Mosaic Expression
Cats: One X-linked Gene with Two Alleles for Coat Color
In cats, one of several genes controlling fur color is located on the X chromosome.
The gene has two versions, or alleles. One form of the gene codes for orange fur
(XB), and the other form codes for black fur (Xb). The orange allele is dominant to
the black allele. Ordinarily, this would mean that an animal inheriting one copy of
each gene (genotype XBXb) should have orange fur. Surprisingly, a female cat
heterozygous at that locus (XBXb) will not be orange. Instead, her coat will be a
patchwork of orange and black, a condition known as tortoiseshell. Why is she not
just orange?
Female Genotypes
There are three possible female genotypes for this locus:
Male Genotypes
Males, having only one X chromosome, carry only one allele for the orange vs. black
fur color trait. The Y does not carry this allele at all, being much smaller and
having fewer loci than the X chromosome. The condition of carrying only one allele
for a particular trait is known as hemizygosity (from the Greek hemi, meaning
"half" and "zygote", referring to the fertilized egg).
There are two possible (normal) male genotypes:
or
Expression of Coat Color in Males
Male color expression is straightforward. Because only one allele exists for the
gene in each cell, it will be expressed. And all males have only B or only b. There are
no heterozygotes, because the Y does not carry the allele.
Expression of Coat Color in Females
In females, the situation is more complicated because of an interesting process
that prevents the female from expressing double the amount of X-linked gene
products as the male, who has only one copy of each X-linked gene.
At a certain point in the embryonic development of every female mammal (including
cats), one of the two X chromosomes in each cell inactivates by supercoiling into a
structure known as a Barr Body. This irreversible process is known as Lyonization;
it leaves only ONE active X chromosome in each cell of the female embryo. Only
the alleles on the active (uncoiled) X chromosome are expressed.
Lyonization is random in each cell: there's no way to predict which of the two X
chromosomes will become inactivated. Hence, any given cell of a heterozygous
female could end up as either of the following:
In the diagram on the left, the black allele is inactivated. In the one on the right,
the orange allele is inactivated.
A heterozygous cat will be a patchwork of these two types of cells. Lyonization
takes place relatively early in development, when the cat is still a blastula, and all
the cells descended from a blastomere with a particular X chromosome inactivated
as a Barr Body will also have the same Barr Body inactivated. That means that all
the skin tissues that arise from a cell like the left one will express orange fur, and
all the skin tissue that arise from a cell like the right one will express black fur.
Hence:


In a patch of skin in which the X chromosome carrying the B allele is
inactivated, that patch of skin will express only the b allele (black fur).
In a patch of skin in which the X chromosome carrying the b allele is
inactivated, that patch of skin will express only the B allele (orange fur).
This is known as mosaic expression.
Here's an overview:
This is why calico cats are almost invariably female.
Can Male Cats Ever be Tortoiseshell?
In rare cases, a male cat can inherit two X chromosomes in addition to his Y
chromosome (Klinefelter Syndrome). If this happens, each cell in the male embryo
will undergo Lyonization, just as a female's would. If the two X chromosomes are
do not carry the same allele (i.e., the cat's genotype is XBXbY), then the male will
express calico coloration, just as a female would. But
he'll also have the disadvantages that go along with
Klinefelter Syndrome.
Real, Live Cats
A tortoiseshell cat is one with black and orange
patches. She is heterozygous at the X-linked coat color
locus.
A calico cat is a tortoiseshell expressing an additional
genetic condition known as piebalding. A piebald animal
has patches of white (i.e., unpigmented) skin. This is
controlled by a completely separate autosomal locus.
The patches may be relatively
large, or
rather small and interwoven:
It's possible that the larger patches are caused by slightly earlier Lyonization
during embryonic development : earlier inactivation will mean that each progenitor
cell will give rise to a relatively larger portion of the body.
Large patch size also could be caused by the same X chromosome being inactivated
in adjacent progenitor cells.