Colour Genetics

Page last modified  18.05.2014

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)
There are only two true colours in sheep - black and brown but other genes  can alter how those colours will eventually look.

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
White spotting can be considered recessive in self coloured sheep, it is incompletely dominant in White - Awt sheep.

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

Self Black with colour modified genetics

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
Age-greying is certainly familial and considered by some to be dominant. 

How to work out your sheeps Colour / Pattern in genetic terms or your sheeps genotype.

At each locus your sheep will have two genes these are inherited from its parents (one from each parent). So if we take the Brown  locus there are only two possible genes black or brown. Some loci can have several genes  like the Agouti locus where there are lots of different patterns but to keep things simple. 

A black (BB) sheep mated to a Brown (bb) sheep = BB x bb

Each lamb will inherit  two genes a  black gene  from one parent and a  brown gene from the other parent but all the lambs will look black and carry the recessive brown gene. 

This is because black is dominant to brown meaning that you will see the black but the brown gene is hidden - the sheep although black to look at is carrying brown. The lambs genotype is Bb, what would happen if you mated it to a sheep with the same genotype?

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.

A white sheep (Awt Awt) mated to a self (aa) sheep = AwtAwt x aa

Each lamb will inherit a white (Awt) gene from one parent and a self (a) gene from the other parent but all the lambs will look white and carry the recessive self gene.

Once the difference between a sheeps outward appearance ( phenotype) and its genotype is known it is possible to calculate with much greater accuracy the  colour and pattern of any lambs.

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.

Awt/a BB x Awt/a bb = a number of variable combinations 

 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.

The Ouessant Sheep originates from the island of Ouessant, part of a tiny archipelago just off the north coast of Finistere, Brittany. The island of terror as it was known to some, was swept by the full force of the atlantic’s weather, the hardy sheep adapted to survive on poor grazing from salty clifftop meadows. It was the women of the island that raised the sheep, renowned for their black wool to weave into cloth known locally as berlinge and their meat with its sweet and delicate taste.

La race "Mouton d'Ouessant" est originaire de l’île d’’Ouessant qui fait parti d’un petit archipel au large du Finistère, Bretagne. L’île de l'épouvante comme c'était connu par certains était balayé par les intempéries de l’atlantique, ces moutons rustiques s'adaptaient à survivre sur les pâturages pauvres des falaises salées. C’était les femmes de l’île qui élevaient les moutons réputés pour leur laine noire à tisser « la berlinge » une étoffe régionale et leur viande avec un goût doux et délicat.

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