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Genetics – Why a Brown Spotted Lamb is Like the Holy Grail

By Wendy Allison, Wairarapa.

In the last article, we looked at the Brown locus – the second of the three main determiners of colour in sheep, and the one that decides whether your sheep’s skin will be black or brown. Today we’ll be looking the third locus, which determines whether or not your sheep has spots.

Muppet is a good example of a sheep that is basically brown, but with a pattern on Agouti that gives her white markings on her face. She is not spotted.

Most sheep are plain coloured – whether that be white, brown, or black, the majority of their colour will be one of those. Any variation comes from their pattern as dictated by the alleles on the Agouti locus that we discussed in the second article.

Some sheep do have spots though. Spots come from the alleles on the third locus, which is imaginatively named the Spotting locus. Alleles on the Spotting locus can create white patches on the sheep’s skin and wool which override the sheep’s basic colour and make it spotted. The spots can be large or small, but they are always white.

In this case, plain colouring is dominant and spotting is recessive so this means a sheep has to have a copy of the spotting gene from each of its parents on the Spotting locus before it will have spots. If you go back to the punnett squares in my first article and substitute dominant plain colour (S+) and recessive spots (Ss) for white (W) and coloured (c), you can work out what combinations will produce spotted lambs.

This is just the first two possible combinations, try the others yourself!

Loki produced one spotted lamb and one dark blue lamb, because both she and the ram carry the recessive spotting allele

In the picture alongside, the ewe is plain self-patterned and the ram is plain blue patterned. Neither parent is spotted themselves, but they are both carrying the recessive allele for spotting, and here is the result. The spotted lamb has large white patches rather than the little flashes of white you’d see if it were a pattern. The skin under those patches is pink, because there is no colouration in those cells at all. While his nose and eyes are still black because he’s black on the Brown locus, if he had a spot on his face that covered the nostrils, his nose would be pink.

 

Cookie is from the same cross, but she has a white spot that includes her nostrils so her nose skin is pink

In this picture of Cookie you can see that her nose is pink but her eyes have black skin around them. This is because her Brown locus alleles are black, giving her a black skin colour as discussed in the article about Brown – but her pair of Spotting (Ss) alleles on the Spotting locus give her pink skin wherever her spots are.

Cookie was bred to a plain, self-patterned ram and her lamb was born plain, because he only has one spotting allele on his Spotting locus. But he would be able to produce spotted lambs if crossed with another sheep that has spotting in its genes.

 

 

 

Now that we understand the three loci and how recessive alleles must have a copy from both parents present to produce the recessive colours, you can see why brown spotted lambs are quite rare. To be brown and have spots, a lamb must have a set of recessive alleles on all three of the loci.

Photo credit: F Vasconcellos

That’s quite a lot of recessive alleles for one little sheep to have! We have not managed to produce one at our place yet, but there’s a chance we might get one from crossing the sheep below.

Spider was sired by a ram that carries the spotting gene so he may be carrying it too. Cookie was sired by a ram that carries the brown gene so she may be carrying it too. Between them, they could potentially produce a brown spotted lamb

Spider

Cookie

In the final article of this series, Genetics – Breaking the Rules, we will look at some exceptions

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