A genetic mutation is a change in the genetic code from what previously existed. While some genetic mutations are advantageous (polled for example), the majority of mutations in nature tend to hinder a population’s success via harmful or lethal means. Mutations of this nature are often referred to as genetic conditions or genetic defects.
Dominant mutations always influence an animal's phenotype, so the mutation can easily be selected for or against. Recessive mutations, however, tend to exist in a population even when harmful to the point of being lethal. This is because animals can carry the recessive gene without showing any signs of it. When carrier animals are mated to other carriers, the resulting offspring have a chance of showing symptoms. Fortunately, technology has evolved to the point where animals that appear normal yet are carriers of recessive genetic conditions can be identified through DNA testing.
A 2 x 2 Punnett Square can be used to illustrate the outcome of various matings. In Example A below, we have mated a carrier sire to a carrier dam, while in Example B we have mated a carrier sire to a non-carrier dam.
In these examples, N is the normal gene while n is the abnormal recessive gene. The cells with single letters contain one copy of each of the sire’s (top row) and dam’s (left column) genes. Since we have used a male that carries the abnormal gene (n) in both examples each Punnett Square has an N and n on the sire side. As explained earlier, in Example A we have mated the sire to a carrier dam (Nn) while in B he is mated to a non-carrier dam (NN). Through the use of Punnett Squares, we can readily visualize what the resulting offspring will look like from our example matings. In Example A, we can see the 4 potential genotypes from the mating are NN, Nn, Nn, and nn — each with an equal probability (1/4) of occurring. Since the presence of N has complete dominance over the expression of n (i.e., N completely covers up the symptoms of n), we know that only the calf receiving nn will show the symptoms of the abnormal gene; the other 3 will appear normal. Because they received the n gene (Nn), 2 of the 3 normal calves in appearance will be carriers of the abnormal gene. In Example B, all of the resulting offspring will appear normal, while half of them (2 of 4) will be carriers. The above examples also work to illustrate other situations where a single recessive is involved, such as polled/horned or red/black.