THE GENETICS OF HABROBRACON JUGLANDIS ASHMEAD 



combination than they do separately, the genes 

 are said to be linked; and therefore, borne on 

 the same chromosome. At the present time four 

 linkage groups (Fig. 1^) are recognizable. 

 These groups may occupy four different chromo- 

 somes or they may be found to be linked and 

 thus occupy fev/er than four chromosomes. In 

 1933 fifty genes v;ere known, and they comprised 

 eight linkas^e groups. It vvas thought then that 

 each group represented a different chromosome. 

 However, since that time, nev; genes have come 

 to light that have linked the second and fourth 

 groups, others have linked the fourth and fifth, 

 others have linked the fifth and eighth, and 

 still others have linked the first and third. 

 It is apparent, therefore, that while the num- 

 ber of known genes has increased, the number of 

 linkage groups' has decreased. 



There is one unusually long sex chromosome 

 known to be at least five hundred units in 

 length. Starting at the left end of this sex 

 chromosome, designated linkage group one (I), 

 the genes are arranged in the order: speckled, 

 reduced, glass, etc. Helsel (1943, 1944) con- 

 ducted experiments in order to check recombina- 

 tion and interference in this region, and to 

 compensate for any viability disturbance or po- 

 sition effects introduced the genes in all pos- 

 sible combinations. She reported 12.07 per 

 cent crossing over betv/een speckled and re- 

 duced, and 12.69 per cent crossing over between 

 reduced and glass. Coincidence is 0.264, but 

 varies markedly as the genes are introduced in 

 different combinations. Linkage of these genes 

 had been shown (P. W. V/hiting, and Benkert, 

 1934) with about 13 per cent crossing over be- 



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