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Essential
Genetics for the Terrified
by Chris Rutt
Part 7:
Chromosomes
Up to now we have taken it
on trust that for the purposes of calculating expected colours, parents
pass just one gene from each pair to their young. How and when does
this happen?
We have after all been talking about genes "going round" in pairs, but
only one half of each pair going to the offspring. This has been a
simplification, which has enabled me to explain some forms of
inheritance so that readers can put their new knowledge to some
practical use. Genes which are in "normal working mode" in fact go
around in long strings, which if we could magnify them enough would
look like a ladder containing MILLIONS of rungs, which has been twisted
through its whole length. These are made of a very complex chemical, of
which most of us will have heard, namely DNA. These ladders contain the
complete plans for the building and working of the whole organism.
However, from time to time these cells, all of which which contain DNA,
need to divide to make new "units". Each of these needs its copy of the
masterplan.
At these times of division, theDNA clumps together, and these clumps
are called CHROMOSOMES. In the case of bodily growth and repair then
the masterplan must be both complete, and an identical copy. However,
when sperm and eggs are being produced, those "cells" MUST have only
HALF the genes necessary for a complete individual so that they can
come together in the fertilised egg to form a complete "blueprint" (as
opposed to two complete, but different, sets of plans, which would be
useless). We can therefore think of it that when cells divide TO GROW,
each chromosome splits down the long axis, each half going to a
different new cell where it reconstructs its other half. This results
in each of the new cells having a complete set of chromosomes, each set
of which is identical to that in the original cell.
In contrast, when the cell is dividing in order to produce sperm or
eggs, we can see this as half of the original chromosomes going into
each of the new cells. So, when sperm and egg come together at
fertilisation, a complete set of chromosomes exists from which the new
organism can grow. The number of chromosomes per cell, and their shape,
are the same within any particular species. Each chromosome can be
considered as shaped like two linked sausages, but two are very
different and these are the controllers of the sex of the individual.
Most books name them the "X" & "Y" chromosomes, though in birds
they are more correctly described as "W" & "Z". PLEASE READ THE
FOLLOWING STATEMENT CAREFULLY AND REMEMBER IT. IN BIRDS, MALES HAVE
TWO "X" CHROMOSOMES. FEMALES HAVE ONE "X" AND ONE "Y". I must
emphasise that the statement is only true of BIRDS. In mammals the
reverse is the case, which fact can make some books on genetics
confusing to the beginner. (Or to the dog breeder who takes up birds in
later life!)
Part
8: Sex
All rights reserved
© 1997-2000 Chris Rutt
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