What makes a calico cat?

prfwaddle at HUGO.FSUFAY.EDU prfwaddle at HUGO.FSUFAY.EDU
Fri Jun 24 06:22:35 EST 1994


Mammals, regardless of sex, generally have one functioning X chromosome 
per cell.  In cats, the gene for orange color is X-linked.  Since males 
normally have only one X, a tom is usually either orange (or cream) or non
orange.  (For simplicity, let us exclude the genetics of cats that are all
white.)  A female will be orange (or cream) if both her chromosomes carry 
the gene for orange color.  She will be nonorange if both do not.  If she 
is heterozygous, some of her hair producing cells will contain a functioning
X bearing the gene for orange.  The other X will be a nonfunctioning Barr
body.  Her other hair producing cells will contain a functioning X bearing 
the gene for nonorange.  Thus, she will have orange (or cream) hairs and 
nonorange hairs.  Such females may be orange and black if full colored or gray 
and cream if not.  Ifs I remember correctly, a tortoiseshell is two colored,
whereas a calico has white patches in addition.  The gene for white patches
apparently has and interaction effect in that calico cats have patches of
orange and nonorange instead of orange and nonorange hairs mixed together.
A calico tom is rare because it must have two X's to be calico and a Y to be 
male.

Calico cats and the XXY condition are easy to find in genetics and general
biology textbooks.  What I don't understand is why X-linked mosaicism has 
not apparently been described in humans, at least not in the books I've 
checked.  About 1 in 40 women ought to be heterozygous for colorblindness.

Has anyone heard of a woman colorblind in one eye?



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