CHAPTER 3 



The Behaviour of Individual Chromosomes 



A. SEX CHROMOSOMES^ 



The mating of a male and a female organism produces males and 

 females in approximately equal numbers ; in this equaHty of number 

 it is analogous to the cross between a heterozygote and homozygous 

 recessive. Correns- first showed that the analogy is more than super- 

 ficial. In Bryonia the male is the heterozygous form and produces two 

 sorts of pollen, a male-producing and a female-producing, while the 

 eggs are all of one kind. Similar sex-determining mechanisms have 

 since been found in most organisms. The action of the sex-determining 

 factors is discussed in Chap. lo. 



In 1902 McClung first discovered chromosomes which formed 

 an xmequal pair in one sex, while corresponding to them in the other 

 sex there was an equal pair. These chromosomes therefore behave like 

 the postulated sex factors and were called the sex chromosomes, the 

 remaining chromosomes being called the autosomes. They have since 

 been found in very many animals and plants. The sex with the unequal 

 pair may be either the male (more usually) or the female (Lepidoptera, 

 birds, some fishes, Fragaria as the only known plant); it is known as 

 the heterogametic sex, and is the sex which is heterozygous. 



I. Types of Sex Chromosomes 



The pair of equal chromosomes are known as the XX pair, the 

 unequal pair as the XY; in cases where the female is the hetero- 

 gametic sex the nomenclature ZZ S ^^ ? is sometimes used, but is 

 probably unnecessary. The sex controlling mechanism depends ori- 

 ginally on the difference between the X and Y chromosomes, but in 

 the course of evolution the Y often loses its importance and the sex 

 determination comes to depend solely on a balance between the X and 

 the autosomes. 



The Y chromosome may be bigger than the X, but is more usually 

 smaller. A complete series of types can be found, ending with types in 

 which the Y is totally absent and the sex determination depends on an 



^ General references: Darlington 1937, chap. 9, Schrader 1928, Wilson 1928. 

 ^ Correns 1907. 



