484 Plant Physiology 



296. Purity of the gametes. As a result of these 

 hybridization studies, Mendel naturally developed his 

 theory of the purity of the gametes, founded substantially 

 in this way : In the case of the hybrid between a tall (T) 

 and a dwarf (S) pea, for example, the male gametes in 

 equal number will carry the character tallness or dwarf- 

 ness, never both ; and so also with the egg cells. Assum- 

 ing large numbers, these gametes, uniting by the law of 

 chance (without selective fertilization), would yield, tall- 

 ness being dominant, 



Tall with tall or TT (homozygote) 



Tall with dwarf or T(S) (heterozygote) 



Dwarf with tall or (S)T=T(S) (heterozygote) 



Dwarf with dwarf or SS (homozygote) 

 from which we get T+2 T(S)+S. The essential features 

 of Mendelism are dominance and segregation, and these 

 phenomena are sufficiently important to-day to receive 

 unusual attention. It has been well shown that many 

 Mendelian character pairs may be expressed conveniently 

 in terms of presence and absence of a single character. 

 Either presence or absence may be dominant. The 

 idea of the purity of the gametes requires modification, 

 at least with respect to certain characters. 



296. Results of segregation. In corn yellow kernels 

 are dominant over white. Indicating the yellow by Y, 

 the white by W, and the white in the hybrid, where it is 

 inevident, by (W), the following diagram indicates the 

 method of segregation in five generations, and the relative 

 number of pure dominants, recessives, and hybrid domi- 

 nants in the hypothetical case where the rate of increase 

 is fourfold : 



