There are a minimum of five loci identified as responsible for producing colour and pattern in sheep. Each locus ( single of loci ) is classified according to its effect. On this page I have identified those loci as they are understood in general and specifically noted their expression as it is found in the Ouessant sheep.
The five main ones are:
1. The Agouti Locus - responsible for pattern
|White ( Agouti white and tan - Awt)|
White is considered a pattern, it helps to think of white as a very boring pattern! There are as many as 17 patterns found in sheep such as mouflon ( black and tan) and badgerface but lets keep it simple and stick to those found in Ouessants.
|Self (Non-Agouti a/a)|
Self this is essentially "no pattern" this is the genotype given to sheep who are a uniform colour all over and don't show any Agouti pattern. It is necessary to include this in the Agouti Locus in order to be able to understand a sheeps genotype, the colour of the self or no pattern sheep is decided by the Brown locus.
White or Awt is dominant to self or non-agouti
2. The Brown Locus - responsible for colour
|Black (self - BB)|
|Brown ( self - bb)|
Black is dominant to Brown
3. White Spotting Locus - responible for areas with no pigmentation in an otherwise coloured sheep. In the Ouessant sheep this is generally restricted to the head and can vary from a few white hairs to a poll spot.
|White spotting - poll spot on a black lamb|
4.Colour Modified Gene - dilution of the base colour in both blacks and browns resulting in a lightening of shade and tone.
Colour Modifying or dilution gene can be considered recessive although its effects can sometimes be discerned in sheep carrying the gene.
|Photo J Schneider|
5. Extension Locus - For the moment not relevant in the Ouessant Sheep
* Age Greying - is included in this section as it is widespread in the breed and can have a significant influence on colour interpretation in many Ouessants. Age greying is the appearance of white fibres in the fleece, the fibres are often noticeably coarser than the usual fleece. The distribution of the white fibres can be limited to specific areas particularly in younger sheep most commonly seen in the britches, around the muzzle and across the shoulders. In the case of the ewe pictured below, the age-greying is evenly distributed throughout the fleece giving a uniform silvered or lightened fleece. It can be seen in some sheep from as early as one year of age.
|Age Grey Black Ouessant Ewe|
Black sheep x Black sheep with genotypes Bb X Bb their lambs will be either BB or Bb or bb
BB = A black lamb only able to pass black genes onto its offspring
Bb = A black lamb carrying brown
Bb = A second possibility for a black lamb carrying brown
bb = A brown lamb
With this example you can see that if you know a sheeps genotype for colour you can start to predict what colour lambs they will have. The same process can be repeated with the other loci with regards to pattern or white spotting etc. All that you need to know is the order of dominance.
At the Agouti Locus the two variables are White (Awt) and Self or no pattern (a) in this case white is dominant to self.
When two loci are combined the number of variables increases for the genotype but the phenotype as a rule remains relatively straight forward.
Lets take the above white sheep ( Awt/a) mated to a sheep of the same genotype only this time in order to predict the colour of the self ( no pattern) lambs we need to know what the genotype is at the brown locus as well.
In this case lets make it that one is Awt/a BB and one is Awt/a bb remember that both sheep will be white as the dominant pattern genotype is white carrying the recessive self.
any lambs that inherit a Awt gene from either parent will automatically be white
So any combination of Awt/a or Awt Awt irrespective of what is also inerited at the brown locus will appear white.
Awt/a BB = white lamb
AwtAwt bb = white lamb
Awt/a Bb = white lamb
Each has a different genotype but their phenotype is the same. There is a one in four chance of a lamb with an a/a genotype.
In this instance an a/a genotype will result in a coloured lamb and the only possible genotype with the above mating would be.
a/a Bb = a self black lamb (phenotype) black carrying brown (genotype)
The expression of genotype ( in otherwords a sheeps phenotype) is regulated by each locus and their dominance relative to each other.
Colour Modifying Gene - In the case of the colour modifying gene its action is on the Brown locus affecting the eventual colour in self coloured sheep. A white sheep with colour modifying genetics is not likely to show any observable effect of the colour modifying gene. The effects of the colour modifying gene can be variable but in the main it can be considered a recessive gene and it requires two genes in order for it to affect phenotype although some sheep may show some slight colour changes with only one gene - possibly as a result of other factors such as fleece type affecting the genes ability to be expressed.
White Spotting Locus - the white spotting gene is considered a recessive gene and is generally only noticeable in self coloured sheep when homozygous ( the animal has two genes for white spotting) however its relationship with the Agouti locus is NOT recessive and this does mean that its effects vary according to whether you have a white or self coloured sheep.
I have put together a montage of white lamb photos to help demonstrate this point as it can be a bit confusing to grasp otherwise.
1. 2. 3.
Lambs one, two and three are all white lambs ( Agouti white and tan) what isn't always appreciated at first glance and it does take some looking to get your eye in is that lamb one and lamb two also show the white spotting gene. Lamb three is considered a white lamb but doesn't have any white spotting.
Instead of considering white as sometimes having little patches of red it is infact the otherway round and the red is the base colour with white spotting overlaid on the top.
The white spotting gene behaves very differently in white (Agouti white and tan) sheep to self coloured sheep where it has very minimal expression in the Ouessant and is also considered as recessive. As is shown by this black lamb
In white sheep the same white spotting gene instead of being recessive is incompletely dominant (expressed as co-dominance)and a single copy of the gene covers the red base colour in a large area of white spotting as can be seen in lamb two. If the sheep has two copies of the white spotting gene the area of white can cover or almost completely cover all the red on the lamb as can be seen in lamb one. Here only very minimal traces of red can be seen at the nape of the neck and the base of the tail. Unlike many modern breeds of sheep where white spotting is used extensively as a mechanism to obtain ultra white fleeces the Ouessant still exhibits a more primitive colouration and recessive and highly coloured red lambs are seen from time to time. In order to select for red lambs a knowledge of the genotype for spotting in both white and self coloured sheep is needed to eliminate the dominating white spotting gene and obtain red lambs.
I have concentrated here on basic pattern and colour expression in the native Ouessant and not included the introduced Agouti greys. I will be updating this page to include more recent observations as time allows.