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Dominance & Recessive

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.


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 circumstanc
es such as molybdenum toxicity, copper deficiency, long exposure to tropical sun, or freeze branding.


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