232 



T. H. MORGAN. 



While no record of the number of flies that were examined was 

 kept, I think more than 30,000 gray and black flies must have 

 been looked over. In no case was the spot present. 



The alternatives then are absolute (or possibly nearly absolute) 

 linkage, or a case of multiple allelomorphism. In practice these 

 alternatives would give the same numerical results. The theoret- 

 ical differences between the idea of absolute linkage and multiple 

 allelomorphism will be discussed later. 



When a spot male was mated to wild females, all the offspring 

 were normal gray without spot. When the FI offspring were 

 inbred the following classes were produced. 



There is one class of females and two classes of males. This is 

 the expectation if spot is due to a sex-linked factor. It is the 

 same kind of result that obtains when a yellow male is crossed to 

 a wild female. When an FI female from the last experiment was 

 back crossed to a spot male the following results were obtained : 



Equality in all four classes is expected and this is practically 

 realized. The spot type seems somewhat less viable than the 

 wild type. The male counts run behind the female, which is a 

 usual result. Here the spot also occurs in the female (Figs. 7 

 and 8). 



A spot female was mated to a yellow male. The 63 daughters 

 were yellow without spot which shows that spot is completely 

 recessive to yellow. The 58 sons were spot, receiving their sex 

 chromosome from the mother. When a pair of these FI flies 

 were bred, the following F 2 classes were produced. 



