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Balancing genetic gain and genetic diversity

Balancing Genetic Gain and Genetic Diversity

The aim in breeding populations is to achieve a high rate of genetic improvement. You can do this by accurate selection and high selection intensity.

When you select fewer animals, selection intensity is higher, resulting in a higher predicted response (remember the breeding equation). However the fewer animals used as parents the more inbreeding, which depresses selection response. This is because of the loss of variation.  

When you design breeding programme you need to consider the balance between genetic gain and genetic diversity.

As you select only the very best animals, a greater genetic merit is achieved however inbreeding is also high.

But if you select a number of rams from different families you limit inbreeding at the expense of genetic progress. 

Sometimes you might be more tolerant of inbreeding. This would be when you can easily buy an outside ram. If your flock is at the top end of the industry the ability to outsource superior genetics is limited. Managing inbreeding becomes extremely important in these situations and the optimum position would be a lower inbreeding coefficient whilst maximising the most amount of genetic gain.

Therefore it is important to consider your position in industry and within your breed and the future of your breeding program.

Inbreeding vs Co-ancestry

Inbreeding is for individuals, coancestry is for the flock

It is important to understand the difference between inbreeding and co-ancestry.

Inbreeding is the practice of mating related animals.

Co-ancestry is when individuals share common ancestors within a population.

For example if we mated 100 females to 1 completely unrelated sire, the progeny would not be inbred. However the co-ancestry of the next generation who are to be used as parents would be very high as they are all by one sire.  This means all of the progeny from that mating will be related.

For long term gain it is important to manage co-ancestry to prevent populations of animals becoming so related that inbreeding depression occurs.

In a population of animals having a handful of inbred individuals is not a problem.