"All sheep
are coloured but some are wearing white pyjamas"
S. Adalsteinsson.
Previous related post on this topicThis is Tiramisu, She is a ouessant sheep. She isn't a crossbred, she doesn't have Dutch, Belgian or German ouessant sheep bloodlines ( acknowledged to have outcrossed)
She does however have a very interesting background, one which I hope to explain without losing myself or others in summing up my explanation for why she has the pattern she does.
First, family relations and what is known about her breeding.
Maternal GrandDam Awt/Aa
Maternal GrandSire Awt/Aa
Dam ??/??
Sire Awt/ Aa
Tiramisu ??/??
Where the genotype is known I have completed it, this is based on their phenotype as a white sheep and also that they have each produced a solid black lamb in other matings, so are known to be Aa. I have not completed Tiramisu's dam as she is of similar pattern to Tiramisu and so its not possible for me to complete her as Awt as I lack the nomenclature to be able to accurately notate her genotype. The entire flock is made up of white or black ouessants and apart from Tiramisu's grand dam and dam no other ewes have produced dark headed lambs in the flock. The sire that fathered Tiramisu's mother is not the same sire to produce Tiramisu. The flock has been in existance for over twenty years, examples of lightly freckled lambs are born but the light speckling is transient and doesn't last. The only traces are some light smudges of darker colour on the extremities.
Currently traditional genetics for primitive sheep would explain her as a white sheep, how else could she fit genetically, she clearly isn't solid colour and according to the genotypes I have listed she has no possibility of inheriting Ag. However her pattern is very reminiscent of some Down breeds in the UK and previous attempts to understand their colouring has determined they are white sheep but there is little firm explanation for the spots and dark colouration.
Could there be an explanation and one that is perhaps not universally accepted but sometimes the exception proves the rule.
Mahler and Denis published a study into "The inheritance of face and leg colour in the mouton Vendeen breed"
They proposed the possible existance of a gene at another locus, not at agouti but extension.
I would suggest that their work was on the right track, but didn't go far enough.
My explanation, All sheep have the potential for a variable and incremental allele at extension. The expression of this allele is seen phenotypically only in white or grey sheep, it may be present in varying degree's however, in all sheep. The pattern of expression of this allele is individual but familial and in the case of breeds with a close or selected genepool can be very specific.
Not only are Down breeds expressing this gene at extension but I would also go further and suggest that all Ag sheep are expressing the same allele at extension to a lesser or greater degree. Ag would more correctly be Awt and a variable gene at extension. Lambs born solid (coloured) but with some degree of fading out, as in Ag would be expressing an increased variable at extension than those born with a white body and dark extremities.
From the tiniest patterns of pigmented spots (denoted as brindling by Mahler and Denis) through to a full Down pattern or Ag sheep and on to a solid colour sheep which is still Awt at Agouti but looks solid ( extension dominant) they are all part of the same variable and incremental allele at extension.
Traditionally dominant black has been looked at as only an on / off switch. A sheep is either ED or E+ . Would this still be the case with an incremental allele at extension? Yes, it is possible for this allele to behave both as a switch and with variable expression. The best analagy I can come up with is, to think of a see-saw, its position and effect is determined by the weight at either end. it can balance or swing between one end and the other or it can switch instantly to one end depending on how its weighted. Depending on the degree of shift to E+ or ED in each sheep at a mating, a variable expression could be seen in the offspring. In sheep with a significant shift to ED its offspring could become extension dominant.
In explaining this variable allele, the pattern of expression is important. It is built up in a series of patches or spots, the size, distribution and joining up of the dots determines the "pattern" . Within families ( or even breeds) where the pattern distribution is closely similar, it can be quickly built up, rather like drawing over the same areas on a piece of paper, it becomes darker and more easily identified as a defined pattern. When outcrossing to unrelated breeds where there is a shift of the allele at extension towards E+, it is easily lost in the cross bred offspring. Hence Mahler and Denis's problem of losing the identifiable Down pattern in their study, however the subsequent backcross determined that the pattern wasn't lost in either the white or black offspring, merely diminished. The cross they had chosen was to a sheep with a shift towards E+ in both the red and black versions and so the resultant offspring were not able to express the full Down pattern, only an earlier stage in the pattern building. In breeds such as shetlands and icelandics where Ag is regularly encountered , there is within the solid colour sheep population a variable but higher degree of shift to ED, that is, whilst within the population the degree of extension can vary, the prescence of Ag sheep keeps the shift towards ED. Where sheep are selectively bred for clear whites without any stray points of colour or patterning the degree of extension is shifted towards E+
At this point I will suggest that the various patterns in different breeds are co-dominant, that is each can be individually expressed however the shift of the allele at extension towards E+ or ED as it effects Agouti determines which is more phenotypically dominant, also by crossing out from close family or breed groups the pattern distributions may be so different as to lose any identifiable pattern and so crosses become a jumble or mix of two patterns that are virtually impossible to distinguish from each other. In essence whilst the prescence of say Ag and mouflon pattern occuring in the same sheep can lead to a difficulty in discerning the two distinct individual patterns, the mistake when it comes to interpreting pattern in Ag and Down sheep is in assuming if they are of the same genetic basis they should be in some way cumulative but the patterns would act as independantly as would Ag and Mouflon.
One further point and specifically in relation to those sheep that carry colour modified genetics. It seems to be the case that in white sheep with a mild shift towards ED , the darker pigmentation is cancelled out or negated by the prescence of colour modified genetics, leading to clearer whites. This may well be an explanation for why some wool breeds when producing solid coloured sheep as in the coloured Wenslydale and the Polwarth show the prescence of colour modified genetics, unwittingly the colour modified genetics have been selected along with sheep who are shifted to E+ .
To return to Tiramisu for a moment. The pattern she displays is quite progressed but its possible to identify this pattern in the large majority of white ouessants with a limited shift to ED, only in her case the pattern and therefore shift is considerably further towards ED noticable both in increased area of distribution and depth of colour and so far more striking, but subtle indications of this basic pattern can be seen in a good number of ouessants. In her case as the normal breeder controls in ouessant flocks of selecting only the clearer whites hasn't taken place the accumulation of a shift towards ED has gone on unchecked and so her dams unusual pattern emerges from an apparently Awt/Aa x Awt/Aa breeding. There is no reason to suspect that there has been an outcross in the flock, the same process of pattern building has been noted in many ouessant flocks where blacks and whites are bred together, "
grubby whites". An explanation for this could be that whilst clear white ouessants are sought after for the breed standard, The majority of ouessants are black, and the incidence of whites has traditionally been extremely low, so it is possible for black ouessants to have maintained a variable degree of shift towards ED and when crossed to whites, some lambs are born showing the beginings of the same pattern seen in Tiramisu or a shift towards ED. I have identified the possibility of two other patterns of extension present in ouessant flocks, it will be interesting to see if its possible to build on these to a similar or greater degree.
So to summarise either the genealogy I have for Tiramisu is incorrect or she is simply a white sheep ( which seems to stretch the definition of phaeomelanin a little) or there is the possibility of the existance of a variable incremental allele at extension. My explanation above will naturally continue to be tested and refined.
Tiramisu's pyjamas only hers have spots on, although technically I suppose they could be holes...............
Mahler and Denis from Proceedings of the world congress on coloured sheep 1989.
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