Re: Rescued kitten has only 3 toes on front paws?

Dear Gayle,

You wanted to know if your kitten has an inherited trait or a mutation. First of all, good for you for adopting a stray. As for your question, there are different kinds of mutations. A mutation refers to a change in the DNA sequence of a gene. If the mutation is passed on to the next generation, then it has been inherited. Using DNA recombinant technology, changes in DNA sequences can be found in genes that have no visible effects.

In the case of your kitten, since you do not have a family history, or pedigree, and don't know what its brothers and sisters are like, it isn't possible to say right off the bat what kind of mutation your kitten has. We can guess, speculate.

It is possible that your kitten has acquired a spontaneous mutation. This is a type of mutation that is not seen in either parent. Or your kitten has (as you asked) inherited the mutation from one or both of its parents. In that case, your kitten's 3 toe phenotype (visible expression of trait) could be the result of 1)a recessive mutation, 2)a dominant mutation or 3) a sex linked mutation.

DNA comes in paired chromosomes. We inherited one from our mother (maternal copy) and one from our father (paternal copy).

1. In a recessive mutationi, the affected individual, your kitten, would have had to get one mutant copy of the 3 toed gene from each of its parents. A recessive phenotype is seen only when there are two copies of the mutant gene. One copy of the mutant gene and one copy of non-mutant gene will not give a kitten with 3 toes. Individuals with recessive mutations could have parents that do not show the same mutation. Your kitten's parents may have normal toes; they are carrying one copy of the mutant gene and one copy of the non-mutant gene.
2. In a dominant mutation, your kitten would need only one copy of the mutant 3 toed gene. The mutant copy can come from either parent (unless it is located on one of the sex chromosomes) or the mutant copy can be a spontaneous mutation; neither parent have a mutant copy of the gene, just your kitten. If it is a dominant mutation, one or both parent cats would have 3 toes.
3. Sex-linked refers to the fact that the mutations are located on the sex chromosomes. Females have two X chromosomes and males have one X and one Y chromosome. In cases where a mutation is carried by the X chromosome, usually only males are affected. Females have another X chromosome that can act to compensate for the mutation; males don't have that option. In humans, conditions such as colour blindess and haemophila are sex-linked mutations and seen mostly in males. The colour of calico cats, orange vs. black patches are sex-linked colour genes carried on the X chromosomes. In this case, almost all calico cats are female. The patches of colour are due to the randomness of which X chromosome colour gene is used in any particular patch of skin.

If the 3-toe mutation is something new to the vet, you can do a few things to narrow down the possibilities. Check the neighbourhood you found to kitten in, ask around. Do you see or has anyone else seen cats/kittens with 3 toes? Is it a common phenotype among the cats in the neighbourhood? That would argue for an inherited trait as opposed to a spontaneous mutation only found in your kitten. As for whether the mutation is dominant, recessive or sex-linked, you would need to keep careful track of all the kittens (progeny) that your kitten has. Construct a pedigree, a family tree for your kitten and keep notes on which of the sons/daughters have 3 toes and which of their sons/daughters have 3 toes etc. With enough information, you can deduce the nature of the mutation. This is how geneticists work to understand human mutations. PLEASE NOTE: I'm not suggesting that you should breed your kitten just to figure this out. I love kittens/cats (I have a cat myself) but population control is very important. There are too many homeless strays out there already. You can ask your vet if he knows of other cases of 3-toe cats, their owners may have a family history for the animals. You can combine all the data for their animals and check for the inheritance of the mutation.

I hope this LONG answer was helpful. You can also check your local library for books on inheritance/genetics. Most of the examples would be about human diseases, but the general principles are the same.

Yours Truly,
Shirley Chan