266 



Radiation, Evolution, and the Position Effect 



one on the other {Hw / + ) have about seventeen extra hairs 

 whereas hairy wing males {Hw / Y) have between thirteen and 

 fourteen (Table 9). In other words, two bands next to one 

 another on one X chromosome produce extra hairs whereas two 



TABLE 9 



Average Number of Extra Hairs on Flies Carrying Various Com- 

 binations OF Bands Involved in the Hairy-wing Locus 



(Adapted from Demerec and Hoover in Genetics.) 



+, a normal wild-type X chromosome with one band. Y, a normal Y 

 chromosome with no bands. Hw, a mutant X chromosome with two bands. 

 D, a small translocated piece of the X chromosome including one band at- 

 tached to the centromere of Chromosome IV. 



similar bands on separate X chromosomes produce no extra hairs. 

 There is an interesting quantitative relationship here, also. A 

 homozygous Hw / Hw female (which has two bands in each 

 chromosome) has more extra hairs than the heterozygote Hw / + 

 (which has a total of three bands), and both have more extra 

 hairs than a Hw male, w^hich has only two bands. The quanti- 

 tative effect can be further enhanced by the presence of an extra 

 segment of the X chromosome. In one case, a piece of the left 

 end of the X chromosome, which included only one of these 



