INHERITANCE 735 



zygote, two have the eyespot (S), two are without it (s). The cross may 

 be represented as follows: 



S X s = S + s 



It is obvious that the segregation of the two characters must have oc- 

 curred at the reduction division. One of the sets of eight chromosomes 

 includes the conditions for producing S, the other the conditions for 

 producing s. 



The results of the crosses in which the parents differ in two pairs of 

 characters may be exemplified in the mating of P. uvella without an 

 eyespot (FpsD) with the large variety of P. /^-^Jf/^m (fPsD) (Fig. 170 A, 

 B) . The two differ in form (F and f ) , and in presence or absence of the 

 papilla (P and p) . The zygote thus carries both these sets of factors; it is 



FpsD 

 fPsD 



The zygote divides into four swarm cells, with reduction. The results 

 are as follows: 



1. From any single zygote two and only two types or combinations are 

 produced. Two of the four cells are of one combination, two of the 

 other. 



2. Different pairs of types are produced in different cases (Fig. 170). 

 About half the zygotes yield again the two parental types, FpsD and 

 fPsD (Fig. 170 C, D). The other half yield FPsD (ellipsoidal wdth 

 papilla. Fig. 170 E) and fpsD (oval without papilla, Fig. 170 F). 



Thus the characters, form, and papilla are inherited independently. 

 The factors F and f are in the two members of one pair of chromosomes, 

 the factors P and p in another pair of chromosomes. 



When crosses are made in which the individuals differ in three pairs 

 of characters (as FpsD X fPsd) or in all the four pairs of characters 

 (FpSD X fPsd), all the four pairs are found to be independent in their 

 distribution. From any single zygote only two types of offspring are 

 produced. But from different zygotes of the three-pair cross, eight dif- 

 ferent combinations are produced; from those of the four-pair cross, 

 sixteen different combinations. The different combinations occur in ap- 

 proximately equal numbers. Thus the factors for the four pairs of char- 

 acters are distributed in four chromosome pairs, which at reduction are 

 assorted independently. 



