Mechanism of Mendelian Heredity 233 



tion produced round and angular seeds in the ratio of 

 3:1 (5474 round to 1850 angular). The explanation 

 is as follows. Let R denote round, A angular character ; 

 the pure breeds of parents have the gametic constitu- 

 tion RR and AA respectively. When crossed, all the 

 offsprings have the constitution RA and since A is 

 recessive this hybrid generation resembles the pure RR 

 parents. The F z generation produces two kinds of 

 eggs R and A and two kinds of pollen R and A in equal 

 numbers, and these if inbred give the following four 

 combinations in equal numbers: 



RR, RA, AR, AA. 



Since RA, AR, and RR all give round seeds the F 2 

 generation produces round seeds to angular seeds in 

 the ratio of 3 : 1 . The two organisms with the gametic 

 constitution RR and RA look alike, yet they are 

 different in regard to heredity. The gametically pure 

 form RR is called homozygous, the impure form RA 

 heterozygous. 



2. W. S. Sutton 1 was the first to show that the 

 behaviour of the chromosomes furnishes an adequate 

 basis on which to account for Mendel's law of the 

 segregation of the characters in the sex cells of the 

 hybrids. If we disregard the cases of parthenogenesis 

 and the X chromosomes, we may state that each 



1 Sutton, W. S., "The Chromosomes in Heredity," Biol. Bull., 1904, 

 iv., 231. 



