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Why are calico cats female? |
First of all what is a calico cat? Calico cats have three
colours of fur in distinct patches. It is a type of cat, not
a breed and in fact, there are at least 16 breeds that have
calico patterns (Cat Fancier’s Association).
Most people have heard that calico cats are always female
and/or that male calicos are lucky. Well, generally calicos
are female and the explanation follows. I guess male calicos
are supposed to be lucky because they are rare.
How do calico cats get their patches?
Well, the orange and black are alleles from the same gene
on the X chromosome. The white spots come from an autosomal
piebald spotting gene that affects pigment cells. Cats have
19 pairs of chromosomes – 18 autosomes and one pair
of sex chromosomes – half from the mother and half from
the father. Every male cat has received a Y chromosome from
his father and an X chromosome from his mother. So, males
only receive one possible choice of orange or black (one X)
and are therefore one colour. In female cats, they receive
one X from their mother and one from their father so they
get two choices for orange and black.
But why are there distinct patches? Why don’t the orange
and black mix together? Well, the X chromosome has lots of
information but the Y chromosome has gradually lost most of
its stuff. Remember that females have 2 Xs and men only have
one so to make things equal, females turn off one of their
two X chromosomes in each cell (= X inactivation). This happens
early in development and once the X has been inactivated,
the same X chromosome stays silent throughout cell divisions,
leading to clusters of cells with the same X “on”
in the adult. So, there will be a patch of cells with the
orange X and another patch with the black X.
Because males only have one X chromosome, they can’t
get these patches. Occasionally (1/3000), a male cat is born
with an extra X chromosome (XXY) so he is able to get the
patches of colour – but he is usually sterile.
Interestingly, it is very difficult to breed calico cats because
the process of X inactivation is random. You can’t predict
what type of patches you will get. The first cat to be cloned
was a calico cat and the resulting clone (Cc) was all one
colour. Either her X inactivation was not re-set from the
one cell that was cloned, or all of the cells decided to inactivate
the X in one way during her development.
June 2005
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