Principles of Genetics and Breeding

10.1.15 Fixation and Loss

The breeding in small populations causes spreading of gene frequency, which is the initial phase.

This phase is followed by steady phase when the gene frequencies are evenly spread out over the range, and as gene frequencies exceed the two limits are equally probable.

Fixation and loss are the limits to the spreading apart of gene frequencies that can be brought about by the dispersive process.

The gene frequency cannot change beyond the limits of 0 or 1 and sooner or later each line must reach one or the other limit.

These limits are the points of no return, because once the gene frequency has reached 0 or 1, it cannot be changed any more in that line.

When a particular allele has reached a frequency of 1, it is said to be fixed in that line, and when it reaches a frequency of 0 it is lost.

When one allele become fixed, no other allele can be present in that locus, and the line is said to be fixed. When an allele is fixed, all individuals in it are of the same genotype with respect to that locus. In course of time, all loci in a line are then genetically identical. This is the basis of genetic uniformity of highly inbred lines.