Sheep - AgRIS

For understanding the inheritance of a trait where only one pair of genes is involved,
following facts (laws) can be stated (I) genes occur in pairs in the body cells of the individual,
(2) one of each pair came from the father and one from the mother, (3) the genes of a pair
separate during formation of sex cells, and (4) on fertilization, the genes are restored to the
paired condition.
The law of chance determines which sperm and which egg will combine at the time of
fertilization to form the new individual. The 1:2:1 genotypic ratio for the offspring of two
heterozygous (Pp) inclividuals works well on paper, but under practical conditions the actual
ratio may be far from the expected. Actually, when a small number of offspring are produced
from such a mating, all might be of one genotype of either PP, Pp, or pp. The larger the number
of offspring produced, the more likely it is that the expected ratio will occur. A similar
condition is observed in the number of boys and girls that may occur in any one family. The
chances of any one baby being a boy is one out of two, for it has an equal chance of being a boy
or girl. Yet, many of us have seen even large families where all the children are of the same
sex. This happens because of the law of chance.
Six different kinds of matings, with reference to parental genotype, are possible when one
pair of genes is involved and where dominance is complete. These are listed as under:
Genotypic of Genotypic ratio Breeding ability of
Parents of offspring offspring
PPxPp { IPP Breedstrue
{ 1 Pp Doesn't breed true
PP x PP All PP All breed true
PP x pp All Pp None breed true
Pp x Pp {I PP Breeds true
{2 Pp None breed true
{ I pp Breeds true
Pp x pp { I Pp None breed true
{ Ipp All breed true
pp x pp All pp All breed true
5.5.2 Incomplete Dominance - Coat colour
Occasionally, modifications of this simple two-gene inheritance, are observed. For
example, the case of coat color in sheep. Three different coat colors appear in the sheep having
red coat colour: redgRR), roan (RW), and white (WW). When a roaan ram (RW) is mated to a
group of raon ewes (RW), the offspring will be colored in the ratio of one red (RR) to two roan
(RW) to one white (WW). It should be noted that this is the same genotypic ratio that is
obtained when dominance is complete, and as, in the example where polled was dominant to
horned. But in this instance, the heterozygote expresses itself phenotypically in a different
manner from either homozygote, and the genotypic and phenotypic ratios are the same, 1:2:1.
This type of gene action is called incomplete dominance, or a blending type of inheritance. One
practical point here is that, even though it is not possible to develop a pure breeding roan herd
(RW), because the roan individuals are all heterozygous a d will not breed true, it is always
possible to produce roan sheep (RW) by mating a red parent (RR) to a white parent (WW).
Many cases of inheritance are known where dominance is incomplete and the genotypic
and phenotypic ratios of the offspring of a mating between heterozygous parents are the same.
For exarnple, palomino horses will not breed true, because they are heterozygous for genes for
color, but they can always be produced by mating chestnut sorrels with pseudo albinos.
Another example is in Comprest Herefords, which are heterozygous for the comprest gene.
When mated, comprests produce offspring in the ratio of 1 normal (cc) to two comprest (Cc) to
one dwarf (CC). Of course, in this instance it is impossible to mate the normals (cc) with the
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