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 10°C. 
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 inthe 
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 F; plants produce triangular 
capsules. When the F2 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 F; plants of the genetic 
constitution CcDd gives Ff, 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 F3 
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 F3. Thus it 
will be seen that 
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