Where there are two or more different genes that can occur at a specific
locus, these are termed ALLELES. An animal can have two different alleles
(heterozygous form) or two the same (homozygous form). This is the case
with the gene pair that determines wool pigmentation in most domesticated
sheep breeds.
If a sheep has only the WHITE allele present, then the animal produces
white wool, and if only the BLACK allele is present then the animal produces
black wool. But if BOTH white and black alleles form the gene pair, the
animal is white. In the HETEROZYGOUS animal, the gene which exhibits the
characteristic it controls by masking the effect of the other in the pair
is termed DOMINANT, and the gene whose characteristic is hidden is termed
RECESSIVE. With most breeds of sheep, white is dominant and colour is
recessive.
To illustrate possible results in breeding programs, capital letters are
used as symbols for dominant characteristics and small letters for recessive
ones. Since white is dominant over black, 'W' stands for the white gene
form, and 'w' will indicate its recessive black form. Thus a homozygous
(pure breeding) white sheep can be illustrated as WW and a homozygous
(pure breeding) black sheep can be illustrated as 'ww'
The mating of two sheep, one possessing the WW combination and the other
possessing the ww form must produce offspring with Ww paired genes, since
one must be contributed by each parent. This new individual will appear
white because the white allele will dominate the black allele. However,
white offspring will be carriers (heterozygous) for the black pigmenting
factor.
These are the only three possible genetic combinations we can have if we
confine ourselves to the precise consideration of black and white:
WW Homozygous White
ww Homozygous Black
Ww Heterozygous white -
but a carrier for the desired black gene.
Breeding Brown Sheep
The recessive allele 'b' which is responsible for the "moorit" or brown
genotype, is to be found in white sheep and coloured sheep. Just as there
is no visible difference between the while woolled heterozygous (Ww) and
white woolled homozygous (WW) sheep, there is no visible difference
between a homozygous black (wwBB) sheep and a black sheep heterozygous
for brown (wwBb). Only sheep HOMOZYGOUS for the recessive allele 'b' are
identifiable as being brown.
Brown sheep are characterised by a deep red-brown (almost purple) nose,
eye socket and lip tissue. This colour contrasts strongly with the definite
black pigmentation evident in sheep producing black or grey fleece. This
skin colour extends to the tongue
and palate but is far less intense,
making the difference in colour between brown and black sheep less
easily discernible.
Facial hair on brown genotypes is also red-brown, as is the hair on the
lower legs. Some black lambs are born with tan hair covering on the face
and legs, but this is shed as the lamb matures. On brown genotypes, the
red-brown hair covering persists and occupies the entire area of pigmented
skin, and there is minimal fading with age.
Black sheep have black hooves, whereas moorits have hooves of deep brown,
but weathering of the outer horn material masks the true colour and is
difficult to perceive.
Black sheep produce a range of fleece colours from jet black to pale silver.
Moorit genotypes produce a similar wide range of fleece colours, from deep
chocolate brown to a very pale fawn. At the extremes of each
range there
can be confusion in identifying moorits. Dark brown fleeces are not
uncommon on sheep of the black genotype, and on the wool table can be
mistaken for moorit fleeces. At the other end of the scale, very pale
fleeces from both moorit and black genotypes can be very difficult to
distinguish from each other, and from white fleeces. The most reliable
means of identifying the moorit genotype is the facial colouring,
considered with other characteristics of hoof material, fleece colour,
and skin and eye colour.
