John Fallon, of Ardgowan School, asks :-
Why are some sheep black and some white when they are the same breed?
Peter Maher, a wool scientist at Lincoln University, responded.
White wool is more in demand because it can be dyed all colours. Hence in most flocks black sheep are culled out. Even after many generations of sheep some genes remain which can produce a black sheep.
The production of coloured pigment in wool is controlled by recessive copies of a gene. Genes exist in pairs along the chromosomes, in all living cells and are the template of life. In the case of recessive genes, both copies of the gene need to be of the recessive type in order for the phenotype (in this case wool colour) to be expressed.
If `W' is the dominant white allele and `b' is the recessive black allele then the WW and Wb genotypes will be white sheep and only the bb genotype will be black. It is almost impossible to select out all of the b genes in a population because most of those genes exist in Wb animals, which appear white. In populations that contain any proportion of the b allele, there will occasionally be animals produced that are black, even if no black animals are allowed to breed. These will come from mating Wb genotype rams to Wb genotype ewes as follows:
Parents Father Mother Genotype Wb Wb Wool Type White White
Offspring Genotype WW(quarter) Wb(half) bb(quarter) Wool Type White White Black
If 11 percent of the genes in the white parents are of the recessive b type then approximately 1 per cent of their lambs will be black.
The black gene can exist in any sheep breed though it is more common in some than others.