MadSci Network: Genetics

Re: What causes a mother cat to produce a litter with many colors?

Date: Mon Jun 21 14:24:51 1999
Posted By: Mark Sullivan, Staff, Molecular and Microbilogy, Fred Hutchinson Cancer Res. Center
Area of science: Genetics
ID: 927128233.Ge

     Well, in the case of your cat "Miss Kitty" the calico it is due to 
what is called Barr Body, or an inactive X-chromosome.  Females have 2 
X-chromosomes, but since the cell only needs to have one active, it shuts 
the other one down so it is not in use.  Now for calico's, the gene for 
black or orange fur lies on the X-Chromosome.  A female cat may inherit one 
gene for Orange fur from her mother, and a gene for black fur from her 
father.  When the zygote(a fused sperm and egg) start to divide to form a 
kitten, the embryonic cells will inactivate one X-Chromosome, but this 
occurs randomly.  Then at a later stage of development, these emryonic 
cells give rise to skin and tissue cells that have the same x-chromosome 
inactivated giving multiple populations of cells that have the same barr 
body(inactivated X-chromosome).  So there may be clusters of cells that 
have an active X-Chomosome that express the orange color, yet another 
cluster of cells may have the black expressing X-chromosome active.  That 
is why calicos have those random black and orange patches of fur and are 
almost always female.  There are sometimes gentic mutations that can cause 
males to express calico patterns such as XXY inheritance.  There may be 
white fur as well, but that is probably caused by a gene on another 
     Now, males also have one X-Chromosome(and one Y-Chromosome), but the X 
is always active because it is the only one present.  So males for the most 
part don't have calico patterns, except in the rare case explained above. 
The patterns on the fur, and other shades of fur are again probably due to 
color genes being expressed on other chromosomes.  Patterns in embryonic 
development, and subsequently tissues are very organized.  That is why cats 
with the genes to express say the Tabby patterns are all very similar in 
pattern.  Minor variations occur due to size of the cat, genes for fur 
color and just individual cell differentiations between one cat and 
another, but the pattern is still roughly the same.  Your cat's litter 
inherited lots of different color patterns from each parent, but some 
colors are dominant over others which is why you saw a mixture.  But the 
question is, are the two calico's female?  I bet they are.  Hope this 
answers your question.

Mark Sullivan     

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