THE GENETICS OF HABROBRACON JUGLANDIS ASHMEAD 



There are ten genes remaining to be located 

 in the ten possible linkage groups, Table II. 

 At present, breeding tests are being carried on 

 in order to test the presence or absence of 

 linkage among these unlocated genes. New genes 

 are still coming to light. These may prove to 

 be located on chromosomes as yet unmarked, or 

 it may be that they will be found to be linked 

 with known groups. The number of genes that 

 may eventually show up through visible effects 

 is incalculable. Table II shows which genes 

 have been so far tested. 



In linkage tests, the x-factor acts as a 

 single gene, occupying a point on the linkage 

 map. The method of making a linkage test with 

 ■the sex-factor may be illustrated by a single 

 example (P. W. Whiting, 1945b). The mutant gene, 

 fused, lies about ten units to the right of x. 

 An orange-eyed female heterozygous for fused, 

 o.xa.+/o.xb.fu, is crossed with a black-eyed 

 fused male, +.xb.fu. The orange-eyed sons are 

 haploid from unfertilized eggs and are fused 

 and non-fused in equal numbers. Sex-linkage is 

 not determinable among these because xa males 

 and xb males are similar. The black-eyed zy- 

 gous diploid or biparental offspring o/+, are 

 either fused or non-fused, and either males or 

 females. From the ratio of the different com- 

 binations the percentage of crossovers may be 

 determined. In the example given, the non- 

 crossover offspring will be xa.+/xb.fu or non- 

 fused females, and xb.fu/xb.fu or fused males; 

 the crossovers, averaging in this case 10 per 

 cent, will be xb.+/xb.fu or non-fused males, and 

 xa.fu/ab.fu or fused females. This is called a 

 tv;o-allele cross. 



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