Inbreeding in Swine, nsif-fs4

Purdue University Cooperative Extension Service West Lafayette, IN 47907 Author: David S. Buchanan, Oklahoma State University, Reviewers: Kreg Leymaster, USDA MARC, Clay Center NE; Ken Stalder, Univ. of Tennessee, Jackson, TN; William Luce, Oklahoma State Univ., Stillwater, OK Inbreeding is the mating of individuals that are related. In the strict sense, all members of a breed are related. As a result, any seedstock producer is practicing some inbreeding. Therefore, we generally reserve the term inbreeding for the mating of animals that are more closely related than the average of the breed. Most breeds of livestock went through a phase of inbreeding early in their development. This resulted from the need to establish color patterns and other aspects of physical appearance. Since one of the results of inbreeding is to establish more genetic uniformity, those traits that have simple means of inheritance can be fixed more easily with the aid of inbreeding. Genetic and Phenotypic Effects Inbreeding can have dramatic effects on a herd. These effects are the result of individuals receiving identical genes from each parent. If the parents are related it is more likely that they have genes that are identical. When an individual receives an identical gene from each parent it is said to be homozygous for that pair of genes. This would be desirable if the gene it received from each parent lead to superior performance. However, most animals carry undesirable genes that usually remain hidden unless the animal has a pair of them. An inbred individual is more likely to have gene pairs with identical members, so is more likely to express undesirable genes. This leads to a decline in performance called inbreeding depression. This phenomenon is well documented in all of the major livestock species. It is essentially the opposite of heterosis which is the advantage gained from crossing lines or breeds. Measurement of Inbreeding Inbreeding in an individual is measured with the inbreeding coefficient. The inbreeding coefficient measures the percent increase in homozygous gene pairs in an individual relative to the average of the breed. If an individual has an inbreeding coefficient of .25, it means that it is expected to have 25% more homozygous gene pairs than a non-inbred individual from the same population. The inbreeding coefficient can have any value between 0 and 1.0 although it is unlikely for it to have a value much above .5 in most herds of livestock. It is fairly easy to have it approach 1.0 in plant species where self-fertilization is possible. There are also some lines of laboratory animals with very high average inbreeding coefficients. A few lines of beef cattle and swine that have undergone intense inbreeding for 40 to 50 years have average inbreeding coefficients of .5 to .6. This takes many generations of full-sib or parent-offspring matings to accomplish. Therefore, an inbreeding coefficient over .5 is unlikely in a purebred herd of pigs. The general formula for an inbreeding coefficient is as follows: