Chapter 7: Inheritance of monogenic traits
- Tessa Jacobs
As we have learned before, traits of animals can have a monogenic or a polygenic background. We can select animals for polygenic traits based on estimated breeding values as explained in the previous chapter. Examples of monogenic traits are: the colour of the animals, dwarfism, extreme muscularity, malformations or severe health disturbances. The alleles determining the expression of monogenic traits may be dominant, intermediate or recessive. For the alleles of monogenic traits, the allele frequencies can be calculated. Characteristics of monogenic traits will be explained first, then breeding aspects of monogenic traits with desired positive effects will be discussed and finally the way monogenetic with undesired negative effects (genetic defects) can be handled in breeding programs will be outlined. The reason to pay a lot of attention to monogenetic traits is that many are determined by recessive /dominant alleles. Then it is impossible to distinguish heterozygotes from homozygote carriers of the dominant alleles. Then you cannot determine the genetic value of all animals for such monogenic traits. That is a problem, whether we deal with positive or negative aspects of the alleles.Â
Characteristics of monogenic traits
To repeat briefly: the loci for a monogenic trait may contain identical alleles: the animal is homozygous for this trait. It got an identical allele from the sire and from the dam. A loci also may contain two different alleles: the animal is heterozygous for this trait. It got an allele from the sire and a non-identical from the dam. Homozygous implies that all offspring of an homozygous animal will get a similar allele from this animal when it is used for breeding and heterozygous implies that offspring gets either one of the two alleles with a chance of 50 %. In heterozygous animals we may have concern with intermediate inheritance (the value for the trait of the heterozygous lies just in between the values of the two homozygous forms) or with dominance / recessiveness. Then the heterozygous animal cannot be distinguished from one of the homozygous forms based on their phenotypes; they are not different.