Chapter VIII 

 LINKAGE 



So far in Habrobracon juglandis there have 

 occurred about one hundred known mutations each 

 involving a different gene. Since the wasp has 

 only ten pairs of chromosomes, it is apparent 

 that each chromosome must bear a number of dif- 

 ferent genes. The position of each gene and 

 its allele or alleles is definite; that is, 

 each pair of alleles always occupies the same 

 locus on the same pair of chromosomes in dip- 

 loid v/asps. Of course, in haploid males, only 

 a single set of chromosomes is present. Crosses 

 have been made involving these mutant types in 

 various ways to test the principles of heredi- 

 ty. Linkage tests are made by crossing various 

 mutant types and counting the F2 haploid sons 

 of the unmated Fl heterozygous females. The 

 azygotic ratios thus obtained approximate gam- 

 etic ratios except as they may be affected by 

 differential viability of the various haploid 

 genotypes. Linkage tests thus made show that 

 maps are very long in crossover units. On this 

 account, despite much work extending over sev- 

 eral years, there is as yet no approximation of 

 linkage groups to the number ten, corresponding 

 to the haploid set of chromosomes. 



If certain gene mutations, such as speckled, 

 reduced, and glass, occur more frequently in 



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