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
|
