FEATURE ARTICLE
EPD-OLOGY
ADDITIVE AND NON-ADDITIVE GENETIC EFFECTS by Darrell Wilkes, Ph.D., International Brangus Breeders Association (IBBA) executive vice president In the last EPD-ology column, we discussed the most important equation in animal genetics. Namely, P = G + E, where P equals the phenotype such as yearling weight; G = the genetic influence on yearling weight, and E equals the environmental effect on yearling weight (think nutrition). We illustrated how vastly different environmental conditions can make two animals appear much different than they really are from a true genetic viewpoint. The computation of EPDs is designed to sort out the genetic effects from the environmental effects. In this column, we attempt to expand understanding of the genetic effects. Genetic effects are broadly divided into two categories: Additive genetic effects, and Non-Additive genetic effects. Any attempt to explain the difference in a single column (as opposed to a stack of books on genetics and statistics) is going
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to over-simplify the subject, but that cannot be avoided. Let’s assume that a trait like yearling weight is affected by only 5 pairs of genes. It is more likely that dozens or hundreds of genes are involved, but we’ll assume that only 5 are at play. Further, we will assume that the large letter for each gene adds 5 lbs of yearling weight, and the small letter adds nothing. Hence, an animal that is homozygous for all the “large” alleles will have a 50-lb yearling weight advantage over an animal that is homozygous for all the “small” alleles. Over time, if a breeder selects aggressively for yearling weight, he or she could “lock in” the large alleles and create a population of animals with a rock-solid 50-lb advantage. AA + BB + CC + DD + EE = 10 + 10 + 10 + 10 + 10 = 50. These gene effects are called “additive” for the simple reason that their individual effects simply add up to a final result. If there was no environmental variation to create confusion, a breeder could compare two animals and know exactly what their true genotype differences are. If animal Alpha was 15 lbs heavier than animal Beta, you would know precisely that animal Alpha had three more “large” alleles than animal Beta. Life as a cattle breeder would then be too easy, and maybe even boring. In the real world, environmental effects come into play which can actually result in an animal with a poorer genotype having a superior phenotype – i.e. an animal with only 5 “large” genes actually having a heavier yearling weight than an animal with 10 “large” genes. It happens every day in every herd, guaranteed. But even with the curve balls thrown in by random environmental effects, those animals with more of the “large” alleles will tend to outweigh those with more “small” alleles. So even though some selection decisions are genetically wrong, there will be a genetic trend over time that is consistent with the selection pressure that is placed on the population. To state the obvious: if you have good data and select for more weight, you’ll get more weight. Note the emphasis placed on the phrase “good data”. If you have poor data or no data, genetic change in the direction that you desire will probably not happen. Environmental effects are not the only thing that can confuse the simple additive model of genetics. Non-additive genetic effects can confuse the picture also. This does not mean that non-additive effects are bad. They actually can be
