THE FACTORS WHICH DETERMINE SEX 675 



been shifted out of the female class by the presence of an extra 

 set of autosomes, and thus the autosomes are proved to play a 

 positive role in sex-production. 



Again, Blakeslee 1 finds in the jimson weed, Datura, many 

 mutations with a 2n, 3n, or even 4?i number of chromosomes. As 

 long, however, as they possess a balanced chromosome number, these 

 mutants are all remarkably stable and healthy, while all those 

 containing an unbalanced chromosome complex, such as a 2?i + l 

 condition, are very unstable, and frequently undersized and often 

 unhealthy in appearance. Thus while the 2n plant will be large 

 and vigorous in appearance, the 2?i + l individual will be half its size 

 and very unhealthy. The presence then of an extra chromosome 

 is capable of affecting the action of all the others. He comes to 

 the conclusion that a species on these grounds must be the result 

 of a whole series of more or less conflicting forces contained in the 

 individual chromosomes acting as a whole, and not the result of 

 any individual chromosome or number of chromosomes acting alone. 

 This conclusion accords with Bridges' work just mentioned, and 

 gives us a totally new conception of the action of the chromosome 

 in heredity. 



The experimental study of the transmission of sex-linked 

 characters is in general accordance with the cytological evidence 

 as to sex-inheritance, an outline of which has been given above 

 Sex-linked characters are those which while ordinarily restricted 

 to one sex, may occasionally pass over to the other. Their 

 inheritance has been studied very fully by Morgan, in the fruit- 

 fly, Drosophila ampelophila, in which as many as fifty sex-linked 

 characters have been identified. In transmission they are supposed 

 to be located in the X chromosome. 



Drosophila usually has red eyes, but Morgan found some males 

 with white eyes. When these latter were mated with red-eyed 

 females, the offspring had red eyes. The heterozygous red-eyed flies 

 when mated together gave rise to 2000 red-eyed females, 1000 red- 

 eyed males, and 800 white-eyed males. White-eyed males with 

 heterozygous red-eyed females gave rise to both red -eyed and 

 white-eyed males and females, the males and females being nearly 

 equal in number ; but the white-eyed individuals, as in the previous 

 mating, owing to less constitutional vigour, were fewer than the 

 red-eyed. White-eyed males and females gave rise exclusively to 

 white-eyed offspring, but red-eyed males of whatever origin when 

 mated with white-eyed females invariably gave rise only to red-eyed 

 females and white-eyed males. 



1 Blakeslee, " Variation in Datura due to Changes in Chromosome Number," 

 Amer. Nat., vol. Ivi., 1922. 



