| Breeding a mutant line to homozygosity can be speeded up if potential mutant heterozygotes are phenotypically identifiable in each generation, meaning that fewer crosses in total have to be made in each generation in order to produce F3 homozygotes. This can be achieved if the original population used for mutagenesis carries dominant genetic markers on their chromosomes that enable individuals carrying one copy of any of those chromosomes to be easily identified. An example showing the inheritance of a pair of chromosomes (blue) that carry a dominant genetic marker (B) leading to abnormal pigmentation is diagrammed here. The corresponding chromosomes from a wild-type animal are in red. Mutagenesis of a parent organism (P) results in a recessive mutation (m) on one member of the chromosome pair. An F1 heterozygote for the mutation is easily identifiable by its color (second row). However, recombination during meiosis means that the marker B may become separated from the mutation m in some of the gametes produced by this individual. So when it is crossed with a wild-type mate, not all the progeny carrying the mutation also carry the pigment marker, and not all pigmented individuals carry the new mutation (third row). |
