HEREDITY 1 93 



As illustrations, I will explain the practical consequences of Mendelian 

 research by two examples only. The Swede, Nilsson-Ehle, was one of the 

 first investigators who tried to use Mendelistic methods t» improve agricul- 

 tural plants. In the cold climate of Sweden some wheat varieties, like the 

 English square-hood wheat, were yielding well but were frozen easily. 

 Other varieties, like the Swedish country wheat, were winter-hard but 

 brought only a poor harvest. Nilsson-Ehle knew that in accordance with 

 the Mendelian law of independent assortment, the breeder is able to com- 

 bine the desired characters of two different parents, like the chemist who 

 combines the atoms to form various molecules or compounds. He crossed 

 the late-ripening, well-yielding, square-hood wheat with the early-ripen- 

 ing, winter-hard, but poor-yielding Swedish country wheat. The resulting 

 Fj generation, however, was very discouraging. It was uniform, in ac- 

 cordance with Alendel's first law, all individuals being late-ripe and poor- 

 yielding, thus combining the two undesirable dominant characters. In pre- 

 Mendelian times the breeder would have been discouraged and probably 

 would have discontinued his efforts. Not so Nilsson-Ehle, who knew that 

 the Fi generation is hybrid, showing only the dominant traits, and that the 

 independent assortment of all characters will appear only in the F^ genera- 

 tion. Self-fertilizing the Fi plants he obtained an F2 generation showing the 

 ratio of nine late-ripe poor-yielding to three late-ripe well-yielding, to three 

 early-ripe poor-yielding, to one early-ripe, well-yielding wheat plants. 

 The desired combination of the two recessive characters, early-ripe, well- 

 yielding, appeared only in the smallest ratio, one in sixteen — but because 

 recessives are always true-breeding, or as it is called "homozygous," Nilsson- 

 Ehle had only to isolate these plants and to destroy all others in order to ob- 

 tain a new true breeding early-ripe and well-yielding variety which after a 

 few years gave a crop large enough to be sold. Thus, by the work of the 

 Mendelist, Nilsson-Ehle, culture of wheat was made possible even in the 

 northern parts of Sweden and large amounts heretofore spent for imported 

 wheat could be saved. 



Another instance shows the importance of Mendelism for the under- 

 standing of human inheritance. Very soon after the rediscovery of Mendel's 

 paper it became evident that the laws found by Mendel with his peas are 

 valid also for animals and for human beings. Of course, the study of the laws 

 of human heredity is limited and rendered more difficult by several obsta- 

 cles. We can't make experiments with human beings. The laws of Mendel 

 are statistical laws based upon large numbers of offspring, while the number 

 of children in human families is generally small. But in spite of these difficul- 

 ties it was found very soon that human characters are inherited in the same 

 manner as the characters of the pea. We know, for instance, that the dark 

 color of the iris of the eye is dominant, the light blue color recessive. I 

 remember a tragi-comic accident connected with this fact. At one of my 

 lecture tours in a small town in Czechoslovakia, I spoke about the heredity 



