510 



BIOLOGY AND HUMAN LIFE 



2D: 2R 



Fig. 214. 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 appear 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, 3, the off- 

 spring will show both dominants and recessives, 

 in the proportion of three to one. This splitting 

 up of the offspring of hybrids into two types 

 showing ancestral factors is almost universal; it 

 is called segregation 



with green seeds (see 

 table on opposite page) ; 

 that is, the two original 

 parental types reappear. 

 There is segregation in 

 the proportion of three 

 dominants to one reces- 

 sive (3:1). By inbreed- 

 ing a second time we find 

 further segregation (col- 

 umn marked "Third Hy- 

 brid Generation"). But 

 here a new fact appears : 

 some of these plants in 

 the second hybrid gen- 

 eration breed true — some 

 of the yellows (dom- 

 inant) and all of the 

 greens (recessive). The 

 greens are found to breed 

 true in every succeeding 

 generation, in spite of 

 the fact that they were 

 derived from yellow hy- 

 brids. Plants (or ani- 

 mals) that behave in 

 this manner are called 

 extracted recessives. Re- 

 gardless of the fact that 

 they have descended 

 from yellow (dominant) 

 plants (first hybrid gen- 

 eration), such recessives 

 are considered pure be- 

 cause they always do 

 breed true. Again, those 



