Genetics – Breaking the Rules

By Wendy Allison, Wairarapa.

In the previous article we looked at why some colours are quite rare – because it’s unlikely for a lamb to receive full pairs of recessive alleles on all three loci. But there are a few exceptions to the rules we just covered, and we’re going to look at these now.

Extension locus – when two black sheep have a white lamb

Because colour is recessive, it’s impossible for two black sheep to have a white lamb. Or, is it?

There is a fourth place on a sheep’s DNA that can affect the animal’s colour. This locus is called Extension. This is another locus where only two alleles are possible:

• Extension Wild (Ew) – this one has no effect on the animal’s colour – it’ll be whatever colour the other three loci determine.
• Extension Dominant (Ed) – this one floods the animal’s cells with melanin and causes both skin and wool to be whatever colour is found on the Brown locus (usually black but sometimes brown), regardless of what’s on Agouti.

Extension Dominant is dominant, so it’s a reversal of the usual rule about white being the dominant colour – and even an animal with the dominant white pattern on Agouti will still be black because the Extension Dominant gene will override it. To be white the lamb would need both alleles on the Extension locus to be Wild. If it has even one Dominant allele, the lamb will be black. Sheep that carry the Wild allele can have white lambs with another sheep that has it.

Extension Dominant is very common in some breeds such as Hebridean or Welsh Mountain sheep but less common in most of the breeds in New Zealand, and it is generally not necessary to understand this for normal breeding purposes.

But what about Arapawas? Heaps of them are spotted!

We discussed back in the first article how recessive genes can stay hidden for a long time, because while they might be there you can’t see them due to the dominant gene covering them up? The flipside of that is that the dominant genes can’t hide because they are always visible in the sheep’s colour.

This means that dominant genes are very easy to remove from your flock – just don’t breed from any of the animals that aren’t the colour you want. In our flock we are breeding out white sheep this way, but if you wanted only spotted sheep you’d simply breed only from the spotted sheep and cull anything that’s not spotted. Because the dominant genes have been removed, eventually you’d only have recessive genes in your flock and all your lambs would be spotted.

So what you have in breeds like Arapawa is a gene pool where most of the dominant plain coloured genes have been removed. If you breed a spotted Arapawa to a plain one, you’ll get a plain lamb – but so many Arapawas are spotted or carrying spots that spots are now more common than plain.

My all-white sheep has a pink nose!

Some breeds of sheep, like Dorsets and Merinos, have all white wool and pink noses. We know from our understanding of the three main loci that most sheep can be white or coloured, have black or brown skin, and be plain or spotted. Only spotted sheep can have a pink nose. So how do you get an all white sheep with a pink nose?

The answer is that these sheep are actually spotted, but instead of having the recessive alleles for colour on the Agouti locus, they have the dominant genes for white pattern. So their wool is all white, and they have the pink nose that comes from spotting. In some cases they are just one big white spot all over. Because they have dominant white on Agouti, most of their lambs are white too. When you breed from a Merino that is carrying a recessive colour gene on Agouti, the lambs will almost always come out spotted.

Some have suggested that the white that comes with spotting is whiter than the white that comes from dominant white. Merino wool is valued for its whiteness, so there may be something in this!

We hope you’ve enjoyed this look at sheep colour genetics. If you have any questions or a sheep with a funny colour that you’d like to work out, please send a picture to webmaster@colouredsheep.org.nz and we’ll see if we can help!