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
|
