The Challenges of the Extension Dominant Gene

A tricky player in the genetics of coloured sheep

As breeders of coloured sheep we typically breed to produce various shades of wool either for our own craft or to sell to others for theirs. It makes sense then, that to a certain degree we want some predictability and ability to control our product – especially if we generate an income from it. But what if I told you that thanks to importations of new sheep breeds into New Zealand since the 1980s, the ability to predict the colour of lambs from your coloured sheep could be at risk?

Before the 1980s all coloured sheep breeds in New Zealand had two genes for colour on a part of their DNA (locus) called Agouti. White is the dominant agouti pattern with all the various shades of black and brown being recessive to white, which is why most sheep in New Zealand are white. To get a coloured sheep you would need both genes on Agouti to be coloured, and because of this you knew if you put two coloured sheep together you would always get a coloured lamb.

Read more about the agouti locus and recessive colour here

Enter the Extension locus – one of the four main genetic sites responsible for determining colour in sheep. All sheep inherit a pair of genes at the Extension locus, one gene from mum and one from dad. Before the 1980s all our sheep breeds had Extension Wild (E+) genes, genes that allowed all the different agouti colour patterns to express. But there is another gene that can appear at this locus, called Dominant (Ed) – this gene “turns off” any pattern from the Agouti locus – including white – by flooding the cells with melanin. And as the name suggests, this gene is dominant so they only need one copy to end up brown or black.

Read more about the Extension locus here

Some newer breeds in New Zealand such as Dorper, Blackshire, Awassi, Damara, and Karakul to name a few, all have Extension Dominant (Ed) genes, and these are being crossed with breeds that have only previously had Agouti colour. To understand how this can affect what colours you end up with, let’s look at some of my previous sheep.

When I first started breeding sheep I didn’t know BCSBANZ existed and I had zero understanding of colour genetics. My goal was heavily spotted coloured sheep but to begin with I just wanted colour, so I started with white Perendale ewes and crossed them with a Gotland ram (Gotland sheep do not carry ED). The Perendale’s white agouti allele trumped the coloured allele from the black Gotland so all the lambs that year were white.

I had failed I thought, so the following year I tried with Dorper and – success! 100% of the lambs were black from those same white Perendale ewes! Little did I know that this was because the Ed Dorper ram only had dominant black genes to give to all his lambs; but all these lambs had also inherited a white agouti allele from their dam which is all she could give. That white gene was still there but not visible in any of the lambs. I kept all these wonderfully coloured ewe lambs and the years that followed were very frustrating… I now wanted to breed in better-quality wool and spotting using E+ coloured rams (traditional rams like Romney), but every spring many of my lambs were coming out white. What was happening? How were two coloured parents producing white lambs? While my Ed black ewes looked the part, they were passing their Perendale dam’s white agouti allele to their lambs which was trumping my agouti-coloured rams.

Here is a photo of a Dorper X Perendale ewe, a heavily spotted Romney X ram and their white and black twins. This ewe passed her one copy of the Ed gene to one lamb, which trumped any agouti alleles that lamb was given, and so the lamb is dominant back. She did not give the Ed to the second lamb, instead passing on a white agouti allele inherited from her Perendale dam which also trumped any coloured agouti alleles passed on from the sire.

So where is the danger I’m talking about? The danger is buying in new stock, let’s say a ram, who is carrying Ed. A fine coloured ram – he could have one copy of Ed and one copy of E+ (let’s assume he is white on Agouti). He would pass Ed on to some of his lambs, and if so, these will be black also and thus could enter your breeding program. If you didn’t catch and eliminate this early enough and the gene spread through your next generations, it would take many, many generations thereafter to truly eliminate it (if at all) and you could have endless amounts of white lambs each spring that weren’t receiving the ED gene. And if lambs are coming out black, you wouldn’t know if they are dominant black or agouti black.

Having Ed in your flock makes it much harder to predict what colour the lambs will be, and Ed colours don’t come in the amazing range of shades and patterns as Agouti colours do either – you just get plain black or brown, sometimes with spotting. Breeders in New Zealand have spent many years developing the huge range of Agouti colours that are now available, and it would be a shame to lose that due to inadvertent introduction of Ed into coloured flocks.

As these new breeds become more popular, many crossbreds are springing up as people breed for “easy colour.” We are already starting to see white lambs popping up in flocks that should be all coloured. If we’re seeking to maintain the range and quality of colours we have available today, we need to be extra careful to avoid accidentally letting Ed animals into our flocks.

We can do this by keeping good records of each mating, so we know the breed and colour makeup of all our lambs. It also makes sense to be certain of the background of any ram purchased for breeding. The Black and Coloured Sheep Breeders’ Association publishes a flock book annually which shows the breeds, rams used, and sheep numbers for all members. There is a commercial section which permits crossbreds along with a stud registry of purebred animals. This allows buyers who purchase from listed BCSBANZ members to have certainty about the breeding of their animals.

– by Daniel Godfrey with support from Wendy Allison