• Question from:eileen girling

Hi again,    I refer the sun newspaper April 27th 2011 page 27 an article called "Ewe been seeing somebody else?" Because on a smallholding near Royston Herts they have 37  white mums ( ewes) and all  were mated to Rowley a white South African Dorper male sheep. But ALL the resulting 60 lambs were all black. So 100 pc black offspring ( from different mums too ).

    I was taught in o level biology 35 years ago that " if the children show it and the parents dont then it is recessive".   So I take it that black ( in sheep) is recessive which would explain why both white parents had black sheep BUT it does not explain the mendelian ratio at all. So I am very interested as to what is happening here.

   First I would have thought black would dominate over white but then this story tells me that white is dominant over black but that both parents all carry the recessive black allele. Which seems okay for the one male Rowley but that all 37 white mums are also heterozygotes for this trait too seems over the odds? Hope I,m explaining well?
  
a Peter Morris of the Nat. sheep association blames a genetic throwback to a jet black gene in the dads ancestry but that would make this black gene dominant and therefore Rowley would show it in his phenotype would he not? Put me out of my misery please. Tks Eileen.

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  • Institut de Biologie du Développement de Marseille-Luminy (IBDML), Marseille
  • Posts: 79

Hi Eileen -
This is truly weird.  I did a little digging, and there is a gene called agouti signaling protein (ASIP) which represses pigmentation, and the black allele of this gene is indeed recessive in sheep.  Basically, the ancestral state was black, but at some point a duplication/rearrangement event occured at the ASIP locus, bringing a copy of the ASIP coding sequence under control of a neighboring gene's promoter.  In this allele of ASIP, the gene is expressed at very high levels, producing a white coat (remember, repressor!).  So that explains why white is dominant - the highly expressed white allele wins.

But, as you point out, this doesn't explain why all the offspring are black.  If both parents were heterozygous at the ASIP locus, you would expect at best 1/4 of the offspring to be black.  My best guess I have is that dad was a chimera, the product of two genetically distinct embryos that got smooshed together in utero early on in development.  These sorts of developmental events are rare but not unheard of.  One of these embryos gave rise to the white wool and epidermis. The other, which was homozgyous for a gene (not ASIP) producing a black coat, gave rise to the germ line.  It couldn't have been ASIP itself, because the mom-sheep all carried the white version, and so there still should have been white offspring.  It would have had to be a modifier that dominantly suppressed ASIP expression. 

Maybe someone on the site has a more elegant theory that doesn't invoke unknown repressors and rare developmental events?

Last edited by Jessica Cande (11th Jun 2011 15:04:50)

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  • University of Bristol
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I am as puzzled as you both are! Your logic Eileen is perfect and one would expect about a quarter of the offspring to be black if recessive and the male should be black if it is dominant. Like Jessica I too can only assume that the dad is genetically black but some other mutation is causing him to have a white coat. I suppose he could have a non-germ line albino mutation as well that causes him to be white and yet allow him to pass  on the dominant black. Sorry no better ideas than that or Jessica's chimera (which would also explain the outcome).

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  • Institut de Biologie du Développement de Marseille-Luminy (IBDML), Marseille
  • Posts: 79

Here is another off the wall theory -
epigenetic silencing of the ASIP locus that is determined by the genotype of the male parent.  Since the coat color of the offspring is determined by the genotype of the dad, Rowley could be white as a result of his dad's genotype, but then Rowley's genotype caused ASIP to be dominantly silenced in all his offspring. It would be very interesting to see what the next generation of crosses looked like!