Genetics – How do we get coloured sheep?

By Wendy Allison, Wairarapa.

People with small flocks of sheep often want to breed coloured (not white) lambs, but don’t know how to do it to get the result they want. This short series is an introduction to sheep colour genetics, to help you understand how it all works so you can start getting those adorable coloured lambs.

As you probably know, DNA is made up of 2 strands twisted together, and the ram and ewe each contributes one strand to the new lamb. These combined decide a lot of things about the lamb, including what colour it will be.

Colour in sheep is governed by three main places, or loci, on the strand of their DNA. These loci have names. The one we’ll be looking at today is called Agouti. This is the one that governs the pattern of white hair and wool on your sheep, and makes it white or not.

When two genes, or alleles, come together, one is dominant. For sheep, being all white is dominant, which means that if you breed a white sheep with a coloured sheep, the white allele will override the coloured one and the lambs will be white. The rule of thumb is:

• White + white = white
• White + colour = white
• Colour + colour = colour

But my white sheep had a coloured lamb, what does that mean?

Just because that coloured allele isn’t showing, doesn’t mean it can’t be passed on to the lamb. And if both parents are carrying a coloured allele and those alleles get together, you’ll have colour + colour = colour.

Um, what?

You can use a little grid called a punnett square to look at how the alleles can combine – this helps predict what colour lambs will be. If a full white sheep has two white alleles (WW) and a coloured one has two coloured alleles (cc*), here are some of the combinations you can get.

*We use lower case for coloured (cc) because it is recessive, and capital for white (WW) because it is dominant.

Note: Notation for this is usually expressed as Awt for white and Aa for black (A for agouti). We have used W and c here to lessen confusion.

Stumpy, the ewe in the below photo, has a white mother and a coloured father, so she’s Wc. She was bred to a black ram (bb) so she has a 50/50 chance of having a black lamb (bottom left punnett square in the above diagram). This year we were lucky – but in general we can assume Stumpy will have 50/50 coloured and white lambs if we keep breeding her to a coloured ram – perhaps she’ll even have one of each!

Of course, this only works if you know what colour genes your sheep’s parents had – often people only find out their white sheep are colour carriers when they have a surprise coloured lamb. That’s one of the reasons it’s worth keeping breeding records.

These colour alleles can stay hidden for generations, and are why even flocks that have been bred to be white for many years can still pop out the occasional coloured lamb. Those of us who like coloured sheep are lucky they do!

You’ll also see that while Stumpy’s lamb is obviously coloured, he’s also not completely black. That’s a pattern, and in a future post we’ll have a look at those.