EXAMPLE 4.19

Mendel’s peas. Gregor Mendel used garden peas in some of the experiments that revealed that inheritance operates randomly. The seed color of Mendel’s peas can be either green or yellow. Two parent plants are “crossed” (one pollinates the other) to produce seeds.

Each parent plant carries two genes for seed color, and each of these genes has probability 0.5 of being passed to a seed. The two genes that the seed receives, one from each parent, determine its color. The parents contribute their genes independently of each other.

Suppose that both parents carry the G and the Y genes. The seed will be green if both parents contribute a G gene; otherwise, it will be yellow. If M is the event that the male contributes a G gene and F is the event that the female contributes a G gene, then the probability of a green seed is

In the long run, 1/4 of all seeds produced by crossing these plants will be green.