What color was Ricki's father?
My cat, Ricki, is a Tortoiseshell shorthaired, non-purebred cat.
Her mother, Ashley, is also a shorthaired, non-pedigree cat, but pure gray.
I don’t know what color Ricki’s father was, but with the help of some
information about cat colors and genetics, I can make an educated
guess. This month, I’ll write some introductory text and start
discussing some of the genes involved. Next month, I’ll finish the
discussion and make my conclusions.
Ashley (with a silly look on her face!)
Chromosomes and Genes
First, a brief discussion of feline chromosomes and genes. The cat has 19 pairs of chromosomes, each pair being different from the other pairs. All cells contain the full complement of 38 chromosomes (19 pairs). A cat gets 19 chromosomes from its mother in the egg and 19 chromosomes from its father in the sperm cell. The two sets are fused together to form the 19 pairs.
Chromosomes carry genes. Genes are the unit of heredity determining all the characteristics – appearance, health, growth -- of an organism, in this case, a cat. Genes occur in specific locations on the chromosomes, and each chromosome carries thousands of genes.
Normally, when a gene reproduces itself, it makes an exact replica. But sometimes, the replication is not exact and a mutation occurs. The original gene and the mutation (or mutations) form a group called an allele. In this group, one gene type may dominate over the others –
this gene is called the dominant gene. What this means is that if the dominant gene occurs on one chromosome, and the non-dominant, or recessive, gene occurs on the other chromosome (remember, chromosomes come in pairs), the dominant gene’s influence will overshadow that of the recessive gene.
Another effect called obscuring can occur when one gene influences another gene. For example, the "all white" gene obscures the effects of the other genes controlling the color of a cat.
Chromosomes also determine the sex of an individual. One chromosome is labeled X, the other Y. If an individual has one X and oneY, that individual is a male. If an individual has two X-chromosomes, the individual is a female. (There is no such thing as YY).
Cats have several genes that determine the color of an
individual and the pattern (markings) of the coat. In this discussion,
I’m only going to hit the high points. The number of genes and how
they combine to produce the wide variety of coloration in cats is
amazing. The genes described below are of particular pertinence for
a Tortoiseshell like Ricki.
One gene is called B, for black pigment. It has a recessive form,
called b. (With genetic symbols the capital letter is used for the
dominant gene.) Another gene is call Agouti, or A. This gene
determines the background color of a tabby cat. A third gene, called
T, controls the type of tabby pattern. Another gene is called full
color or C. There are several mutations of C, indicated by c with a
superscript letter. (The various mutations of a gene are distinguished with
different superscript letters.) For example, a pointed cat like the
Siamese has cs.
Yet another gene, the dense pigmentation gene (D), determines how
strongly the color appears in a hair. There are a couple of different
genes that control the presence of white. Finally, there is another
gene for orange or red, called O. O is a special gene because it is
sex-linked. More on that next month.
How Genes Combine
The black pigment gene, B, is a good example for describing how genes from the mother and father combine. Because B has a mutant, recessive, form called b, there are 4 different combinations of how these genes can appear in an individual. An individual cat will have 2 of these genes, because the cat has two chromosomes and the black pigment gene occurs once on both of them
An individual could have two B genes, or two b genes, or a B and a b, or a b and a B. When the cat becomes a mother or father, only one of its two black pigment genes will get passed to its offspring. If the cat is BB, then it will always pass on a B. If it is bb, it will always pass on a b. If it is bB or Bb, it could pass on either a b or a B.
So for a given kitten, if its mother and father both passed it a B gene, the kitten will be BB. If both parents gave it a b, it will be bb. If the kitten got one b and one B, it will be bB or Bb. A table shows how this works.
Combinations of the black pigment gene
If four kittens each have one of these combinations of genes for the black pigment gene, what color will they be? Because B is dominant over b, any combination with capital B will be black, so three kittens will be black. Only bb will be chocolate, or one out of four.
The dense pigmentation gene, which controls how densely the pigment granules are deposited in individual hairs, works the same way. There are four combinations of D and d.
- DD – full color
- Dd – full color
- dD – full color
- dd – dilute color
Now, combining the black pigment gene and the dense gene gives 16 combinations. Here are some examples.
- BBDD – black
- BbDD – black
- BbDd – black
- bbDD – chocolate
- bbDd – chocolate
- BBdd – blue (gray)
- Bbdd – blue (gray)
- bbdd – lilac
The Reader’s Digest Illustrated Book of Cats, The Reader's Digest Association (Canada) Ltd.,
Montreal, 1992, pp. 142-151.
A Standard Guide to Cat Breeds, Richard H. Gebhardt, Grace Pond, and Dr. Ivor Raleigh, Eds.,
McGraw-Hill Book Company, New York, 1979, pp. 15-23.