436 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



bleeding (haemophilia), which occur chiefly in males, but are never 



transmitted by males to their sons but only through their daughters to 



their grandsons. 



Morgan and his pupils have described between forty and fifty 



characters in DrosoDhila which are sex-linked in heredity; they also 

 have discovered a large number of other 

 Mendelizing characters in Drosophila which 

 are not sex-linked but which nevertheless are 

 inherited in groups, characters in the same 



FIG. 89.- Drawing group showing coupling when introduced in a 

 showing the four pairs 



of chromosomes seen in cross from the same parent, and repulsion 



the dividing egg of Droso- when introduced from different parents. The 



phila- (AfterDr.C.E.V. num ber of these groups exactly corresponds 



with the number of the chromosomes and 



Morgan believes that their genes are located in the chromosomes, an 

 hypothesis which seems reasonable but which would be severely strained 

 if an additional group of characters should be discovered. There are 

 three groups of the non-sex-linked characters. (See Fig. 90.) In one of 

 these referred to as Group II (the sex-linked group being called Group 

 I), are found variations known as black body and vestigial wings respec- 

 tively, together with some thirty-five other variations. In Group III 

 are found the variations known as pink eye, spread wings, and ebony 

 body, together with some twenty other variations. In Group IV are 

 included as yet only two characters, bent wings and eyeless, which how- 

 ever show linkage with each other. No inherited characters have been 

 discovered in Drosophila which are not inherited in one or another of 

 the four linkage groups. 



TABLE II 

 RECIPROCAL CROSSES OF WHITE-EYED AND YELLOW-BODIED FLIES 



Male Female Male Female 



P Yellow-red X Gray-white Gray-white X Yellow-red 



Fj Gray-white Gray-red Yellow-red Gray-red 



P / i Gray-white: i Gray-red: i Gray- white: i Gray-red: 



1 \ i Yellow-red i Gray-white i Yellow-red i Yellow-red 



DROSOPHILA TYPE AND POULTRY TYPE OF SEX-LINKED INHERITANCE 



i. Drosophila type. The same type of sex-linked inheritance 

 which is found in Drosophila is found also in man, in cats (inheritance 

 of yellow color), and in the plants, Lychnis and Bryonia. The essen- 

 tial feature of the "Drosophila type" of inheritance is this. In a race 



