What makes a calico cat?

Bill Purves purves at jarthur.cs.hmc.edu
Mon Jun 13 18:41:40 EST 1994

In article <bowersox.771542380 at taweret.Colorado.EDU>,
Paul Bowersox <bowersox at taweret.Colorado.EDU> wrote:
>If I recall correctly, it has to do with a female X chromosome getting
>turned off, except the same one is not turned off in all the cells,
>resulting in a mosaic... but I don't know exactly.

As you say, the gene in question is on the X chromosome.  A female
heterozygote is calico; homozygotes for either allele are solid color.
Only one X chromosome is expressed in a given patch of cells, the
other X being reduced to a Barr body.  (Thus doth nature make both
males and females viable, even though males are X-chromosomally challenged,
being only half as good as females.)

>PS -- You might even comment on how a male calico cat might come about, if
>you want to enlighten me on that score as well! 

A male calico cat doesn't come about... the male is hemizygous for this
locus and will express only the color determined by the allele it
carries on its single X chromosome.


Bill Purves
Department of Biology
Harvey Mudd College
Claremont, CA 91711

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