HEREDITY 1 9 1 



Now, Mendel self-fertilized the hybrids of the first generation, dusting 

 the pistils of the flowers with their own pollen and obtained thus the second, 

 or Fo generation of hybrids. In this generation the recessive characters, 

 which had seemingly disappeared, but, which were really only covered in 

 the Fi generation, reappeared again and in a characteristic and constant pro- 

 portion. Among the Fg hybrids he found three red-flowered plants and one 

 white-flowered plant, or three smooth-seeded and one-wrinkled-seedcd 

 plant, or three plants with yellow cotyledons and one with green ones. In 

 general, the hybrids of the F2 generation showed a ratio of three dominant to 

 one recessive plants. Mendel derived from the behavior of the F^ generation 

 his second experimental law, the so-called "Law of Segregation." Of course, 

 the characteristic ratio of three dominant to one recessive may be expected 

 only if the numbers of individuals are large, the Mendelian laws being so- 

 called statistical laws or laws vaUd for large numbers only. 



The third important experimental law Mendel discovered by crossing 

 two plants which distinguished themselves not only by one but by two or 

 more pairs of hereditary characters. He crossed, for instance, a pea plant 

 with smooth and yellow seeds with another having green and wrinkled 

 seeds. The first, or F^, generation of hybrids was of course uniform, show- 

 ing both smooth and yellow seeds, the dominant characters. Fi hybrids 

 were then self-fertilized and the second hybrid, or F^, generation was 

 yielded in large numbers, showing all possible combinations of the parental 

 characters in characteristic ratios and that there were nine smooth yellow to 

 three smooth green to three wrinkled yellow to one wrinkled green. From 

 these so-called polyhybrid crossings, Mendel derived the third and last of 

 his experimental laws, the "Law of Independent Assortment." 



These experiments and observations Mendel reviewed in his lecture. 

 Mendel's hearers, who were personally attached to the lecturer as well as 

 appreciating him for his original observations in various fields of natural 

 science, listened with respect but also with astonishment to his account of 

 the invariable numerical ratios among the hybrids, unheard of in those days. 

 Mendel concluded his first lecture and announced a second one at the next 

 month's meeting and promised he would give them the theory he had elab- 

 orated in order to explain the behavior of the hybrids. 



There was a goodly audience, once more, at the next month's meeting. 

 It must be admitted, however, that the attention of most of the hearers was 

 inclined to wander when the lecturer became engaged in a rather difficult 

 algebraical deduction. And probably not a soul among the audience really 

 understood what Mendel was driving at. His main idea was that the living 

 individual might be regarded as composed of distinct hereditary characters, 

 which are transmitted by distinct invisible hereditary factors — to-day we 

 call them genes. In the hybrid the different parental genes are combined. 

 But when the sex cells of the hybrids are formed the two parental genes 

 separate again, remaining quite unchanged and pure, each sex cell contain- 



