Linecrossing is the practice of mating animals that are of the same breed, but from different lines (bloodlines are established by the process of linebreeding). Some breeders call this process outcrossing. However, since some producers use outcrossing to refer to crossbreeding, we have chosen the term linecrossing for clarity.
Going back to our imaginary horses again, Tumbleweed is a good example of linecrossing. Even though Dust Devil comes from an inbred bloodline, he was mated to an unrelated mare of the same breed (Sunflower) to produce Tumbleweed. Because of this linecrossing, Tumbleweed has no significant inbreeding. If we were to trace his pedigree back over many generations, we would probably find related ancestors, but their contribution to his genetics would be so small as to be negligible.
How It Works
Because the primary purpose of linebreeding is to encourage genetic consistency in a given population, linebred animals typically have many like genes, as explained earlier in this series. Bloodlines within the same breed will have many similarities, but they will also have differences, as each breeder has shaped the genetics of his animals to reach his own unique goals.
When animals of two different bloodlines are mated, the genes that they share in common will continue to stay alike in their offspring. For example, suppose we cross two bloodlines of Angus cattle. The sire and the dam come from families with very different production characteristics, but they are both Angus and they are both black (represented by the formula BB). While their production traits will be paired in new ways in their calf, it will, like them, be black and obviously an Angus. The unique physical characteristics that make an Angus an Angus will be passed on in spite of the linecrossing. The reason is illustrated by the genetic formula for black color: the sire, having two copies of the B gene, will pass one copy of the B gene on to the calf, and the dam will do likewise. Thus the calf will also have the formula BB.
However, because there are differences between each bloodline, the offspring of a cross between two lines will not always receive pairs of like genes from its parents. For example, in a cattle breed where both black and red coat colors are accepted, a breeder might mate a black (BB) bull to a red (bb) cow. The resulting calf would look black, but because its formula would be Bb an element of genetic variability would be introduced. This calf would possess traits of both bloodlines, and might pass on characteristics of either to its future offspring.
Linecrossing is primarily used as a way to promote hybrid vigor in purebred animals. As linebreeding is continually practiced, the inbreeding coefficient gradually rises, and inbreeding depression can become a result. Many breeders will then use a linecross to “bring in outside blood.” This brings the inbreeding coefficient back down to virtually zero in the resulting offspring, and the breeding program can continue. In some livestock breeds, such as Texas Longhorn cattle and White Leghorn chickens, linecrossing is the most common way that producers raise animals that are viewed as typical of the breed. The resulting hybrid vigor imparts size to the Longhorn and incredible egg-laying ability to the Leghorn.
Another way that linecrossing can be useful is in preserving genetic diversity within a specified population, making it a very important tool in the hands of conservation breeders, those who preserve rare breeds of livestock. A common practice in conservation breeding is to divide the population into similar lines, maintain those lines separately for the most part, and cross the lines every so often. This strategy tends to keep a broad range of genetic variation available within the breed, even if its numbers are low. Maintaining genetic variation is critical in rare breeds to avoid inbreeding depression and ultimate extinction.
The main drawback of linecrossing is variability, particularly in subsequent generations. Any time that unlike genes are paired, a certain level of instability is introduced into the population. For example, take a look at the Bb calf we mentioned above. If, after he has matured into an adult bull, he is bred to a BB cow, his offspring could be either BB like his mate or Bb like himself. If this bull is bred to bb cow, there will an equal chance of producing a Bb calf or a bb calf. And if the bull is bred to another Bb cow, they will have a 25% chance of producing a BB calf, a 50% chance of producing a Bb calf, and a 25% chance of producing a bb calf.
This is why some producers feel that animal breeding is such a shot in the dark. The genetic variability inherent in breeding linecrossed animals will probably result in some animals that are regarded as brilliant, some that are merely average, and some that are of terrible quality, no matter what objectives they would be required to meet. For this reason, linecrossed White Leghorns are typically not expected to reproduce themselves—they are butchered after their egg-laying days are over.
However, because some level of hybrid vigor is necessary to ensure the survival of a population, many breeders use linecrossing in combination with other tools, particularly linebreeding. Linecrossing is used to counter the rising inbreeding coefficients of a linebreeding system, while linebreeding is used to cement desired traits into the bloodline despite the variability of linecrossing. Neither tool is a silver bullet.