Factors that change it and implications for meat goat breeders

Mendelian principles again explain genetic mechanisms operating in individual goats. However the charge to meat goat breeders is not to change individuals, but populations of individuals (herds or breeds). In describing an individual for some simply-inherited trait, reference might be made to the specific genes which that individual possesses or the one-locus or two locus genotype might be described.

To describe populations genetically the answer is to use gene and genotypic frequencies. A gene frequency or allelic frequency is the relative frequency of a particular allele in a population. It is a measure of how common that allele is relative to other alleles that occur at that locus. Relative frequencies range from zero to one. If an allele does not exist in a population then its gene frequency is zero. As an example in a herd of black Angus cattle, if there are no “red” alleles, the gene frequency for red is zero. Likewise in that same herd the gene frequency for the “black” allele is one. In a black Angus herd in which an occasional red calf is born, the frequency of the red allele will be something greater than zero. The sum of the frequencies of the various alleles (two or multiple) at a single locus in a population must equal one. Not all alleles are of equal merit or desirability.

Role of selection

From a population genetics standpoint, the effect of selection is to increase the gene frequency of favorable alleles. When replacement does are chosen the owner is attempting to retain those animals with the best sets of genes and reject those with poorer sets of genes. As a result, offspring in the next generation should have, on average, better sets of genes than the current generation. Another way of saying better sets of genes is to say better breeding values. Gene frequencies, average breeding values, and mean (average) performance are really tied together. Although selection is not the only force that can change gene frequencies in a population, it is the most powerful one available to most meat goat breeders.

Simply-inherited versus polygenic traits

Simply-inherited traits are affected by only a few genes. Only a single locus or at most, a few loci are involved in their expression. There are two common secondary characteristics of simply-inherited traits:

• They tend to be “either / or” or categorical in nature.
• They are typically affected very little by environmental factors.

Polygenic traits are affected by many genes, and no single gene is thought to have an overriding influence. Examples include growth rate, feed efficiency, and ribeye area of the carcass.

• Phenotypes for polygenic traits are usually described by numbers, e.g. 0.45 pounds per day in gain, 40 pound weaning weight, 1.89 square inch ribeye.
• Phenotypes for polygenic traits are typically quantitative or continuous in their expression rather than either / or.
• Most, although not all, polygenic traits are also quantitative traits.
• Polygenic traits are clearly affected by environmental factors to varying degrees.

Be aware that there can be some crossover between the secondary characteristics of simply-inherited and polygenic traits. Size is an example wherein mature size is clearly polygenic but some dwarf body types are simply-inherited and controlled by a major gene as well.

Threshold traits

There is a special category of traits which are polygenic in control but in which the phenotypes are expressed in discrete categories, not different from simply-inherited traits. Two such traits that come to mind are dystocia and fertility, as reflected in conception rate (pregnant versus not pregnant).

These threshold traits present special challenges for the breeder. Fertility is believed to be influenced by many genes and is therefore polygenic. But the trait may be measured in only two phenotypes; pregnant or nonpregnant. Dystocia is measured in only three to five categories although there is evidence that it is polygenic. Threshold traits are no different from quantitative polygenic traits in regard to genotype. How do we deal with this? Well think of a threshold trait as having a continuous but unobservable underlying scale of expression; a liability scale. Think of an animal’s liability for a threshold trait as the sum of its genetic values for the trait. On the liability scale there is a point above which animals exhibit one phenotype, and below which they exhibit another.

Simply-inherited and polygenic traits have a great deal in common as noted below:

• Genes affecting both kinds of traits are subject to the same Mendelian mechanisms. Mendel’s laws of segregation and independent assortment apply equally.
• Dominance and epistasis affect gene expression for both kinds of traits.
• The basic tools of goat breeding, selection and mating, are the same for both types of traits. Selection focuses on increasing the frequencies of favorable alleles.

However, different approaches to genetic improvement are taken between the two types of traits. This different approach is primarily a function of the number of genes involved. The more genes affecting a trait, the more difficult it is to observe the effects of individual genes, and therefore the less specific information we have about those genes. The amount of available information affects the way we characterize genotypes and therefore determines the breeding technology to be used. To add to this difficulty, consider the fact that relatively few studies have been made on the specific genetics of meat goats. There is somewhat more for dairy goats, but even so not abundant amounts. Test matings may be advisable in some cases to more clearly identify or describe the genotype.

Polygenic traits are affected by so many genes that it is extremely difficult to observe the effects of specific loci and specific alleles at those loci. It is impossible then to explicitly identify an individual’s many – locus genotype for a polygenic trait. The logical alternative is to characterize the net effect of the individual genes influencing the trait. That is, to quantify the individual performance and breeding value for a trait. This requires statistical tools including concepts such as heritability and accuracy. The terminology will change from A1A2 or Bb used for simply-inherited traits, to the terminology of polygenic traits, e.g. EBV’s, EPD’s, ACC’s and the like.