508 Introduction to the Study of Science 



facts. Mendel, by testing through several generations, found a 

 dwarf pea which bred true to type and did not exceed eighteen 

 inches in height, and a tall pea that had a length of six feet. 

 These he hybridized. The plants of the first generation of the 

 cross-fertilized seed (Fig. 158) were all tall ; in fact they were 

 taller than the tall parent plant, growing to a height of about 

 seven and one half feet. This is the usual result of crossing 

 two varieties the strong character is considerably reinforced. 

 Because the unit character, tallness, appears exclusively in all 

 plants of the first generation, Mendel described this character 

 as dominant. Because the other unit character, shortness or 

 dwarfishness in this case, disappears entirely in the plants of 

 the first generation, it is described as a recessive character. 

 As any one may prove by hybridizing plants having opposite 

 and distinctive characters, the dominant character will always 

 appear exclusively in the offspring of the cross-fertilized parents. 

 But unit characters are never blended in the hybrids, and, how- 

 ever combined with one another, they retain their integrity 

 a'nd are transmitted unmodified to the descendants. 



Succeeding generations. The seeds of the tall hybrids were 

 collected and planted. The plants when grown proved to be 

 of two varieties, some tall and others short. There were no 

 intermediate lengths of plants, as would be the case if the two 

 unit characters had blended to form an average. The seed of 

 this generation was planted ; the seed of the short peas grew 

 true to type ; but the seed of the tall peas produced two varie- 

 ties, in the ratio of three tall to one short. So it occurred in 

 all succeeding generations. The seed of the dwarf peas re- 

 produced true to parent, while the seed of the tall plants split 

 in the same ratio of three tall to one short. 



245. The formal law. Let us assume that the total number 

 of seeds produced by the first cross-fertilized plant was twenty. 

 These when planted grew into tall plants only. The number of 

 seeds produced, it may also be assumed, was 256. These when 

 planted produced 190 tall plants and 66 dwarf plants. The 



