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Black-yellow Mosaics

from 'Robinson's Genetics for Cat Breeders & Veterinarians, 4th edition'

All of the numerous coat colors of mammals are produced by genetic modification of two basic pigments:

Black and brown do not vary a great deal although the brown can be so dark as to be called chocolate. On the other hand, the red to yellow series is subject to notable variation as indicated by their descriptions. However, genetically, all of these are termed yellow as the differences are merely due to the depth of color.

Domestic animals

All of the usual domestic animals sport the above colors, sometimes in more than one form. The two pigments are probably derived from the same precursor substance, but a gene controlled switch early in the embryonic biochemical pathway causes either black or yellow to be produced. The normal pathway is the production of black pigment and the production of yellow pigment is caused by a mutant allele of the normal gene. The result is a completely yellow individual – a red tabby in the case of the cat. Another way of viewing the change of color is to say that it is devoid of black pigment. For this reason, ‘yellowing' alleles were designated as ‘non-extension of black' or, simply, as ‘yellow'. The symbols for these genes and alleles are F for the production of black pigment and e for the production of yellow pigment.

Typically, such alleles are inherited as recessive to black pigmentation. For example, the familiar yellow varieties of the guinea pig, hamster and rabbit are all caused by recessive alleles. In the cat, the genetic situation is different. There the mutant allele for yellow is borne on one of the sex chromosomes, the X. The cat was, for many decades, unique in having a sex-linked yellow. Now, a similar mutant has been found in the Syrian hamster. The inheritance of sex-linked yellow is not as straightforward as recessive yellow but the expected outcomes can easily be worked out. They are usually presented as a table of matings and of any expected progeny. The interesting aspect is that the allele can create a distinctive color pattern in the female in certain matings. The result is the well-known black and yellow tortoiseshell.

Normal gene

Denote the normal gene by the symbol O and the sex-linked yellow allele by o Using this convention, the tortoiseshell has the genotype Oo. That is, the color is heterozygous for the genes O and o. The pattern is engendered as if some of the body cells are producing black pigment while others are producing yellow. There is little order in the production so that the pattern is extremely irregular. The above supposition is correct and is a peculiarity of sex-linked genes. In each cell one of the X chromosomes ceases to function. In each cell, one of the X chromosomes ceases to function. In the case of the tortoiseshell, with its heterozygous genotype Oo, if the X carrying O is inactivated, the cell will produce black pigment due to the active o gene. But if the X carrying o is inactivated, the cell will produce yellow pigment, due to the active O allele.

Initially, which of the two chromosomes ceases to function is a matter of chance. Once one of the chromosomes has become inactivated in a cell, the same chromosome in all of the cell's daughter cells remains inactivated. As the cat develops, the cell lineages intermingle, resulting in the mixed up pattern of the typical black and yellow tortoiseshell. As a consequence of the sex-linked inheritance, it would be expected that all tortoiseshell cats are female. In the normal course of breeding this expectation is borne out. Rarely, however, tortoiseshell males make an appearance. Usually, the tortoiseshell color on a male is caused by abnormal sex chromosome constitutions. Typically, they possess more than one X chromosome. Most are sterile; only a small minority are fertile.

The majority of breeders and veterinarians are probably familiar with mutation in the germ cell tract as this is how all of the colors of cats originated. However, mutation can occur in the cells that make up the body. Most of the time, these would pass unnoticed but they can become noticeable under certain circumstances. One of these circumstances is that of the uncommon tortoiseshell male. That mutations can occur in the body cells may seem novel. However, each cell of the body contains a full set of genes although not all are functional at a given stage of development. However, most genes are functional during the active phases of embryonic growth and this is where the effect of a mutation is most likely to be seen.

On rare occasions, yellow colored variations of animals may have variable sized patches of black. This is due to a mutation for e to F in an embryonic body cell. Or, in the particular case of the cat, it is due to a mutation from O to o. Now, the group of cells carrying the o gene will produce black pigment, as opposed to all of the other cells carrying O that will produce yellow pigment. The size of the black patch depends on the stage of embryonic development at which the mutation occurred, the earlier the occurrence, the larger the patch. Mosaics with small spots would be expected to be more frequent, as the older the embryo, the more cells there are in which a mutation could occur.


These curious black and yellow animals are known as mosaics, a black/yellow mosaic to be precise. A female mosaic would not excite much curiosity as it is likely to be taken as an ordinary tortoiseshell. On the other hand, a male mosaic is likely to be a focus of comment, the more so because he would be behaving as a fertile male tortoiseshell. The mosaics could occur in a litter from any mating in which yellow offspring are to be expected. However, one mating in particular would distinguish the mosaic from the tortoiseshell. This is the pairing of two red tabbies. All of the kittens would be expected to be red tabby. Hence, if an apparent tortoiseshell is produced it is almost certain to be a mosaic, even if the individual is a female. Both sexes will be fertile and probably breed as red tabby because the mosaic condition is a property of the body cells and is rarely heritable as such.

A number of probable cases of mosaicism in pedigree cats have been found in the Netherlands recently. This has generated a certain amount of discussion as to whether or not these should be registered as tortoiseshell. These are red tabby with small spots of black on various parts of the body. They are almost certainly black/yellow mosaics. In addition, the small size of black areas would indicate that the mutation occurred late in embryonic development. A proposal to register these cats as ‘red with black spots' to distinguish them from ordinary reds and tortoiseshells is being discussed in one registry. The smallness of the black spots indicates that they will breed as ordinary red tabbies.

‘Robinson's Genetics for Cat Breeders & Veterinarians', Fourth Edition
Carolyn M. Vella, Lorraine M. Shelton, John J. McGonagle, Terry W. Stanglein
Butterworth-Heinemann, ©Reed Educational and Professional Publishing Ltd 1999; ISBN 0 7506 4069 3

note from the preface: 'This book continues the pioneering work of the late Roy Robinson both in his three editions of Genetics for Cat Breeders (1971, 1977, 1991) and in his many articles. It seeks to expand the scope of these works in assisting the practicing veterinarian who is dealing with cats and cat breeders.'

The Art of Registering a Blue Silver Tortie Mackerel Tabby Male Cat
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