BIF 2019: The Next Generation of Genetic Tools

John Genho
Neogen GeneSeek

Why do we use a crossbreeding system? To gain the advantage of heterosis, the cross outperforming the parent average. Retained heterosis is the amount of this advantage that is maintained after multiple generations of mating crossbreed animals (not mating the purebred parents).

Dominance, over dominance, and epistasis likely all contribute to heterosis.

In general, traits with higher heritabilities have lower heterosis and traits with low heritability have higher heterosis.

Why don't commercial producers use crossbreeding? The average herd size is 40 head and 9% of herds have >100 head. It is harder to implement a crossbreeding program in a smaller herd.

Researchers in Canada have developed genomic indicators of retained heterosis. They use the genomic data to infer the breed proportion of the animals. They can then square these breed proportions, add them up, and subtract from zero to estimate the retained heterosis. Another approach is to count the number of heterozygote loci.

The genomic indicator of retained heterosis significantly predicted the heifer pregnancy and stayability of crossed cows in the University of Alberta data. Genho also looked at this in King Ranch and Eldon Farms data, and the genomic measures of heterosis and retained heterosis predicted heifer pregnancy and stayability.

You can't fix heterosis in a herd by selecting for it. Mendelian segregation means that each generation of mating crossbreeds you end up with homozygous loci in the progeny. These homozygous loci result in decreased heterosis.


Joe Paschal said…
You are right on. You cannot select in the traditional sense for heterosis! I haven't read the Canadian work but heterosis is the sum of thousands of genetic interactions, I doubt there are major genes as was once considered in additive genetic action. The reason that so many of the American breeds (those with Brahman genetics) and the American Tweeners (those more recent with solely Bos taurus)was to provide those with smaller herds that could not create their own crossbreds an opportunity to use heterosis more easily and less expensively. However, no matter the current re-emphasis on heterosis or hybrid vigor, the breed effect should not be ignored. The Lasaters and the Klebergs and many others understood that.
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