Principles of Genetics II: Crossbreeding and Heterosis
D.H. “Denny” Crews, Jr., Ph.D., P.A.S.
Crossbreeding is defined as the mating together of parents from different
breeds to produce crossbred or hybrid progeny (offspring). The breeds used in a
crossbred mating are assumed to be dissimilar in genetic history, and
essentially unrelated.
Crossbred progeny arising from the mating of
purebred parents from two separate breeds are referred to as F, progeny, and for
traits which are under the control of many genes, might be expected to perform
at a level intermediate to the two parent breeds. For example, assume that two
individuals from different breeds are mated which have weights of 30 and 40
pounds. Then their progeny, if they weighed 35 pounds, would have performed at a
level equal to the average of the parents. Complementarity is used to describe
this result of crossbreeding where the progeny perform at the average parental
level. One of the reasons crossbreeding is used in animals is to combine
desirable traits from different breeds in the crossbred progeny. That is, two
parental breeds are mated which complement each other with respect to important
traits. Breeds may be selected for use in crossbreeding to combine different
traits which are desirable, or to moderate the expression of a trait in the
crossbred progeny. It is known that increased growth performance is often
associated with increased birth weight. For example, in most livestock,
increased growth rate is desirable, but increases in birth weight can eventually
cause problems with giving birth, called dystocia. Crossbreeding may be used to
improve growth rate, but at the same time, changes in birth weight must be
considered. Therefore, breeds to be used in crossbred mating systems must be
chosen carefully.
Color patterns and conformation traits are also
affected by crossbreeding. The structural defects or genetic illnesses
associated with a certain breed may be corrected by crossbreeding with a breed
which does not exhibit the defects. This can usually be done quickly, since
coloration and genetic defects are typically under the control of fewer genes
than traits such as weight or growth rate. The resulting hybrid may have reduced
incidence of genetic defects, but also possess some of the desirable
characteristics of both parental breeds.
Another result of crossbreeding
is referred to as hybrid vigor, or heterosis. Using the example previously
mentioned, if the progeny weight was 37 instead of 35 pounds, then the progeny
performed at a level above the parental average. The extra weight, defined as
the difference between progeny performance level and parental average is assumed
to be due to heterosis. In this example, the difference between progeny and
parental average is two pounds. If the two pounds in extra weight is divided by
35 pounds (the parental average), this calculation shows that weight was
increased by about 6% due to heterosis. For traits that are not highly
heritable, heterosis levels tend to be higher as a result of crossbreeding. For
some traits in livestock, heterosis levels in excess of 25% are not uncommon. It
is also possible for the progeny to perform at a level lower than the parental
average. In livestock species, heterosis is an important tool used to improve
traits such as growth rate and reproductive efficiency. Scientists have shown,
for example, that crossbred beef cattle are often more fertile, have increased
longevity and grow at a faster rate than straight-bred or purebred
contemporaries. It is also likely that survival and vigor of puppies can be
improved through crossbreeding. The present challenge in higher animals is that
no one has adequately described the basic mechanism of heterosis.
A good
example of how heterosis affects traits that are expressed in discrete units can
be described using litter size or perhaps progeny survival. These are examples
of traits that do not exhibit continuous variation as do weight, height, or
growth rate. It is not possible, for example, for a female to give birth to
3-1/2 progeny. However, the effects of heterosis are often largest with traits
related to reproduction and mortality such as these. If a purebred female from
breed A (with an average litter size of four) is mated to a male of the same
breed, that female would be expected to give birth to the number of progeny
which is average for that breed. Some of the progeny may be expected to die due
to dystocia, disease, or other problems. If that same female is mated to a male
of breed B (which also has an average litter size of four), the parental average
is still four. However, due to heterosis generated as a result of this crossbred
mating, the female may give birth to more than four progeny. More important than
the possible increase in number of progeny is the vigor of those progeny. Recall
that the terms heterosis and hybrid vigor are sometimes used interchangeably;
that is, the hybrids are more vigorous. Even if the female described in this
example gives birth to four progeny, those progeny will probably have a higher
survival rate than if she had been mated to a male from her own breed. This is
because crossbreeding has caused a novel “pairing” of genetic material, and more
importantly, removed all possible inbreeding levels in the progeny.
The
breeds are assumed to have different forms of similar genes. That is, the genes
controlling a trait in breed A are different from the genes that control the
same trait in breed B. When these breeds are crossed, the progeny have genes
from both breed A and breed B, instead of only from A. For example, if there
were genes present in breed A that had a negative effect on progeny survival,
those gene pairs could be broken with crossbreeding, and the genes from A would
be paired with genes from B, resulting in a reduction of the negative effects.
The negative phenotypic effects of undesirable gene combinations within breeds
increase with inbreeding, and with many generations of mating within lines or
families of animals. In livestock species, where purebreds are maintained,
careful breeding plans are used to avoid or minimize the inbreeding that results
from mating of related individuals within the breed.
Purebreeding and
line breeding are used to maintain distinctly separate breed types, but
crossbreeding uses those distinct types to improve production, survival, and
longevity through the use of both complementarity and heterosis. The choice of
mating system depends on sound selection practices which allow for the combining
of desirable traits from different breeds and the generation of
heterosis.