Resistance: It's a Gene Frequency Thing
by Will Getz
Fort Valley State University (Georgia)
Sometimes in the interest of simplifying biology, an understanding of the true system and relationships is lost. In breeding, some traits are simply inherited meaning they are influenced by few genes, each with a relatively large effect, and expressed through some kind of dominance/recessive mechanism of gene action or gene expression. Those traits are usually expressed in only a few visible or measurable categories such as colors, color patterns, or presence and absence of some feature of the anatomy.
Resistance of internal parasites to the effect of a particular type of anthelmentic or resistance of the host (sheep or goat) to the effects of a particular type of internal parasite, are not simply inherited. Instead the evidence suggests those traits are influenced by many genes; each with a small and probably not an equal effect; and the individual effects are cumulative across all genes affecting the trait. Other traits such as average daily weight gain are expressed and inherited in a similar manner. In other words there are degrees of resistance which is a reflection of the genetic variation (differences) within those traits. Genetic variation is raw material to be used to change genetic resistance.
Most sheep and goat owners have observed that some animals appear to be more resistant to the effects of the local farm worm burden. Likewise it is known that there is genetic variation in the degree to which worms can tolerate the effects of an anthelmentic. While the feeding program and nutrition, sanitation and general health, as well as levels of climatic and management stress are sources of variation in those characteristics, the common denominator is the presence of genetic variation.
To illustrate that point, suppose 20 genes (10 pairs of genes) have some influence on the expression of resistance. Also suppose that each gene contributes 5 units of resistance to the organism (goat, sheep, or worm). It becomes obvious then that the organism with more units of resistance will be better able to withstand any challenge.
Within a herd or flock if there has not been much direct or indirect selection for resistance, the number of genes conveying resistance among all the animals in a herd or flock are distributed in what is referred to as a normal distribution which tends to look like a bell-shaped curve. In other words there are relatively few animals or worms with very few (low frequency) genes creating resistance to the effects of the anthelmentic or the effects of worms on the host, and likewise there are relatively few worms or animals with a large number (high frequency) of genes creating biological resistance. In a situation where there has been little effective selection for resistance most (68 percent) of the herd or flock will be located in the middle of the curve and the frequency of genes providing resistance would be in the moderate range.
In situations where selection is applied, assuming there is a reasonably high correlation between the way resistance is measured and the genetic makeup of the organism, the number of genes for resistance will tend to increase. In the case of worm resistance to anthelmentic this outcome is undesirable while in the case of sheep or goats with the ability to resist the effects of worms it is a desired
outcome. Because genetic variation exists within most populations the possibility exists to change gene frequency through selection
How does selection occur?
In the worm population any time a deworming event takes place selection occurs. Those worms with few genes available to provide resistance to the anthelmentic of choice will die and no longer be part of the breeding population. Those worms with a higher frequency of genes providing resistance will survive, mate, and produce the next generation with a higher average level of resistance. Gradually the frequency of genes providing resistance in the whole worm population on a farm becomes high enough that the anthelmentic of choice no longer works.
Likewise, genetic variation exists within the goat herd or sheep flock (within all breeds) where some animals have a relatively low frequency of genes providing a positive biological reaction (probably within the immune system) against the effects of exposure to worms. Other animals will have a relatively high frequency of genes providing a positive response to attack. By keeping records on such measurable characteristics such as body condition score, fecal egg count, and FAMACHA© scores it becomes possible to identify most of the animals that have the higher frequency of genes for resistance and hence would be the more desirable breeding animals.
Summarizing then, resistance is a polygenic trait, controlled by a relatively large number of genes, each with a small but probably not similar effect. It is possible to make changes in gene frequency within either the worm population or within small ruminant populations through selection. It takes time, turning generations, and deliberate action. The results are liberating for landowners raising sheep or goats for profit.