Dominant color from the Extension locus E
The final of the four BASE pairs is the Extension locus, designated as E. Again, there are only two possible alleles at this locus: E+, which is called extension wild, allows all the previously discussed BASE pairs to operate as they have been described, and Ed, called extension dominant, which does not allow Agouti to express pattern, flooding the wool follicles with eumelanin based on the B-locus (black or brown) and allowing spotting to express normally. It requires only one dominant allele at extension for the entire sheep to be colored – even if the agouti locus contains two copies of white/tan (AwtAwt) or any other combination of alleles. The possibilities at extension are:
- E+E+ – called extension wild, allows all BASE pairs to express, allowing for color and pattern differences within the flock. Every lamb will inherit one copy of extension wild from a sheep with this genotype.
- E+Ed – called extension dominant with a single copy, does not allow the agouti locus to express in the sheep, producing an entirely brown or black sheep with or without spotting, based on the genetics at S. Half of the lambs of a sheep with this genotype will inherit extension wild and could display pattern, and the other half will inherit extension dominant and not allow for display of agouti pattern.
- EdEd – often called double dominant, does not allow the agouti locus to express in the sheep, producing an entirely brown or black sheep with or without spotting, based on the genetics at S. All of the lambs of a sheep of this genotype will inherit one extension dominant allele and will be solid brown or black based on their genetics at the B-locus – agouti display will not express in any of these lambs.
There is no way to identify the agouti pattern alleles of a dominantly colored sheep with only a single copy of Ed except through the careful tracking of progeny who carry no dominant color. There is no way to easily identify the agouti pattern alleles of a double dominant sheep, since all offspring will be solid black or brown, possibly with spotting.
The phenotype of a sheep that is AaAa E+E+ and that of a sheep that is A_A_ EdE+ or A_A_ EdEd is identical – there is no discernable visual difference. For this reason, careful breeding and lambing records should be maintained if the shepherd wishes to track these genetics. There have been studies that have identified advantages to recessive color including increased fertility, improved mothering skills, lower levels of internal parasites, and less hoof issues (Adalsteinsson 1970, 1975). These many survival traits are linked to the co-dominant and recessive alleles of the agouti locus and are more prevalent in recessively colored sheep than those that are colored due to a dominant allele at extension.