INHERITANCE 195 



two unequal elements (XY) in the male. The same is true 

 of man and numerous genera of mammals. In the vast 

 majority of cases the male is the heterogametic sex. We have 

 seen, however, that in moths and birds the female is the hetero- 

 gametic sex. In birds the chromosomes are too small and 

 numerous to provide satisfactory material for investigation, 

 though recent work of Hance shows that the female is hetero- 

 gametic. In some moths (Seiler) there is a pair of dissimilar 

 elements in the female which are equally paired in the male. 



The coincidence between the genetic and microscopic 

 data has been illustrated still further by the phenomenon of 

 '' non- disjunction " described by Bridges in connection with 

 several sex-linked nutant characters of which our original 

 instance of white eye-colour will serve as an example. There 

 appeared among the white-eyed m.utant stock of Drosophila 

 certain strains of which the females when crossed to normal 

 red-eyed males gave a certain proportion of red- eyed males 

 and w^hite-eyed females in addition to the usual red-eyed 

 females and white-eyed males alone. When the v/hite-eyed 

 female offspring of such abnormal crossings were mated back 

 to red-eyed males, they in their turn gave all four classes — 

 red-eyed males and females, white-eyed males and females. 

 The white-eyed females behaved like their mothers, giving 

 abnormal results in all cases. Half of the red-eyed females 

 gave normal and half abnormal results in crossing. Of the 

 male progeny the red-eyed individuals were normal, whereas 

 only half the white-eyed individuals were normal, the remainder 

 begetting daughters whose progeny was exceptional. Bridges 

 found that in the F.i abnormal white females the chromosome 

 complex of the dividing cells show^ed a Y element in addition 

 to the XX pair. This is explicable on the understanding 

 that at reduction of the egg in a certain proportion of cases 

 the X elements failed to disjoin, so that the ripe egg contained 

 either two X elements or none at all . If we represent the sperms 

 of a red male as X' or Y, two additional types of individuals 

 will result from fertilisation by a Y or X' sperm respectively : 

 an XXY or white female, and X'O or red male. This accounts 

 for the exceptional individuals in the F.i, and accords with 



