From: ktruett@gems.vcu.edu
Newsgroups: rec.pets.cats
Subject: The possibility of XYY cats
Date: 25 Jul 92 12:17:23 -0400
Organization: Medical College of Virginia

It would be almost impossible for a cat or human to have a XYY chromosomes.

For a cat (or person) to be XXY, there are two possibilities:
1.  Non-dysjunction (see below) in mother so that the egg's chromosomal 
    complement is XX.  Normal paternal chromosome so that the sperm's 
    chromosomal complement is Y (or X).  Thus the offspring is (XXY or XXX).
2.  Non-dysjuction in father so that the sperm's chromosomal complement is XY.  
    Normal maternal complement so that the egg's chromosomal complement is X.
    Thus the offspring is XXY.
Because the father does not have two Y chromosomes, it is not possible for him 
to give his son YY, thus making the possibility of XYY very small.

As a disclaimer:  there are cases where cell division goes awry after 
fertilization, so the the chromosomes don't separate properly and the two 
resulting cells have different chromosomal complements.  Each cell then 
continues to divide, giving each daughter cell it's (now incorrect) 
chromosomal complement.  The result is called a mosaic.  If the Y chromosome 
divided improperly, then one daughter cell could be XYY, and the other would 
not recieve a Y, and thus would be X.  All subsequent cells would be one or 
the other.  However, this is not a true XYY because not all the cells possess 
that complement.

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Non-dysjunction (this is not meant to insult anyone's intelligence, but 
rather to explain a genetic term to those who are not familiar with it.)

Normally, during meiosis (cell division to produce sex cells --as opposed to 
mitosis which is normal cell division), all chromosomes replicate (so that the 
cell has 2 copies of each chromosome).  Then the cell divides twice.  The 
first division results in two cells which contain the original complement of 
chromosomes.  The second division occurs without the chromosome replicating 
themselves, therefore each daughter cell (sperm or egg) contains half of the 
original number of chromosomes.  When fertilization occurs, the number of 
chromosomes is "back to normal".  
In each parent cell, the chromosomes split up so that one copy of each 
chromosome goes into each daughter cell.  If this splitting up goes wrong, 
then one of the cells can end up with an extra chromosome and the other 
would be missing a chromosome.  This erroneous division is refered to as 
non-dysjunction.  It can occur with any chromosome (not just the sex 
chromosomes), but nearly every case is fatal and will spontaneously abort.  
Human disorders which are the results of non-dysjunction include Down's 
syndrome (trisomy 21), Klinefelters syndrome (XXY), and Turners syndrome (X).
A few others are very, very rarely seen and the children are always severly 
mentally retarded and die very young (usually within days/months).

The calico males have Klinefelters (XXY).  They could be fertile if they are 
actually mosaics (some cells are XXY and some are XY) and the non-dysjunction 
was not present in the developing gonads. 

Kim Truett
Dept. of Human Genetics
Medical College of Virginia