THE GENETICS OF HABROBRACON JUGLANDIS ASHMEAD 



for X and fused but only two phenotypically 

 separable ratios. 



Fraternities of the close-cross type may now 

 be called two-allele fraternities, those of the 

 outcross type three or four-allele fraternities. 

 A third allele brought into the cross by a fused 

 male, xc.fu for example, always masks sex-link- 

 age, since all diploid offspring are hetero- 

 zygous for X and hence female, fused and non- 

 fused in approximately equal numbers (Fig. 16E) . 

 Diploid males reappear with inbreeding, and 

 their presence is always associated with obvious 

 sex-linkage of fused as shown by Bostian ( 1939) . 



Sex determination, is in a sense, polygenic, 

 but because of no crossing-over within the seg- 

 ment, the various groups of genes act as a sin- 

 gle series of allelic factors . Just as the many 

 sex-producing genes distributed among the 

 X-chromosome of Drosophila segregate as a unit 

 from the Y without crossing-over, so the domi- 

 nant female-producing genes and the recessive 

 male-producing genes of the x factor in Habro- 

 bracon segregate without crossing-over from 

 their homologues. Males, as determined by the 

 various alleles (whether azygous or homozygous) , 

 or females as determined by any of the hetero- 

 zygous combinations, are always phenotypically 

 similar (P. W. Whiting, 1945b). 



Whether the sex alleles ever mutate has not 

 been established with certainty. They appear 

 to be very stable. Extensive breeding within a 

 two-allele stock has resulted in two-allele 

 fraternities almost exclusively, as evidenced 

 by the presence of diploid males. With certain 

 very rare exceptions, the fraternities lacking 

 diploid males are small, so that the lack is 



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