Re: why is the clone not exactly the same as the thing from which it was cloned

Hello Nihal,

There are a couple phenomena at work here.  One is why clones are not
completely identical in general, and the second has to do specifically with
the genetics of calico cats.

The characteristics of an organism, in this case a cat, depend not only on
its genetic makeup, but also the environment in which it developed.  In
other words, the organism is a product of how its genes interact with the
environment.  Now, the term “environment” includes a lot of things in the
life of a cat, beginning in the mother cat.  

In the case of Rainbow the cat and her clone, “cc”, they were probably
carried in the uterus of different “mother” cats.  Even if Rainbow and her
clone were carried in the same mother cat, it was at a different time. 
This means that the mother cat was younger when carrying Rainbow, and
perhaps was eating different foods.  Any difference like this can have an
effect on the developing cat.

Once cc was born, she was no doubt raised in a somewhat different
environment than Rainbow, which probably will not affect appearance, but
could very well affect personality.

Even “identical” human twins are not completely identical, even though they
have the same genes and developed at the same time in the same environment.
 Why?  
Perhaps one had a slightly better blood supply from the placenta, perhaps
their physical positioning within the uterus created ever so slightly
different environments.  Furthermore, an organism like a human or a cat is
a fantastically complicated system of parts, and for the development of two
such complicated systems to be absolutely identical is extremely unlikely,
even with the exquisite instructions of the genome.

Now, in the specific case of the calico cat, there is a special
circumstance that makes it virtually certain that any two clones will look
quite different.  In mammals, the sex of the organism is determined by the
X and Y chromosomes (also known as the “sex chromosomes).  In a nutshell,
the organism can have two X chromosomes, in which case it will be female,
or one X and one Y chromosome, in which case it will be male.  In calico
cats, one of the important genes for coat color is located on the X
chromosome.  This means that females carry two copies of this gene, while
males only carry one.  Now, females only need one X chromosome to be
active, so at some point during their development, each cell turns off one
of the X chromosomes.  

Importantly, each cell does this independently and at random, so half of
the cells will have one copy inactivated, while the other cells have the
other copy inactivated.  As the cells continue to divide as the cat
continues to develop, this leads to patches of cells that all have the same
copy turned off.  The cat then is made of numerous patches that were
determined at random.  These patches correspond to the different color
patches seen in calico cats.  Because the arrangement of the patches is
random, there is almost no chance that any two cats will have the same
arrangement, and so they will have different coat patterns.

There is a more eloquent explanation of X chromosome inactivation here: http://www.thetech.org/genetics/ask.php?id=141

Alex Brands
Lehigh University