Mendel’s experiments didn’t end here. Next, he let
these F1 plants—all yellow-seeded—self-pollinate.
Again, he let the seeds mature, planted them, and recorded the seed color of the resulting
plants. This second generation of offspring is called the F2 generation. Now
what results would you expect? 
Another surprise!
About one-fourth of the offspring had green seeds! How can
yellow-seeded parents produce green-seeded offspring?
These results were again consistent with the results of
his experiments on other traits. In every case he found that approximately one fourth of
the plants in the F2 generation exhibited the other expression of the
trait—the expression that didn’t show up in the F1 generation.
From these results, as well as the results from
experiments on other plant traits over the course of ten years, Mendel came to a
remarkable conclusion—remarkable in its creativity as well as its accuracy. Mendel
concluded that there were two "hereditary factors" for every plant trait
studied.
Furthermore, he postulated that one of the hereditary
factors was dominant over the other one. He termed the trait that was consistently
expressed in the first crossing (the F1 generation) the dominant trait. The trait that was
not expressed in the F1 generation, but that reappeared in the F2
generation he called the recessive
trait.
Mendel further theorized that during the formation of egg
and sperm, these hereditary factors are segregated from one another. The parent plant has
two hereditary factors, but only one of each pair goes to each gamete produced.
Then, during fertilization, two gametes unite. The egg
donates one hereditary factor; the sperm the other, so the resulting zygote, like the
parent plants, contains two hereditary factors. Mendel concluded that a recessive factor
can be carried through a generation unchanged, even when it is not expressed. This is
called Mendel’s principle of segregation, and explains how a trait can be
hidden, but not lost. We’ll use some diagrams to make things clearer.
