INDEPENDENT MENDEL1AN INHERITANCE 



81 



These two cases illustrate very well how closely the results of Men- 

 delian investigations fulfill mathematical requirements, and their signifi- 

 cance cannot be doubted when it is considered how little difficulty is 

 experienced in classifying this particular kind of material. Nevertheless 

 the mathematical requirements are very often not fulfilled on account 

 of the action of external conditions of various kinds. Here as elsewhere 

 the disturbing influence of biological factors must ever be kept in mind 

 in judging the significance of the application of any strict mathematical 

 tests. 



Dihybridism. When two pairs of factor differences are involved in 

 a hybrid the same laws apply in segregation and recombination as apply 

 in the monohybrid. The two pairs of factors segregate independently 



Fio. 38. Maize ear showing ^2 segregation of grains in the ratio of 3 purple, 1 white. 



of each other and give character combinations in F% to be expected on the 

 basis of chance factor distribution. In maize there are varieties which 

 have a deep purple aleurone color which gives the entire grain a black 

 appearance. When such varieties are crossed with certain white varie- 

 ties which possess no aleurone color the FI is purple and in F 2 the grains 

 are in the ratio of 3 purple : 1 white. An ear displaying such F 2 segrega- 

 tion is shown in Fig. 38. The factors involved in this case are W for 

 pigment production in the aleurone layer and w for no pigment pro- 

 duction in this tissue. The hybrid Ww since it is a monohybrid will, 

 therefore, give in F% genotypes in the ratio 1 WW : 2 Ww : 1 ww, which are 

 distributed in two phenotypes in the ratio 3 purple : 1 white. We have 

 shown similarly how starchy corn when crossed with sweet gives a starchy 

 FI and in F 2 3 starchy : 1 sweet. Here the factors involved are S for 

 starchiness and s for sweet. A purple sweet corn, therefore, will have 

 the genetic constitution WWss with respect to the above factors, and a 

 white starchy corn, the genetic constitution wwSS. 



When a purple sweet corn is crossed with a white starchy corn the 

 FI will be purple starchy it will display the dominant characters of both 

 parents to the exclusion of the recessive characters, white and sweet. 

 From the purple sweet corn the FI receives gametes of the genetic consti- 

 tution Ws and from the white starchy wS. Consequently its genetic 



