20 SEX-LINKED INHERITANCE IN DROSOPHILA. 



HOW THE FACTORS ARE LOCATED IN THE CHROMOSOMES. 



A character is in the first chromosome if it is transmitted by the 

 grandfather to half of his grandsons, while, in the reciprocal cross, the 

 mother transmits her character to all her sons (criss-cross inheritance) 

 and to half of her granddaughters and to half of her grandsons; in 

 other words, if the factor that differentiates the character has the same 

 distribution as the X chromosome. If, however, a new mutant type 

 does not show this sex-linked inheritance, its chromosome is determined 

 by taking advantage of the fact that in Drosophila there is no crossing- 

 over in the male between factors in the same chromosome. For 

 instance, if a new mutant type is found not to be sex-linked, its group 

 is determined by the following tests : It is crossed to black, whose factor 

 is known to be in the second chromosome, and to pink, whose factor 

 lies in the third chromosome. If the factor of the new form should 

 happen to be in the second chromosome, then, in the cross with black, 

 no double recessive can appear, so that the F2 proportion is 2:1:1:0; 

 but with pink, the mutant type should give the proportion 9:3:3:1, 

 typical of free assortment. 



If, however, the factor of the new form is in the third chromosome, 

 then, when crossed to black, the double recessive and the 9:3:3:1 

 proportion appear in Fo. But wMien crossed to pink no double recessive 

 appears in F;., and the proportion 2:1:1:0 occurs. 



If these tests show that the new mutant does not belong to either the 

 second or third chromosome, that is, it both with black and with pink 

 the 9:3:3:1 ratio is obtained, then by exclusion the factor lies in the 

 fourth chromosome, in which as yet only two factors have been found. 



We propose to give in a series of papers an account of the mutant 

 races of Drosophila and the linkage shown in their inheritance. In this 

 paper we shall consider only the members of the first chromosome, 

 descnbmg a large number of new mutants with their linkage relations 

 and summarizing to date all the linkage data relating to the first 

 chromosome. In later papers we propose to consider the members 

 of the second, third, and fourth chromosomes. 



The list at the top of page 21 gives the names of the factors dealt 

 with 111 this paper. They stand in the order of their discovery, the 

 mutant forms reported here for the first time being starred. 



In each experiment the percentage of crossing-over is found by 

 dividing the number of the cross-overs by the sum of the non-cross- 

 overs and the cross-overs, and multiplying this quotient by 100. The 

 resultmg percentages, or cross-over values, are used as measures of the 

 distances between loci. Fhus if the experiments give a cross-over 

 value of 5 per cent for white and bifid, we say that white and bifid lie 5 

 units apart in the X chromosome. Other experiments show that yellow 

 and white are about i unit apart, and that yellow and bifid are about 

 6 units apart. We can therefore construct a diagram with yellow as 



