X-Linked Inheritance of Orange in Cats
Orange (yellow) cats and non-orange cats can be males or females, but the tortoiseshell pattern, consisting of a patchwork of orange and non-orange, occurs almost exclusively in females. The basis for this phenomenon is the X-chromosomal location of the Orange (O) locus. In genetic parlance, the O locus is X-linked. Two alleles are known to occur at the O locus: O results in an orange hair coat, and produces non-orange, the particular color of the non-orange hair depending upon genes at other loci. For purposes of this discussion, we will assume the cats to be described have the genotype aaBBCCDD at the relevant autosomal loci. That is, the non-orange hair will be non-agouti, black, non-dilute.
Since females normally have two X chromosomes, three genotypes are possible: OO (orange), (non-orange), and O (Tortoiseshell). Males, having only one X chromosome and a Y chromosome, are hemizygous for X-linked genes and can normally have only two genotypes: OY (orange) and Y (non-orange). In rare instances, a phenotypic male cat has both orange and non-orange patches in the coat. These animals have been shown to have a variety of chromosome anomalies in which there are at least two X chromosomes and a Y chromosome.
X-Chromosome Inactivation - The Lyon Hypothesis
In the tortoiseshell or calico female cat, the two alleles at the O locus are expressed in an unusual way. In contrast to previous examples in which one coat color allele was completely or incompletely dominant over another, at the X-linked O locus, both the Orange and the Non-Orange alleles are fully expressed, one to the exclusion of the other, in different areas of the coat. The coat seemingly is a mosaic in which either the O allele or the is active in a given area. This phenomenon, also observed with X-linked coat color genes in the mouse, was hypothesized by Lyon (1961) to be the result of random X-chromosome inactivation. According to her hypothesis, one of the two X chromosomes in each cell of early female embryos is inactivated; following this event, the same X (either maternal or paternal) is inactive in all descendants of a given cell. X-chromosome inactivation occurs at random, so that on the average, the maternal X is active in one-half of the body's cells and the paternal X is active in the other half. The female tortoiseshell cat is thus a mosaic for its X linked coat color genes. In approximately one-half of the coat the chromosome bearing the orange allele is active, and in the other half, the non-orange allele is active. This gives a random admixture of dark (usually black) and orange patches.
Studies of X-linked traits including a number of biochemical polymorphisms in man and other mammals have amply confirmed the Lyon Hypothesis. The major questions remaining concern the completeness of inactivation of genes on the X chromosome and the chromosomal mechanism responsible for turning off the genes involved. It appears that a few loci may escape inactivation, but the mechanism involved is still poorly understood.
The tricolor or calico cat is a tortoiseshell cat with piebald white spotting. The presence of spotting in some way causes the black and orange areas to coalesce into larger patches. The tortoiseshell or calico patterns are seen in both agouti (tabby) and non-agouti cats. In cats homozygous for the Siamese dilution allele,,
the orange (flame) and tortoiseshell color patterns are seen only on the points.
Click here to see an example of a calico cat.
Characteristics of X-linked Inheritance
The pattern of inheritance of X-linked traits differs from that in autosomal traits in a number of ways that are clearly demonstrated by X-linked Orange:
The results of reciprocal crosses differ:
In a mating between an orange male (OF
) and a black female (
), all females produced are tortoiseshell and all males are black.
The reciprocal mating of a black male (Y
) and an orange female (OF
) gives a different result: all females are tortoiseshell and all males are orange.
Male to male transmission does not occur.
It is apparent in the examples just given that males transmit their color types to their daughters and not their sons. This is because a male receives his X chromosome from his mother and the Y chromosome from his father.
The phenotype associated with the less common allele will be found predominantly in males.
This phenomenon is most apparent when one allele is rare, as in the hemophilia, but it is also evident in the orange phenotype of cats. In many parts of the world, the frequency of the O allele is about .10 (one in ten X chromosomes will have the O allele at the O locus, while the remaining nine-tenths will have the allele).
Under conditions of random mating, the frequency of the orange phenotype in males will be equal to the frequency of orange-bearing X chromosomes or 0.10. Since the orange phenotype in females requires the presence of two O alleles, its frequency will be 0.10 x 0.10 or only 1 in 100.
Click here to see an example of an orange cat