PLANT HYBRIDIZATION BEFORE MENDEL 313 



If now both the male and the female germ cells bear the char- 

 acters of either the dominant or the recessive in equal numbers, 

 and cross at random, then 



D X r = Dr 



and 

 r X D = rD 



Then, when the F^^ hybrid, which we have been calling Dr, 

 but which is also rD, produces its offspring by self-fertilization, 

 we should get four kinds of combinations of germ cells in equal 

 numbers as follows : 



1. D X D =DD 



2. D X r = Dr 



3. r X D = rD 



4. r X r = rr 



i.e. 25% 25% 25% 25% 



DD Dr rD rr 



It is plain that this combination would be a germ-cell analysis 

 of the actual visible result already stated in the ordinary form 

 of a ratio. 



25% 50% 25% 



DD : Dr : rr 



In order to prove his assumption, "that the various kinds of 

 egg and pollen cells were formed in the hybrids on the average 

 in equal numbers," Mendel carried out the following experiment. 



He crossed two forms of peas which were different in both the 

 form and in the color of the seeds. Hence, following the symbols 

 previously used : 



A round seeds a wrinkled seeds 



B yellow seeds b green seeds 



The Fi hybrids thus produced we>e sown, and there were also 

 sown seeds of both of the two parental types. AB and ab crosses 

 were then made as follows : 



1. The Fi hybrids were crossed with pollen of AB (round yellow) 



2. The Fi hybrids were crossed with pollen of ab (wrinkled green) 



3. AB (round yellow) were crossed with pollen of Fj hybrids. 



4. ab (wrinkled green) were crossed with pollen of F^ hybrids. 



