Mendel's Explanation 247 



would qualify the statement of these principles somewhat, 

 he would go farther than Mendel and point his finger more 

 precisely to just what it is in the germ cells that brings about 

 the observed results. As to the principle of unit character, 



(^ ®. 



a 3 



(5) (°) 



■ a 



■ D 



(^ ($ @ ($ (5 ,@ (°) 





■ ■ 



D 



Fig. 



®® ®'® ®'®- ®® 



64. Mendel's Explanation of Segregation 



Two parents, P, that differ with respect to a given character (here represented by dif- 

 ference in size) differ also as to the germ cells (small circles) which they produce. The 

 pure recessive can produce only one kind of gamete, carrying a determiner for the reces- 

 sive trait. The pure dominant likewise produces but one kind of gamete, carrying a 

 determiner for the dominant trait. Now the hybrid, Fx, although resembling the domi- 

 nant parent, has the capacity to produce two types of gametes: that is, each gamete is 

 " pure " for one or the other of the contrasting characters, regardless of whether it is 

 produced in a pure bred individual or in ^ hybrid. The oflfspring of the hybrids, F.2, 

 results from the four possible combinations of the two types of gametes: (i) egg and sperm 

 both dominant; (2) egg and sperm both recessive; (3) egg dominant and sperm recessive; 

 (4) egg recessive and sperm dominant. This assumption of Mendel's agrees with the ex- 

 perimental facts: the offspring of hybrids are one recessive for three dominants, only one 

 of the three, however, being " pure " in its constitution, as shown by the results of further 

 breeding. 



it has been shown that while Mendel was right in view of the 

 limited number of experiments he carried out, both the ob- 

 served constitution of the germ cells and the observed trans- 

 mission of hereditary traits would make this assumption of 



