136 GENETICS IN RELATION TO AGRICULTURE 



On account of these relations the expected Mendelian behavior of 

 this factor in crosses with normal flies is obscured in cultures grown 

 on dry food, but with moist food Mendelian expectations are completely 

 fulfilled. 



Moreover, the variability in the expression of the abnormal condition 

 of the abdomen is not connected with any variability in the factor itself 

 but is merely an expression of a variable reaction of the factor to the 

 environment. Normal flies possessing the factor for abnormal abdomen 

 when given moist food produce offspring just as abnormal as those from 

 abnormal flies. The factor itself is invariable just as in a chemical 

 system the elements which are in the system are invariable but may 

 produce different results according to the dilution, temperature, and other 

 conditions under which the reaction is going on. 



The reduplicated stock in Drosophila shows similar relations to en- 

 vironmental conditions. The characteristic feature of this mutation is 

 the production of extra legs or parts of legs. At normal temperatures 

 very few flies show this condition, but when strains are grown at 10C. 

 a high percentage of them show supernumerary legs. As with ab- 

 normal abdomen and moist food, so Miss Hoge has shown that with 

 temperatures below 10 these flies satisfy Mendelian expectations when 

 crossed with normal strains, but at ordinary temperatures of cultivation 

 the phenomena are entirely obscured. 



Duplicate Factors. A number of cases are known where similar or 

 identical effects are produced by factors located in different loci in the 

 germinal substance. A case in point which has been subjected to excel- 

 lent analysis is that for capsule form in the common shepherd's purse 

 (Bursa). When the form having flattened triangular capsules is crossed 

 with that having top-shaped seed pods, the FI plants produce triangular 

 capsules. When the F z is grown approximately 15 produce triangular 

 capsules to one which produces top-shaped capsules. 



Such a result may be explained by assuming that two recessive factors, 

 c and d, combine to produce the top-shaped capsule. The top-shaped 

 race then is of the genetic constitution ccdd, and the contrasted tri- 

 angular-shaped race is CCDD. The factors C and D are fully dominant 

 and produce identical results, namely plants bearing the typical tri- 

 angular-shaped seed pods. Consequently selfing FI plants of the genetic 

 constitution CcDd gives F 2 , 15 plants with triangular pods to 1 with 

 top-shaped pods. The checkerboard for this case is shown in Fig. 61. 



If this analysis is valid for the inheritance of capsule form the F 3 

 and subsequent generations should display a characteristic type of 

 behavior as shown in the checkerboard. In each square is given the 

 ratio in which the particular genotype should segregate in F 3 . Thus it 

 will be seen that 



