Dominant and
Recessive Genes:
In the example of Milking Shorthorn colours, each gene of the pair
(R and r) produced a visible effect, whether paired as identical
genes (two red or two white) or as two different genes (red and
white). This is not true of all genes. Some of them have the ability
to prevent or mask the expression of others, with the result that
the genetic makeup of such animals cannot be recognized
phenotypically with perfect accuracy. This ability to cover
up or mask the presence of one member of a set of genes is called
dominance. The dominant gene masks the traits coded for by the
recessive gene.
The polled trait
is dominant over the horned trait. Thus, if a pure polled bull is
mated with horned cows (or vice versa), the resulting
progeny are not midway between the two parents but are
polled. Likewise, not every hornless animal is
homozygous for the polled trait, many of them carry a
recessive gene for horns. A simple breeding test can be used to
determine whether a polled bull is homozygous or
heterozygous. The breeding test consists of mating the polled
bull with a number of horned females. If the bull is homozygous for
the polled trait, all of the calves will be polled. Heterozygous
sires produce half polled offspring, on the average while half have
horns like horned parents.
Dominance often
makes the task of identifying and discarding all animals carrying an
undesirable recessive factor a difficult one. Recessive genes can be
passed on from generation to generation, appearing only when two
animals, both of which carry recessive factor, are mated. Even
then only one out of four offspring produced, on average, will be
homozygous for the recessive gene and demonstrate that trait
phenotypically. |
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Incomplete or
Partial Dominance
There are varying degrees of dominance, from complete dominance to
an entire lack of dominance. In the vast majority of cases dominance
is neither complete nor absent, but incomplete or partial. The
results of crossing a trait with horned animal are clear cut because
the polled character is completely dominant over its
allele (horned). If a cross is made between a red and white
Milking Shorthorn, the result is a roan (mixture of red and white
hairs) colour pattern. In the latter cross, the expression of a gene
is such that it does not cover the allele (incomplete dominance);
the roan colour is the result of combined expression of a pair of
genes, none of which is dominant. Dominance is not always simply the
result of single-factor pairs. The degree of dominance depends on
the animal's whole genetic makeup together with the environment to
which it is raised and the various interactions between the genotype
and the environment. Environment has little effect on hair colour
except for extreme circumstances
such as molybdenum toxicity, copper deficiency, long exposure to
tropical sun, or freeze
branding.
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