MadSci Network: Genetics

Re: Is there a parallel between XXY in human males and male Calicos?

Date: Tue Nov 1 12:16:25 2005
Posted By: Allen Gathman, Faculty, Biology, Southeast MO St. U.
Area of science: Genetics
ID: 1130840912.Ge

As you are probably aware, the tortoiseshell (black/orange) color
combination in cats is due to inactivation of one X chromosome in each cell
of female mammals.  The coat color gene O/o is located on the X chromosome.
  Individuals with only the O allele have orange fur containing
phaeomelanin, which is a reddish pigment.  Ones with only the o allele have
fur containing melanin, which is dark brown to black in color. (The Cat
Fanciers site has a nice page about color inheritance in cats.)  A
heterozygous female has one X chromosome with the O allele and one carrying
the o allele.  Early in embryonic development, one X chromosome is
inactivated (randomly) in each cell; descendant cells continue to have that
chromosome inactive and the other active.  In the skin of the adult, then,
there are patches of cells descended from a single embryonic cell, and some
patches have an O on the active X, and produce orange fur, while others
have an o on the active X, and produce black fur.  This color pattern is
called tortoiseshell, and if white spotting (controlled by another gene
that isn't sex-linked) is also present, the pattern is called calico.  

Males normally have only one X, so they can't be calico or tortoiseshell,
just orange or black all over.  In rare instances a male has two X
chromosomes and a Y chromosome, and then X-inactivation takes place just as
it would for a female.  This produces a tortoiseshell or calico male, if he
is heterozygous for the O/o gene.  XXY cats are usually sterile (Centerwall
and Benirschke, 1975).   The XXY condition is called Klinefelter's syndrome
in humans, and has similar effects, except that no skin or hair color genes
appear to be located on the human X chromosome, so you don't get calico

You can find lots of information about Klinefelter's syndrome (mainly in
humans, though, as well as on X inactivation by searching PubMed Central.  

Centerwall WR, Benirschke K. An animal model for the XXY Klinefelter's
syndrome in man: tortoiseshell and calico male cats. Am J Vet Res. 1975

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