INHERITANCE 189 



are also linked, and the percentage of crossing-over between 

 a and c is in linear relation to the percentage crossing- 

 over between a and h on the one hand, and h and c on the 

 other ; similar phenomena appear to hold in the sweet pea, 

 where linkage was first discovered by Bateson and Punnett 

 (1906) ; (2) if the gene a segregates independently of d, then 

 b and c which are linked with a also segregate independently 

 of d. Thus in the fruit fly all the mutant genes can be classified 

 in four groups such that members of a given group show linkage 

 inter se and independent segregation with respect to members 

 of other groups. The genes of Drosophila thus appear to 

 be associated in four pairs of material units. 



Sex-linked Inheritance.— One group of linked characters 

 in Drosophila is of special importance to a consideration of 

 the general applicability of the gene hypothesis, and is equally 

 important because of the Hght which it sheds on the problem 

 of sex- determination. A single instance will suffice to make 

 clear the characteristic feature of this group. In the wild 

 fruit-fly the eye is red ; there is a mutant form with white 

 eyes. A red-eyed female crossed with a white-eyed male 

 yields an F.i composed exclusively of red-eyed individuals ; 

 but in the F.2, which consists of three reds to one white, all the 

 females are red- eyed, and all the whites are male. Now when 

 a pure red-eyed male is crossed with a white-eyed female 

 the result is quite different ; all the females in the F.i as 

 before have the dominant red eye ; but the males are white- 

 eyed. When the F.i are mated inter se^ equal numbers of 

 white-eyed and red-eyed females and males are produced. 

 The inability of the male to transmit red to his offspring of 

 the same sex is readily explained on the asumption that the 

 red gene is linked to something which, if present in the zygote 

 in duplicate, leads to the production of a female, and if present 

 in the zygote unpaired (diagram) leads to the production of 

 a male ; the red-eyed male produces sperm of two kinds, one 

 bearing the " red " gene destined to fertilise an egg which 

 must become a female, and one which cannot bear the red 

 gene and which is destined to lead to the production of another 

 male (Fig, 42). This implies that sex itself is predetermined 



