The stages in genetic analysis

In the previous chapter sex-linked inheritance was discussed. In sex-linked inheritance the way in which certain characters are transmitted and expressed depends on the sex of the parents and offspring. The mechanisms involved depend on the genes for these characters being carried on one of the sex chromosomes, usually on the Z chromosome in the fowl since it is much larger than the W chromosome. In general, characters will show linked inheritance if the genes responsible are on the same chromosome, whether on the autosomes or sex chromosomes. The closer the genes are on the chromosome the stronger is the linkage. Linkage between two genes is a measure of the probability of them being transmitted together to offspring. An important goal of genetic analysis is to determine the positions of all the genes on the chromosomes and how their transmission and expression are controlled. This heightened understanding would enable genetic predictions to be made more accurately.

The process can be divided into a number of stages. (i) The first is to establish that a character is a genetically inherited character, and that it is inherited in Mendelian fashion. Some characters may be environmentally controlled. Others may be polygenetic, i.e. controlled by several genes (these are considered further in Chapter 9) and although each gene follows the normal Mendelian pattern, the overall analysis is rather complex.