Question:

 Is there a real difference between inbreeding and line breeding? Isn’t inbreeding a problem that we should avoid? After all, it reduces for genetic diversity and can lead to problems, like inherited diseases and physical problems. Yet I sometimes see references to line breeding in certain breeds and families as if it’s a normal. Some people even say it’s a good thing. What am I missing?

Answer:

Inbreeding is the mating of related individuals. This is a very simple definition. Of course I could add a variety of complicated scenarios to expand on this explanation, but these are not necessary for answering your question. Mating a horse with a relative increases the probability that genes that are identical by descent in each related parent will pair (be homozygous) in the resulting offspring. Indeed if these genes have harmful consequences when homozygous (recessive inheritance), then those consequences will be expressed in the offspring. This is, basically, inbreeding depression. (Again, this is a simplification.) This is the primary reason breeders generally avoid inbreeding.

Line breeding is a form of inbreeding. The distinction is that with line breeding a single ancestor appears more than once in the pedigree and on both sides (sire and dam) of the pedigree. The goal is to concentrate the genetics of that ancestor in the new individual or lineage. The purpose of line breeding is primarily to fix the favored characteristic(s) of a specific individual into a new group of animals so that it can be propagated reliably in the future. The difference between inbreeding and line breeding is simply that inbreeding is the general term for mating of relatives while line breeding is a specific form of inbreeding.

The impact of inbreeding

The main impact of inbreeding is that it lowers genetic diversity within the group. This decreased diversity can lead to an increase in homozygosity. This, in turn, increases the risk of inherited diseases and can lead to inbreeding depression—a general loss of vigor and fertility. Inbreeding depression is one of the great difficulties encountered in attempting to preserve rare breeds. Small population size always leads to some degree of inbreeding. Careful planning and management in breeding decisions is essential for preventing inbreeding from overwhelming preservation efforts of rare breeds.

The potential benefits of careful inbreeding

Despite its potential for general negative consequences, inbreeding is a valuable and powerful tool in the development of domestic breeds. Inbreeding is the best way to establish a specific desirable trait in a population. It also increases the predictability of the inheritance of the trait. The key is the intensity of the inbreeding employed. This refers to how closely related the individuals used in a cross are related to each other. First-degree relatives (brother/sister, parent/offspring) share 50 percent of their genes with each other. This means that the probability that any gene will be homozygous in the offspring is 25 percent. Mating of first-degree relatives creates a very high degree of inbreeding depression, leading to the quick extinction of the lineage.

You can slow the rate of inbreeding depression and increase the chance that harmful variants of genes will be eliminated from the developing lineage by using less closely related individuals. This is why line breeding usually involves using the common relative at the third or fourth level of the pedigree. Inbreeding can be valuable in livestock development if certain rules are followed.

E. Gus Cothran, PhD 

Professor Emeritus

Texas A&M University

College Station, Texas

E. Gus Cothran, PhD, is Professor Emeritus at Texas A&M University. He received his Bachelor of Science and master’s degrees from North Texas State University. He earned his doctorate at the University of Oklahoma. His primary research interests include the heredity basis of equine congenital defects; interrelationships of inbreeding and population genetics of feral horses.