704 HEREDITY AND CELL DIVISION 



abnormalities.) Fig. 544 shows the inheritance of eye-colour 

 in Drosophila. Here red-eye (#) is dominant to white-eye (O). 

 The cross between a wild-type homozygous red-eyed female and 

 a white-eyed male will therefore give equal numbers of males 

 and females, all red-eyed. But the females are heterozygous ; 

 bred with white-eyed males (i.e. back-crossed to the male parent) 

 they produce all four possible types in equal numbers. Bred with 

 a red-eyed male, the heterozygous females will give red-eyed and 

 white-eyed males in equal numbers, but all the female offspring 

 will be red-eyed, for although half of them will bear the white- 

 eye gene from their mother, its effect will be overcome by the 

 dominant red-eye gene which they must inherit from their father. 

 Where a recessive sex-linked gene is rare, therefore, its effect 

 will hardly ever be seen in the homogametic sex, for the character 

 can only appear in this as the result of mating between two indi- 

 viduals carrying it, and the chances of such a union are small. 

 This is the case for colour-blindness in man, which is inherited 

 like white-eye colour in Drosophila, and is observed in only six 

 females for every thousand males. 



Sometimes the Y chromosome carries a gene which has no 

 counterpart on the X ; its effect will be apparent in the hetero- 

 gametic sex only. An example is the normal allelomorph of barred 

 feathers in chicks. The two X chromosomes of the male (the 

 homogametic sex) cause a pattern which is visible on hatching ; 

 the female, which is necessarily heterozygous, has a different 

 appearance, and so the chicks can be sexed without examination 

 of their genitalia. In all these types of sex-linkage there can be 

 no crossing-over between X and Y chromosomes, and this is 

 confirmed cytologically. In some animals, e.g. gnats, parts of 

 the chromosomes do cross over, and genes situated here do not 

 show complete sex-linkage. In most animals the Y is relatively 

 empty, and contains very few genes. 



CYTOPLASMIC INHERITANCE 



For a long time it was generally assumed that all biological 

 inheritance was Mendelian, and that the chromosomal genes were 

 the sole determinants of development. There are strong theoretical 

 objections to this view — it would seem to require that there should 

 be no difference between the cytoplasms of different species of 

 animals, which is absurd — and there is now increasing evidence 



