RECESSIVE BLACK PIGMENTATION IN MERINO SHEEP - Fact Sheet

What sheep (and ourselves!) look like is primarily controlled by our genes, but is also affected by our environment (including our climate, our diet and our experiences).

Particular things about us can be determined by one pair of genes or by numerous pairs of genes the important thing to know is that these genes generally come in pairs, because one is on each of a pair of chromosomes.

The particular genes can vary and when they are different each is called an allele. The diagram (left) shows a pair of chromosomes (pink) with one pair of genes (blue) and the alleles of those genes (yellow and orange).

The various colour patterns of sheep across the world are very complex, with quite a few different genes contributing to coat colour, each with their own group of alleles.

We are only going to consider one of the colour patterns found in merino sheep and this is caused by recessive genes.

It is called self-colour black and the sheep is generally black over the entire body (pictured).

Other genes will also influence the colour, causing it to be lighter of mottled or with other patterns, which we will not cover here.

The sheep in these examples will be one of three types:

White sheep that do not carry any allele for self colour black shown as two white circles.
White sheep that do carry one allele for self colour black shown as one white circle and one black circle.
Black sheep that carry both alleles for self colour black shown as two black circles.

The black allele is recessive and for the sheep to appear black, both of the genes must be this same allele.

The white allele is dominant, so whether the sheep has two white alleles or one white and one black allele, the sheep will appear white.

Example 1: Here we see what happens when two white sheep are mated, but one of them is carrying a black allele. All lambs will be white, but half of those lambs will be carrying a black gene.

Example 2: This shows the result when two white sheep are mated, but both of them are carrying a black allele. Three out of four lambs will be white, but two of those (half the lambs overall) will be carrying a black gene, and one lamb in four will be black.

Example 3: This shows what happens when we mate a white carrier sheep with a black sheep. Half will be black, the other half although white, will all be carriers of the black gene.

Can you breed a white lamb from a black ewe?

Of course, but the ram must be white! If the white ram is a carrier of one black allele (such as example 3 above), then you have a 50:50 chance of a black or a white lamb. If the ram carries only white alleles, then all the lambs will be white. However, no matter what the white ram is, all the white lambs will be carriers of a single black allele, because their mother's alleles were both black.

Remember

The examples provided above have been described very simply to provide an introduction for those with little or no previous exposure to genetics -- in reality, other genes and alleles are involved to alter the final colours shown by the sheep.