Natural Science Forum / Biology / Biology / April 2004

I was trying to avoid using the word "alleles", not knowing just how
elementary or advanced your study is so far.   The idea is that each
gene is a specific location (gene locus) on a chromosome that codes
for a particular protein.  (That is a drastically out-of-date and
oversimplified notion of a gene, but that is a different story).
However the genes can differ in form, giving different alleles for
each gene.  So we all have genes for eye color, but some have the
alleles that produce a lot of pigment and have dark eyes while some
have alleles that produce much less pigment and have light colored
eyes.  The difference, of course, is in the details of the genetic
code and therefore in the sequence of amino acid in the protein.
Still, the overall structure of the gene is so similar and the number
of agreements in the genetic code so overwhelmingly larger than the
number of changes, that the code for one individual is a good start to
understanding things.  For eye color, the protein is one that is
involved in producing the dark pigment.  If you have a mutation in
that gene, the defective protein can synthesize the pigment so your
eyes don't have any -- they look blue.  (Again, an oversimplification,
but it illustrates the point).

As someone else has pointed out, the variations between humans are
really quite trivial compared with the enormous similarities.  For
example, I am now looking at the set of wood chairs around my dining
room table. They look identical.  However a close inspection shows
that the pattern of wood grain differs from chair to chair.  Each
chair is, in a sense, unique.  So which is it?  Are the chairs really
the same or are they really different?  Think of the variations
between individuals like the variation in the grain.  The initial goal
of the genome project is to get an idea of what shape and size the
chairs are, to see how the structure of the seat and the back and the
arms and the legs serves the function of being a chair.  Meanwhile,
people are starting to look at the fine details, the grain structure.
The details are of great medical importance, just as the grain pattern
might influence the strength of that section of wood.

The terminology is confusing.  Everyone has the same "eye color
genes".  However, different people have different alleles for eye
color, different forms of those genes.

Your second question is about every cell having all the genes to make
every kind of cell.  You are again correct:  in each cell only a
fraction of the genes are expressed.  Each cell type expresses a
different set of genes.  The mechanism to switch genes on and off is
really the key to understanding how the genome functions. One way of
turning genes on and off is by binding proteins to them.  Another way
is to modify them chemically (methylation). It is one thing just to
make a list of all the proteins that a cell can possibly make.  It is
a very different thing to make a list of which proteins any specific
cell makes at any particular point in the life of the organism.  That
is the move from genomics to proteonics.  We are really just beginning
to understand that story. The next ten to twenty years should be very
exciting on that perspective!

It really would be a mess to try to isolate just which genes any cell
would need.  Besides, many genes are not needed all the time but might
be very important sometime later for that cell.  So it is simply a lot
simpler and easier for every cell to have all the genes, but just shut
off the unused ones.