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 Drosophila which are sex-linked in heredity; they also 
have discovered a large number of other 
Op Mendelizing characters in Drosophila which 
aX are not sex-linked but which nevertheless are 
inherited in groups, characters in the same 
Fic. 89.— Drawing  s;oup showing coupling when introduced in a 
showing the four pairs 3 
of chromosomes seen in cross from the same parent, and repulsion 
the dividing egg of Droso- when introduced from different parents. The 
ee (After Dr.C.E.V. number 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. go.) In one of 
these referred to as Group II (the sex-linked group being called Group 
J), 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 
REcrPROCAL CROSSES OF WHITE-EYED AND YELLOW-BODIED FLIES 
Male Female Male Female 
P  Yellow-red X Gray-white Gray-white X Yellow-red 
F, Gray-white Gray-red Yellow-red Gray-red 
F,! 1 Gray-white: 1 Gray-red: 1 Gray-white: 1 Gray-red: 
|. 1 Yellow-red 1 Gray-white 1 Yellow-red 1 Yellow-red 
DROSOPHILA TYPE AND POULTRY TYPE OF SEX-LINKED INHERITANCE 
1. 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. Ina race 
