So, 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.

The 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).

Is 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 Human Genome Project).