LINKAGE 



In crosses involving three sex-alleles, sex- 

 linkage cannot be determined because all the 

 zygous offspring are heterozygous for x and 

 are, therefore, female; xa/xb X xc gives xa/xc, 

 xb/xc. Fecundity is higher here, and the ratio 

 of females to haploid males is doubled because 

 the poorly viable diploid males are replaced by 

 females. In this case, if the cross is made 

 between a female heterozygous for fused and a 

 fused male, fused and non-fused females are 

 produced in equal numbers. 



Three kinds of fraternities from heterozy- 

 gous females by fused males are therefore dis- 

 tinguishable as follows: 



Diploid Haploid 

 Females Males Males 



Non-fused Non-fused Non-fused 

 fused fused fused 



(1) two-allele 9/1 1/9 1/1 



(2) two-allele 1/9 9/1 1/1 



(3) Three-allele 1/1 1/1 



Linkage tests involving two or more mutant 

 types fall into two general categories, (1} re- 

 pulsion tests, (a X b), and (2) coupling tests, 

 (a.b X +). The mutant factors may be intro- 

 duced from different parents (repulsion tests) 

 or the double miutant type may be crossed with 

 wild-type (coupling tests). It is advisable to 

 make these reverse crosses to check deviations 

 that result from viability differences, but 

 thus far most tests have been of the repulsion 

 type as this is the more convenient way to test 



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