Implications of Improper Management of Known Genetic Abnormalities
STILLWATER, Okla. (Oct. 9, 2014) — Perhaps you have heard the story about a respected animal breeder whose reputation was founded on the successful raising and marketing of linebred stock. When asked to explain the difference between linebreeding and inbreeding, the veteran stockman said, “When it doesn’t work, it’s inbreeding.”
Alison Van Eenennaam explained why genetic abnormalities occur. She also discussed ways to avoid problems through management of what she called “loss-of-function alleles.”
Whatever you prefer to call it, according to geneticist Alison Van Eenennaam, inbreeding is the mating of animals more closely related than the average relationship of animals in a population. It is applied to produce progeny that are more uniform and homozygous for superior genes. In the vernacular of our storied breeder, that’s what you get when it works. However, genes associated with abnormal phenotypes or embryonic death also may become homozygous. That’s what you don’t want.
A University of California–Davis (UC–Davis) beef cattle specialist, Van Eenennaam explained to attendees of the Applied Reproductive Strategies in Beef Cattle (ARSBC) symposium Oct. 8-9 in Stillwater, Okla., why genetic abnormalities occur. She also discussed ways to avoid problems through management of what she called “loss-of-function alleles.”
Van Eenennaam explained how many genetic abnormalities are associated with recessive genes. Alternative forms of the same gene (alleles) may not function normally, or at all. Mutant alleles that “don’t work” are called loss-of-function alleles. An abnormality occurs when an individual inherits a loss-of-function allele from both parents. If an individual inherits the loss-of-function allele from just one parent, that individual will appear normal, but will be a carrier of the loss-of-function allele and is capable of passing it on to progeny.
With inbreeding, one or more common ancestors contribute alleles to both sides of an animal’s pedigree. Van Eenennaam said that is the reason why deleterious recessive alleles often are identified with widely used sire lines. She emphasized that it is not because these sires carry more deleterious alleles. Rather, it is because these sires are so widely used, through artificial insemination (AI), that they are more likely to appear on both sides of a pedigree. That increases opportunity for an individual to inherit a deleterious allele from both sire and dam.
“Because carriers appear normal, recessive alleles can increase in frequency in a population more easily than undesirable dominant or additive alleles. It is only when two carriers mate that there is the possibility of producing offspring that have, by chance, inherited both of the non-functional alleles from their parents,” explained Van Eenennaam.
She further explained how it is likely that all animals carry recessive genetic conditions in their DNA. Genomic technology has allowed for the discovery of recessive alleles associated with specific physical abnormalities. It is likely that more will be discovered. Other recessive conditions are lethal, resulting in fertility problems such as increased calving interval or missed heat periods.
Van Eenennaam said genotyping of individual animals allows determination of whether an individual possesses alleles associated with known abnormalities. Their occurrence can be managed by avoiding the use of sires identified as carriers. However, this strategy does not allow their use for genetic improvement of traits for which the sires have superior merit. Another way to manage the occurrence of deleterious genetic conditions is to avoid matings between animals identified as carriers.
For implementing the latter strategy, said Van Eenennaam, a decision-making tool, called MateSel, has been developed. This software program is designed to optimize mating selections, showing the tradeoff between genetic gain achieved by mating the very best sire to the very best dams irrespective of their relationship to each other, and genetic diversity, which is maintained by avoiding or minimizing inbreeding.
According to Van Eenennaam, its Australian developer, Brian Kinghorn, is in the process of modifying MateSel so that it can be used to optimize the rate of genetic gain with a key objective being to reduce both affected offspring and frequency of the lethal alleles. Thus, it should help ensure that the value of high genetic merit carriers is maintained and those genetics are available to appropriately utilize in breeding programs.
“Using mate selection software to avoid carrier matings achieves a better outcome than just indiscriminately avoiding the use of carriers,” concluded Van Eenennaam.
Van Eenennaam spoke during Thursday’s ARSBC session focused on use of genomics for reproductive improvement. For more information, visit the Newsroom at www.appliedreprostrategies.com to view her PowerPoint or listen to her presentation.
Comprehensive coverage of the symposium is available online at www.appliedreprostrategies.com. Compiled by the Angus Journal editorial team, the site is made possible through sponsorship by the Beef Reproduction Task Force.
Editor's Note: This article was written under contract or by staff of the Angus Journal. To request reprint permission and guidelines, contact Shauna Rose Hermel, editor, at 816-383-5270.