Emerging Technologies
Proposed Strategy for Selection Against Recessive Genetic Defects Through a Combination of Inbreeding and DNA Markers
Abstract:
Recessive genetic defects are currently on the minds of many cattle breeders. The relatively rapid development of diagnostic DNA tests for recessive defects appears to be a major recent technological advancement. However, the attitude of breeders and breed associations toward recessive defects seems relatively unchanged.
One of the great untapped opportunities made possible by recent developments in DNA marker technology is the systematic elimination of recessive defects through planned inbreeding. However, this will require a transition by the beef industry toward a less emotional and more pragmatic and proactive paradigm for managing genetic defects. The proposed system would involve producing between 16 and 32 progeny by sire-daughter matings for highly influential sires. This would result in an average of two to four affected progeny for any gene at which the sire carried a recessive allele. Both the affected and unaffected progeny would be used to map any defects discovered to a relatively small proportion of the genome. Sets of DNA markers surrounding the region(s) containing the defect would be developed and used in selection of descendents and collateral relatives of the original sire.
If this mating system were to become common practice for highly influential sires in the beef industry, the occurrence of recessive defects at economically significant frequencies would probably be reduced substantially. This approach would require breeders to view recessive defects as just one more trait group to be considered in a balanced breeding program.
If we look hard enough, most cattle probably carry recessive defects. Cattle known to carry genetic defects should not be considered defective. Instead cattle in which the defects have been identified and mapped should be preferred over those in which the defects have not been identified.
It seems likely that rigorous implementation of this system would uncover many unfavorable alleles that are less serious and have not previously been recognized as recessive defects. Examples might include metabolic irregularities that produce general unthriftiness and poor performance. Elimination of such defects might reduce variation and improve mean performance, especially in straightbred cattle. This approach would require a new class of DNA-testing services, including products that are customized for specific families within breeds. There would also be a need for new infrastructure to store and analyze the information and provide useful selection and mating tools derived from it. Additional requirements of the new infrastructure include identifying relatives of the original sire that should be tested because they are potential carriers and progressive reduction of the size of the genomic region(s) in which the defect may be located. It would be neither necessary nor practical to identify the mutations that cause these defects. Selection would be limited to close relatives of the sire in which the defect was identified and mapped. Insufficient information is available to predict the consequences of this mating system accurately. However, potential advantages may include fewer devastating genetic defects, increased uniformity and improved performance of beef cattle.
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