A. SEX C H R O M O SO M E S1 The mating o f a male and a female organism produces males and

females in approximately equal numbers; in this equality o f number it is analogous to the cross between a heterozygote and homozygous recessive. Correns2 first showed that the analogy is more than super­ ficial. In Bryonia the male is the heterozygous form and produces two sorts o f pollen, a male-producing and a female-producing, while the eggs are all o f one kind. Similar sex-determining mechanisms have since been found in most organisms. The action o f the sex-determining factors is discussed in Chap. io.