CALVIN B. BRIDGES 



649 



fact that half the females were vermilion although there had been no 

 vermilion in the maternal ancestry. The amount of crossing over be- 

 tween this vermiHon and forked (19.9 per cent) was sHghtly less 

 than the amount expected from ordinary vermilion and forked. But 

 there was no crossing over observed between the vermilion and the 

 lethal; that is, there were no vermilion sons. This set of facts im- 

 mediately suggested that here was another case of "deficiency"^ — 



TABLE III. 



Offspring of Eight Females, Constitution 



(i 



itution \ - 



tb g f 



S s g 



and Vermilion Forked Males. 



Mar. 26, 1916. 



from Culture 3,680, 



TABLE IV. 



Offspring of Two Garnet Daughters, Constitution 



S - 



s g 



Mar. 26, 1916. 



\S th 



3,680, and Vermilion Forked Males. 



I from Culture 



that the process which gave rise to the lethal was not simple mutation 

 but a process characterized by the ' inactivation^' or loss of the genes of 

 a whole section of the X chromosome. The garnet forked father of 

 Culture 3,302, according to this view, had produced a sperm whose 

 X was "deficient" for the vermilion locus and for a section of chromo- 

 some long enough so that one or more genes necessary for the life of 

 the male individual were rendered inoperative. The deficiency of 



