Hey Tib,
I looked up a definition and example of eucaryotic gene expression in Calico cats for you.
here goes. Reference link: http://www.ajpe.org/legacy/pdfs/aj5802204.pdf
Here are some details. It is not that hard to follow...I hope...just use a little imagination:
THE CASE OF THE CALICO CAT
This is a cat with the coat color commonly referred to as
calico. (A colored overhead, slide, or best of all, a living,
purring cat in the classroom, is shown at this point.) Those
of you who have had such a cat know one crucial fact about
them; they must be spayed, or you will end up with lots of
pretty calico kittens. In other words, all calico cats are
female. The coat color is a result of the sex of the cat, and not
the other way around.
Recall that there are two X-chromosomes in each cell in
this cat, one maternal and one paternal, except for the germ
cells, the eggs, which are haploid. On each X-chromosome,
there is a gene that can code for either yellow or black coat
color. During the development of the cat embryo, at the
stage of 64 cells, one of the two X-chromosomes in each cell
is inactivated, leaving only one X in working order. This Xsilencing
is random within the embryo, with the choice of
which X is inactivated in a particular cell unaffected by the
choice of that cell’s neighbors. From that point on in the
development of the cat embryo, all the daughter cells of each
of the original 64 will continue to have that same X-chromosome
inactivated, and so the same active gene for coat color,
either black or yellow, depending on that initial inactivation.
In general, these “sister” cells tend to remain close to each
other during later stages of embryonic development. The
graphic result of such a patchwork of clones of cells, expressing
either black or yellow coat color, is this strikingly marked
animal.
Only female cats have two X-chromosomes; only female
cats can demonstrate the calico coloring. The white
coat color often seen in calico cats in addition to the yellow
and black patches is a result of an autosomal influence on the
final coat markings
Night! ;)
