HEREDITY AND HUMAN IMPROVEMENT 375 



character. The dwarf peas thus arising produced nothing 

 but dwarfs, the dominant character never reappearing. 

 It was found also that one-third of the tall peas produced 

 nothing but tall plants, the recessive character never appear- 

 ing. These were called the pure dominants; two-thirds 

 of the tall peas, however, when interbred produced tall and 

 dwarf in the ratio of 3 to i. Two- thirds of the tails were 

 therefore mixed as regards the tall and dwarf characters. 

 It is customary to write a formula for the second genera- 

 tion of hybrid crosses in the following way: iDD + 2DR 

 + iRR. In the case of our second generation of peas one- 

 fourth would be pure dominants or DD's, two-fourths 

 impure, DR's, and one-fourth pure recessives, RR's. 

 Similarly the progeny of the yellow peas resulting from 

 crosses of the yellow and green varieties are found to 

 produce yellow and green peas in the ratio of 3 to i. Of 

 the yellows one-third were pure yellows, and two-thirds 

 produced both yellow and green peas, whereas the greens 

 being the pure recessives produced nothing but green peas. 

 The most significant thing about Mendel's law is that 

 contrasted characters come to be segregated out in the 

 second generation of hybrids in definite numerical ratios. 

 Characters such as greenness, yellowness, tallness, short- 

 ness and many others apparently behave as units capable 

 of being combined and separated again without losing their 

 identity. 



Where organisms differing in two pairs of contrasted 

 characters are crossed each pair behaves as a rule inde- 

 pendently of the other. Thus when a tall, yellow pea is 

 crossed with a dwarf green variety the first generation is 

 tall and yellow; the second generation is constituted as 

 follows: 9 tall yellowy tall greeny dwarf yellow :i 

 dwarf green. Where several pairs of characters enter into 

 the combination the ratios are still more complex. 



