One of the benefits of performing DNA testing on our cats is that we are able to identify and establish their genotype, or their genetic state, at various positions in the DNA that have been associated with certain diseases. The presence of a particular DNA combination at that site could tell us that the cat is at risk of developing a particular disease, or that it is a carrier for the mutation but won’t develop the disease, or that it is clear of the mutation all together. By knowing this information, we can make appropriate care choices for the tested cat.
Using the results of the tested cat in combination with the test results of potential mates, we can also proactively leverage DNA testing to help produce healthier kittens. We achieve this by avoiding tom x queen combinations that could produce kittens with an at risk genotype.
By knowing the parents’ genotype and the mode of inheritance of the particular disease in question, it is easy to determine the probability of any kitten born in a litter of being clear, carrier, or at risk for a recessive disease. Remember that recessive conditions require both parents to contribute the mutation to a kitten to produce an at risk state. In the case of dominant conditions, the only potential outcomes are clear and at risk because dominant diseases only require one copy of the mutation from either parent.
It can be helpful to depict the probability that a particular mating will produce kittens at risk of a particular mutation by graphing them on a Punnett Square. In this example, we have a hypothetical condition where N is the normal state, n is the mutated state, and only ‘n/n’ kittens will be affected because the mode of inheritance is recessive. Keep in mind that N/n kittens will not be affected but carry the mutation and can transmit it to future generations.
Each parent, depending on its genotype, will contribute either the ‘N’ or the ‘n’ form of the gene to a kitten. This in turn will result in that particular kitten’s own genotype of N/N, N/n, or n/n (clear, carrier, at risk respectively). Each of the four squares shown for each of the six possible matings in the Figure represents a 25% chance for producing a kitten with that genotype. Thus, the matings resulting in one, two or four red squares will on average produce litters containing 25%, 50% and 100% affected kittens, respectively.
For example, breeding an N/N tom to an N/n queen can only produce kittens that are N/N or N/n - none of the kittens would be susceptible to this theoretical recessive condition (2 green squares and 2 yellow squares). On the other hand, breeding an N/n tom to an n/n queen gives a 50% chance that a kitten will have the condition, since kittens can be either N/n or n/n (2 yellow squares and 2 red squares). All kittens from the mating of two n/n parents will be n/n and thus likely be susceptible to the condition (four red squares).
It is worth pointing out that a carrier can still be a part of a well-managed breeding program. A cat that is a carrier for a recessive mutation can be safely bred to a clear cat; the mating will produce a 50:50 ratio of clear and carrier kittens. By using a “test and replace” program, you can test the litter and keep a clear kitten to replace the carrier parent, thereby retaining the carrier parent’s good genes in your gene pool while successfully removing the known mutation from your breeding program.
Figure: Six potential matings and the potential genetic outcomes for a theoretical disease that has a recessive mode of inheritance. Green are clear, yellow are carrier, red are affected.