140 



PHYSICAL BASIS OF HEREDITY 



and curved wings were crossed to wild-type females. The 

 Fi female produced in tMs way would be heterozy- 

 gous for the three mutant factors involved in the cross. 

 Such an F-^ female was then bred to a male pure for the 

 three recessive genes, black, purple, curved; and her 

 offspring were kept at a given temperature until they 

 emerged as flies, and then if necessary for some days 

 longer in order that as many eggs as possible might have 

 matured under the specified temperature. Controls of 

 sisters and brothers were made in each case and kept at 

 average "normal" temperature. In the table that fol- 

 lows crossing over between black and purple is indicated 

 as "1st crossover," and between purple and curved as 

 ' ' 2nd crossover, ' ' and between both as double crossover. 

 Ten different temperatures were tested. At 5° C. the 

 eggs did not hatch, and at 35° C. the females were sterile. 

 In the seven intermediate temperatures the results were 

 those recorded in the next table. 



■pr- 



At the two lower temperatures the crossover value is 

 high, i.e., little crossing over occurs. At the next three 

 temperatures (17.5°, 22°, 29° C.) the crossing over value 

 is much less, while at the last two temperatures 29° and 



