GENES IN THE MALE OF DROSOPHILA. 1 99 



appear in the Fo generation. The explanation of this is clear on 

 an examination of the first analysis. Since there is no crossing 

 over in the male, each gamete in the male will contain one or the 

 other of the dominant factors, hence no double recessive zygote 

 can appear, and every zygote must contain at least one dominant. 

 This means that there is one class with two dominants, one with 

 the one, one with the other. If we assume that the factors are 

 carried by specific chromosomes, and that the factors for black 

 and for vestigial (and hence for their normal allelomorphs) lie 

 in the homologous chromosomes, we can explain the results just 



i 



given on the basis that no interchange of factors occurs between 

 these chromosomes in the male even though in the female inter- 

 change takes place. Why an interchange should take place 

 in one sex and not in the other cannot be stated, but I should 

 not be surprised if a thorough cytological study would throw 

 some light on the subject. 



The converse case is that of two recessives entering from the 

 same side, in which case all the expected classes appear in F^. 

 An example with its analysis will make this clearer. Gray, long- 

 winged females (double dominants) were mated to black vestigial 

 males (double recessives) and gave gray long winged FI offspring. 

 These FI flies, mated, gave in F: 



Gray Long. Black Long. Gray Vestigial. Black Vestigial. 



246 9 1 8 63 



The following analysis shows why the result to be expected 

 here is different from that in the last case. Half of the gametes 



Gray long 9, BV u BV g . 



Black vestigial cf , bv g bv g . 



F! cf and 9, BV g bv g . 



Gametes of F! b V g B V g bv g Bv g Eggs 



B V g bv (J Sperm 



bV g BV g , gray long, bV g bv g , black long, 



BV g BVg, gray long, BV g bv g , gray long, 



bv g BVg, gray long, bv g bv g , black vestigial, 



Bvg B V g , gray long, Bv g bv g , gray vestigial. 



