Inbreeding

Inbreeding is the mating of animals that are more closely related to each other than the average relationship within the population concerned. Such coatings tend to make the offspring more homozygous, on the average, than if their parents were of average relationship to each other. Genes occur in pairs. If both members of a pair are alike they are said to be homozygous; if they are different they are said to be heterozygous. Thus, inbreeding increases the proportion of pairs of homozygous genes, or determiners of heredity.

The results achieved by corn breeders with inbreeding and crossing of inbred lines seemed to justify investigations into the possibilities of speeding up livestock improvement by establishing inbred lines and testing the usefulness of these lines in various types of crosses. Hence, Much work has been initiated in recent years. The major projects in this field are being conducted cooperatively by the Bureau of Animal Industry of the Department of Agriculture and various State experiment stations through the Regional Swine Breeding Laboratory, whose headquarters is in Ames, Iowa; the Western Sheep Breeding Laboratory at Dubois, Idaho, and the United States Range Livestock Experiment Station at Miles City, Mont. Work at Miles City is primarily with range beef cattle, but a limited amount of work is also under way with swine. Extensive work with swine, sheep, and cattle is also in progress at Beltsville.

Since inbreeding is the most powerful tool the breeder has for establishing uniform strains or families that are distinct from each other, and since much experimental work is now being conducted to determine how best to use it in livestock improvement, many readers may wish to know how the amount of inbreeding is measured.

The method that is now used almost exclusively was developed by Sewall Wright, formerly of the Department. His formula is: F,=Y( 1/2 ) n+n'+l ( 1 + Fa)

The formula appears more technical than it actually is. F stands for the coefficient of inbreeding of an animal, which is to be calculated. The Greek letter Y. (sigma) represents all the hereditary contributions to the inbreeding, but has no numeral value of its own. For example, if two or more ancestors contribute to the inbreeding, the contribution of each is calculated and then all are added together to obtain the coefficient of inbreeding. The fraction 1/2 is the animal's relationship to each of its two parents; n stands for the number of generations between the sire and a common ancestor; n' stands for the number of generations between the dam and a common ancestor. The factor ( 1 +Fa) represents the influence of a common ancestor, if that ancestor is itself inbred. If the common ancestor is not inbred, this part of the formula is omitted.

To illustrate, suppose an animal, A, has the following ancestors:

The animal has the same grandsire, (D), on both the sire's (B) and dam's (C) sides of the pedigree. Thus D is a common ancestor of both parents of A. Since there is only one generation between B and D, and also one between C and D, the value for n and n' in the formula are 1 and 1.

The third power or the cube of 1/2 is 1/8, which is expressed as 12.5 percent, and is the coefficient of inbreeding.

This coefficient indicates the increase in the proportion of homozygous pairs of genes that can be expected, on the average, in matings where there is one common grandparent, as compared with matings where there is no common ancestor. If we suppose that random breeding had been practiced in a herd, and that 50 percent of the pairs of genes were homozygous and 50 percent heterozygous, then 12.5 percent inbreeding would imply that 12.5 percent of the heterozygous pairs were homozygous in the new individual (12.5 percent of 50 is 6.25)—so, 56.25 percent of the pairs would be homozygous, while 43.75 would be heterozygous.

If the common ancestor, D, in the above example had already been inbred, for example 25 percent, then the factor (1 + F.) would have been (1 +0.25) or 1.25, and the inbreeding of animal A would have been 12.5 X 1.25 or 15.625, usually shortened to 15.6 percent.

Inbreeding does not create nor destroy any genes—it merely permits more of them to occur in homozygous pairs. Genes that favor development of both desirable and undesirable characters may become homozygous. Inbreeding thus uncovers many recessive genes that would otherwise remain concealed by their dominant-pair mates, or alleles (a recessive gene is one that is not able to express itself when it occurs as the pair-mate of a dominant gene, hence only the effect of the dominant gene is seen). Recessive genes generally have less desirable effects than dominant genes, so there is usually some degeneration in the average merit of individual animals when inbreeding is practiced. The chief danger of intense inbreeding, therefore, is that it may make undesirable genes homozygous so rapidly that it will be impossible to discard all the individuals that are homozygous for them.

The chief advantages of inbreeding are: It helps to uncover undesirable recessive genes so that animals possessing them may be culled; it may be used to develop uniform and distinct families so that interfamily selection may be more effectively practiced; new and often superior groups of animals may be produced by combining two or more inbred lines; it increases prepotency by increasing the chances that animals will pass on their traits to their offspring; and it is useful in maintaining a high relationship of stock to an especially desirable ancestor.

The extent of the experimental work that is being undertaken to test the possibilities of using inbreeding in livestock improvement can best be shown by details from some places where the work is being done.