Diversity - Equal emphasis is placed on long (coancestry) and short-term inbreeding (progeny inbreeding), but no emphasis is placed on genetic Index. This is the only strategy available when Index is not available for all your available animals.

• Balanced + Gain - This strategy is similar to Balanced Diversity but with a small emphasis on genetic Index (gain). This is an ideal strategy if maintaining diversity and avoiding inbreeding is most important but some improvement in genetic Index is desired.

• Middle Road - All of long and short-term inbreeding and genetic Index have equal emphasis. This strategy is ideal if all 3 are equally important and neither of diversity or genetic Index are more important than the other. This strategy will still result in good genetic gain on Index but will not be as aggressive as the Performer strategy below.

• Performer - Choose this strategy when your main focus is to make rapid genetic gain on Index. X'Aim will still try to keep long and short-term inbreeding as minimal as possible, but you should expect both of these to result in higher values than any of the other strategies.

Why is the Performer strategy not the highest point? In theory, we could be just slightly higher in Index, but this would come at the cost of considerably increased coancestry, which isn't worth the small increase in Index. This illustrates particularly well the Genetic Tradeoff we are trying to illustrate here. Striking the right balance between genetic gain, inbreeding and coancestry is at the heart of a good mating strategy.