Following on from the article in the June Newsletter, Part Two was originally going to cover how colour genetics affects breeding decisions before looking in more detail at the variations to the basic colours.
But Part One has led to some debate on social media, and there are new proposals to update the Breed Standard, so I’ve been asked to say more about white Ouessants. Colour variations will now form Part Three.
I’d repeat that, although trained as a scientist, I’m not an expert on Ouessant Sheep, or on colour genetics, so remain grateful to Diane Falck, of the French Society, for her advice and permission to use her photographs. Links to a two part ‘primer’ on the colour genetics of Ouessants (which I played a very small part in developing) and a more detailed scientific paper by Diane may be found at the foot of this article. The paper covers the science in more depth, using the appropriate terms; in what follows I’ll try to use non-technical expressions wherever possible.
In Part One we learnt that the genes in two places are the key to understanding the basics of Ouessant colour: the Brown locus, which controls the base colour of the fleece (black or brown), and the Agouti locus, which controls which fibres in the fleece express the base colour.
Black or Brown
At the Brown locus there are only two different genetic variants; one producing a black base colour and one producing a brown. The gene for black is dominant to the gene for brown. This means that if both variants are present in a sheep (remember every gene is paired) then the black gene will prevail and the sheep will be black. It will look exactly the same as a black sheep with two black genes.
So, at the Brown locus there are just 3 genetic combinations possible:
- A black sheep with two genes for black (Black/Black)
- A black sheep that has one gene for black and one for brown (Black/Brown)
- A brown sheep that has two genes for brown. (Brown/Brown)
So, what happens when you mate a black ram with a brown ewe? Well, that depends on whether the ram has two black genes or one black gene and one brown gene. We know that the brown ewe must have two brown genes.
If the ram has two black genes then, as every lamb gets one gene from each parent, and the ram can only supply a black gene and the ewe a brown gene, it follows that every lamb will have one black and one brown gene (Black/Brown). In other words, 100% of the lambs from this pairing would be black lambs who are carriers of brown.
But if the ram has one black and one brown gene then the pairings of one gene from each parent can be shown diagrammatically as:
And we might expect 50% of the lambs to be black (carrying the brown gene) and 50% to be brown.
A similar analysis for the pairing of a black ram and a black ewe, each of which carries the brown gene is shown here:
We can expect a 25% chance of producing a black lamb with two black genes, a 50% chance of producing a black lamb with one black gene and one brown gene and a 25% chance of producing a brown lamb. 75% of lambs born to this pairing will be black and just a quarter will be brown.
White or Coloured
There are several genetic variations at the Agouti locus, but only two need concern us today; one that suppresses the expression of the base colour of the sheep, so that the sheep appears white and one that allows the full expression of the base colour (either black or brown). The gene for white (more accurately for White/Tan) is dominant to the gene for base colour, so that a sheep with one white/tan gene and one base colour gene will be white.
So, when dealing with base colours alone, there are again 3 genetic combinations possible at the Agouti locus:
- A white sheep with two genes for white/tan (White/White)
- A white sheep with one gene for white/tan and one that allows the expression of the base colour (White/Coloured)
- A black (or brown) sheep with two genes for the expression of base colour (Coloured/Coloured)
Again we can ask: what happens when you mate a black (or brown) ram with a white ewe? And again the answer depends on whether the ewe has two white/tan genes or just one white/tan and one gene that allows the base colour to be expressed.
If the ewe has two white/tan genes then, as with the first example above, every lamb will have one gene for the expression of colour and one for its suppression and as white/tan is dominant, every lamb will be white, carrying the coloured gene.
If the ewe has only one white/tan gene then the position is as shown here:
There is a 50% chance of a white lamb (which will carry the gene allowing the expression of colour) and a 50% chance of a black (or brown) lamb.
Mating two sheep that each have one gene for white/tan (White/White) and one for the expression of colour (Colour/Colour) produces similar results to the third example above: 75% will be white sheep and 25% will be black or brown.
You can get a hint as to the genetic make up of your sheep by checking their pedigree in the flock book. Once they’ve produced offspring you’ll have a better idea of their likely characteristics, and make decisions about breeding to stand a better chance of producing the colours you want in your flock. There are more examples – and fuller explanations – of the process in Diane’s colour primer, listed at the bottom of this article.
Since Part One of this article appeared there has been comment, on social media and elsewhere, about the status of white sheep in the Ouessant population. Some of the comments have been less than helpful, including suggestions that the display of tan pigmentation in white lambs (see Part One) is the result of cross-breeding or ‘impurities’ in the Ouessant gene pool (It’s not) or that white is a late introduction to the breed and that nineteenth-century photographs of lighter-coloured Ouessants are, in fact, of brown sheep (It isn’t, they are not and it is the brown base colour that, if anything, is a later introduction to the breed).
Since sheep were first domesticated, centuries ago, commercial breeders have selected for desirable characteristics, including the whitest possible wool. As a result, modern, improved breeds of sheep rarely display tan. But the Ouessant is an unimproved sheep – it has been described as the most primitive of the French native breeds – and it’s not surprising, therefore, that Ouessant lambs can display a surprisingly wide range of tan colouration at birth. This completely natural and normal display is produced by the pigment phaeomelanin, and is quite separate from the black or brown base colour of the sheep, which is a result of the pigment Eumelanin. For more detail on this see Part One, but a couple of points are worth repetition:
The amount of tan pigment displayed by a lamb is variable. Some authorities believe that the amount of display is controlled by genes at the Spotting locus, although recent research suggests that the situation is more complex and other genes / loci are involved. Regardless of the mechanism, there is no ‘right or wrong’ answer as to how much tan pigmentation a white Ouessant lamb may display:
The second thing to note is that the shade or hue of the tan pigment (phaomelanin) can be very variable. Although, as shown above a tan, or even a fox-red colour is common in Ouessants, other colours are found, ranging from dove grey to blackish brown and from a buttery cream to bright orange as illustrated in this picture from Diane’s paper:
The Breed Standard has always allowed for the display of tan in white sheep, both in lambs and as a base colour to the adult fleece. Following changes to the French breed standard, an updated UK standard is being presented to the AGM. This makes clear that the display of tan in lambs may cover the entire body and that, while the display usually fades over time, it may not do so completely. This is not a change to the standard; rather a clarification of the description to remove confusion.
The proposed Breed Standard has also been updated to note that the hooves of white sheep may be light or dark. This, too, reflects a change to the French standard which, in turn, reflects reality.
This picture of a class at the French society show demonstrates that many white Ouessants have darker hooves and the topic is also covered here
in a blog by Dominique Morzynski.
We hope to have a good discussion of the new standard, and the status of white sheep, at the AGM, and shall return to variations in the colours of Ouessants in the third (and definitely final) part of this article!
In the meantime, our recommended reading includes:
Author: David Clements