446 



ELEMENTARY BIOLOGY 



2D: 2R 



3D: IR 



FIG. 238. Mendel's Law of Segregation 



When two individuals with a pair of alternative 

 characters are mated, the offspring will all have 

 the character of one of the parents ; this char- 

 acter is called the dominant one, and the alterna- 

 tive character is called the recessive. The hybrid 

 offspring of such a mating is represented in the 

 diagram by F^. Offspring of this kind resemble 

 the dominant parent D, but experiments show 

 that there is a real difference. If such a hybrid 

 is mated with one of the pure dominant type, /, 

 the next generation will all be dominant. If 

 such a hybrid is mated with an individual of 

 the recessive type, 2, the offspring will consist 

 of dominants and recessives, in about equal num- 

 bers. If two such hybrids are mated, j, the 

 offspring will show both dominants and reces- 

 sives, in the proportion of three to one. This 

 reappearance of recessives in the offspring of 

 hybrids that seem to be dominants is called 

 segregation, or splitting up 



1. He crossed hybrids 

 with plants of the yellow- 

 seeded parent variety. 



2. He crossed hybrids 

 with plants of the green- 

 seeded parent variety. 



3. He crossed hybrids 

 with hybrids. 



The results of these 

 crosses are indicated in 

 Fig. 238. 



This fact of splitting 

 up into the two ancestral 

 types has been found to 

 be quite general among 

 all plants and animals that 

 have been tested, and it is 

 called the Law of Segre- 

 gation. The idea is that 

 the hybrid plant, no mat- 

 ter how much it may re- 

 semble one of the parents 

 (with respect to one or 

 more particular charac- 

 ters), does not constitute 

 a pure kind of organism, 

 inasmuch as it cannot 

 reproduce itself in off- 

 spring all having the 

 same character. 



The plants resulting from 

 the mating of hybrids (that is, 

 the segregated yellow-seeded 

 and green-seeded individ- 

 uals) were experimented with 



