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Good Horsemanship

Hybrid Vigor

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We've talked about hybrid vigor before, and the importance in bringing in some diversity.

This could be beneficial in helping to breed spavin out of the breed; put better bone on the horses, or even strengthen gaitedness.

"Hybrid vigor or increase in size, yield, and performance found in hybrids, especially if the parents have previously been inbred. The application of heterosis has been one of the most important contributions of genetics to scientific agriculture in providing hybrid corn, and vigorous, high-yielding hybrids in other plants and in livestock."

Heterosis:

[] Heterosis occurs when unrelated strains or breeds are crossed.

[] Heterosis is the recovery of performance depressed by inbreeding in the parent populations.

[] The degree of heterosis for the same trait varies between strains, breeds and environments.

[] A hybrid dam improves her offsprings' chance of conception and survival through maternal heterosis. This individual's own hybrid status improves its growth through offspring heterosis.

[] Heterosis is highest for low heritability traits of reproduction and survival but lowest for high heritability traits of post-weaning growth and carcass fatness.

[] The performance of a crossbred can be predicted knowing the average performance and degree of heterosis of the breeds crossed.

The members of the same population (eg. breed or strain) are usually related and therefore somewhat inbred. When two different populations are crossed, the level of inbreeding in the offspring falls to zero and there is an improvement in those traits which suffered from inbreeding depression in the parent populations. This improvement is heterosis.

As with inbreeding depression, heterosis is most often seen in low heritability traits, eg. those connected with reproduction, early growth and survival. It occurs least often in high heritability traits, eg. carcase characters. Heterosis is usually greater if the genetic difference between the crossed populations is wide. Thus the crossing of breeds should give more hybrid vigour than the crossing of strains within the same breed.

By Elaine Reynolds:

Inbreeding increases homozygosity. Breeders commonly use inbreeding to set type"; this means that they will mate animals that are related and share desirable characteristics, so that those characteristics will become homozygous, and all offspring of the inbred animal will inherit the genes for those characteristics.

This method works very well and quickly. The dogs so bred will exhibit these characteristics and "breed true". This means that as long as they are crossed with others of this type their offspring will also be this type. This is often done to try to preserve the characteristics of the ancestor that both parents have in common.

Heterosis-the phenomenon of an increase in vigor of individuals which have a high degree of heterozygosity.

There are two theories of hybrid vigor, or heterosis. These theories are not mutually exclusive and both contribute to the incresed health of individuals that have a high degree of heterozygosity.

The first is the dominance theory of heterosis. This theory states that hybrid vigor is presumed to result from dominant growth and fitness factors. This means that there are "bad" recessive genes that are presumed to determine a healthier individual than the dominant ones, that there are suboptimal recessive genes that result in a less vigorous individual when they are homozygous.

From this theory we have developed ways to inbreed, discover the genes that are undesirable in the line and eliminate them from the genepool, theoretically ending up with a purged genepool and healthier dog. As long as the undesirable traits that we are removing are easily identified in the pups and we can cull them before they are placed, this works fairly well, too, for a time.

The second theory is the overdominance theory of heterosis. In this theory, heterozygosity itself is the reason for hybrid vigor. A visual model of this is the palomino color in horses where the recessive homozygote is red, and the dominant homozygote is cremello. It is the heterozygote, the palomino, that is the desirable phenotype!

The overdominance theory of heterosis is particularly important if we are to understand the genetic cause of problems with the immune system. The genes that control the immune system must be heterozygous for the resulting individual to inherit a healthy, vigorous immune system.

The genes that control the immune system pass down through generations as "haplotypes" These are groups of alleles that lie closely together on a chromosome and are seldom separated by crossing over during the formation of gametes.

These genes must be in the heterozygous state for the individual who inherits them to have a genetically healthy immune system.

When inbreeding occurs, the chances for one individual to inherit an identical haplotype from each parent, thus ALL the genes that control the immune system are homozygous. This homozygous state results in an individual which has a seriously compromised immune system.

This occurs in every breed, line, strain of dog (or other life form) it is not the result of a "bad recessive gene" that is carried in a certain line and must be eradicated. The eradication of such "bad genes" (simple autosomal recessive genes) often results in the narrowing of the genepool and increases the likelihood that the immune system haplotypes of the parents will be the same one, and the individual will be homozygous for all the immune system genes.

Reducing the diversity in the genepool proportionatly increases the chances for an individual to inherit identical haplotypes from each parent. The result is tremendous suffering.

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