Moving up from genes to inheritance of bodily traits?
you ever hear the saying that nothing is so obvious as something that
has been pointed out? Well, this certainly applies to the discoveries
on inheritance by Gregor Mendel (see Story Time). Many inherited traits pass from parents
to offspring in a very clear and simple way. However, it is only simple
with hindsight. It was not so simple for Mendel, because he had no
prior information about how things worked.
how did Mendel figure it out? He kept careful records of what happened
in the offspring when he bred one kind of pea plant with another kind of pea
plant. But then he thought hard about
what the data meant.
only way his data made sense was to assume that inherited traits
can be coded for by
one of two variations. In peas, Mendel found seven traits, such as
a smooth or rough surface, that were inherited this way. Today,
we know that these variants are the genes, which are carried on
structures in the cell nucleus called chromosomes. If the image of a cell
nucleus is magnified greatly, most chromosomes can be seen to have
the shape of the letter X (as on the right). Sometimes, though,
they look like the letter Y (as in human males).
there a rule for chromosome number?
The general rule is that the more chromosomes a species has, the
more developed it is. That should not be surprising. More complex
life forms need more genes and more different kinds of proteins.
With more genes, there is a need for more chromosomes to contain
those genes. Fruit flies, for example, only have four chromosomes
while humans have 46. Chromosomes work in pairs. Therefore, each
human cell has 23 pairs of chromosomes. In each pair, one comes
from the male parent and one from the female. Each chromosome contains
hundreds, sometimes thousands, of smaller pieces of information
called genes. The smallest human chromosome, identified as number
21, has been completely described in terms of its genes (see the
section below on genes and also