12 



BIOLOGY AND HUMAN LIFE 



A 



ai0— - 



Qi 



e-- 



Fig, 215. The law of 

 segregation 



A hybrid produces germ cells of 

 two kinds with respect toapairof 

 contrasting characters — one kind 

 bearing the elements needed for 

 developing the dominant charac- 

 ter, and the other kind bearing 

 the elements that result in the re- 

 cessive. If two individuals, A and 



yellows which upon self-pollenation 

 yield yellow-seeded plants that bear 

 yellow-seeded offspring are also 

 pure. These lines are shown in the 

 table by being printed in capital let- 

 ters. In each generation, then, some 

 of the apparently dominanj: plants 

 will behave like hybrids and split 

 up again when they reproduce ; one 

 out of every three dominants will 

 turn out to be a pure dominant ; and 

 the recessives will remain pure, 

 or capable of reproducing their 

 characters. 



An attempt to explain how this seg- 

 regation is brought about during re- 

 production is illustrated in Fig. 215. 

 This explanation was not known to Men- 

 del but has been worked out since his 

 time by careful experiments with plants 

 and animals and by studies of the 

 changes which take place in the cells at 

 the time the germ cells (gametes) are 

 formed, at the time of fertilization, and 

 in later development. 



374. Combinations of characters. 

 In the meantime we must not for- 

 get that every organism is made up 

 of many characters. After showing 



B. both hybrid and both showing 



the dominant character, are mated, they may give rise to three kinds of offspring. 

 The germ cells given off by A are of two kinds, Cg and a^ having the factor for 

 dominance, while a-^ and a^ bear the factor for recessiveness. In the same way, B 

 bears two kinds of germ cells. The two kinds of eggs can combine with the two kinds 

 of sperms in four ways: (i) a recessive egg combines with a recessive sperm; (2) a 

 recessive egg combines with a dominant sperm; (3)3 dominant egg combines with a 

 recessive sperm; (4) a dominant egg combines with a dominant sperm. As a result, 

 half the offspring are again hybrid and the other half pure; and the pure are likely 

 to be dominants and recessives in equal numbers. Note that the hybrids resemble 

 the dominant grandparent, giving the appearance of one recessive to three dominants 



