We've talked about hybrid vigor before, and the importance in bringing
This could be beneficial in helping to breed spavin out of the breed;
better bone on the horses, or even strengthen gaitedness.
"Hybrid vigor or increase in size, yield, and performance found in
especially if the parents have previously been inbred. The application
heterosis has been one of the most important contributions of genetics
scientific agriculture in providing hybrid corn, and vigorous,
hybrids in other plants and in livestock."
 Heterosis occurs when unrelated strains or breeds are crossed.
 Heterosis is the recovery of performance depressed by inbreeding in
 The degree of heterosis for the same trait varies between strains,
 A hybrid dam improves her offsprings' chance of conception and
through maternal heterosis. This individual's own hybrid status
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
 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
and therefore somewhat inbred. When two different populations are
the level of inbreeding in the offspring falls to zero and there is an
improvement in those traits which suffered from inbreeding depression
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
survival. It occurs least often in high heritability traits, eg.
characters. Heterosis is usually greater if the genetic difference
the crossed populations is wide. Thus the crossing of breeds should
more hybrid vigour than the crossing of strains within the same breed.
By Elaine Reynolds:
Inbreeding increases homozygosity. Breeders commonly use inbreeding to
type"; this means that they will mate animals that are related and
desirable characteristics, so that those characteristics will become
homozygous, and all offspring of the inbred animal will inherit the
for those characteristics.
This method works very well and quickly. The dogs so bred will exhibit
characteristics and "breed true". This means that as long as they are
crossed with others of this type their offspring will also be this
This is often done to try to preserve the characteristics of the
that both parents have in common.
Heterosis-the phenomenon of an increase in vigor of individuals which
high degree of heterozygosity.
There are two theories of hybrid vigor, or heterosis. These theories
mutually exclusive and both contribute to the incresed health of
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
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
they are homozygous.
From this theory we have developed ways to inbreed, discover the genes
are undesirable in the line and eliminate them from the genepool,
theoretically ending up with a purged genepool and healthier dog. As
the undesirable traits that we are removing are easily identified in
pups and we can cull them before they are placed, this works fairly
too, for a time.
The second theory is the overdominance theory of heterosis. In this
heterozygosity itself is the reason for hybrid vigor. A visual model of
is the palomino color in horses where the recessive homozygote is red,
the dominant homozygote is cremello. It is the heterozygote, the
that is the desirable phenotype!
The overdominance theory of heterosis is particularly important if we
understand the genetic cause of problems with the immune system. The
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
"haplotypes" These are groups of alleles that lie closely together on a
chromosome and are seldom separated by crossing over during the
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
immune system are homozygous. This homozygous state results in an
which has a seriously compromised immune system.
This occurs in every breed, line, strain of dog (or other life form) it
not the result of a "bad recessive gene" that is carried in a certain
and must be eradicated. The eradication of such "bad genes" (simple
autosomal recessive genes) often results in the narrowing of the
and increases the likelihood that the immune system haplotypes of the
parents will be the same one, and the individual will be homozygous for
the immune system genes.
Reducing the diversity in the genepool proportionatly increases the
for an individual to inherit identical haplotypes from each parent. The
result is tremendous suffering.