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Essential
Genetics for the Terrified
by Chris Rutt
Part 8: Sex
In the previous section I
introduced you to the idea of chromosomes, which are the microscopic
transporters of genes into new cells. We saw that in the making of sex
cells (sperm and eggs) only half of the chromosomes go into the gamete
(shorthand for a sex cell) and when the gametes combine at
fertilization a complete set of chromosomes is formed, which therefore
contains half the master plan of each parent. We also saw that the sex
of the new individual is fixed by two particular chromosomes and that
two "X" chromosomes results in a cock, and an "X" & a "Y" in a hen.
In determining the probable sexes of chicks, we could, if life were
simple, think of these as a pair of genes for sex with "Y" being
dominant to "X", resulting in a female. Two "Y"s are of course
impossible, since the male can NEVER provide one of these chromosomes.
As practice, do a grid for this combination.
| Parents
Contribution |
X |
X |
| X |
XX |
XX |
| Y |
XY |
XY |
Not very informative you
say? It only shows that we expect half cocks and half hens, which is
what commonsense and experience makes us believe, without having to go
to the trouble of drawing grids. However, it serves other purposes also.
First, we have not so far considered many pairings in which the two
partners were not identical genetically, and this grid shows how the
method can be applied to such pairings in the same way. For revision,
try doing a similar grid for a pairing of a Green split for Marine
mated to a Marine. Then do one for a Jade paired to a Light Green. The
second aspect of this grid's usefulness comes when we remember that "X"
& "Y" are NOT genes, but chromosomes containing many thousands of
genes, SOME OF WHICH ARE GENES FOR COLOUR MUTATIONS.
Part
9: Sex & Linkage (1)
All rights reserved
© 1997-2000 Chris Rutt
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