PRINCIPLES OF HEREDITY Q^\ 



Mendel then took the seeds produced by this first generation of the 

 cross (called the first filial generation, abbreviated Fi), planted them and 

 had the resulting plants fertilize themselves to produce the second filial 

 generation, the Fo. He found that both the dominant and the recessive 

 characters appeared in this generation, and upon counting the number 

 of each type (Table 10) he found that, whatever set of characters he 

 used, the ratio of plants with the dominant character to those with the 

 recessive character was very close to 3 : 1. From such experiments Mendel 

 concluded that (1) there must be discrete unit factors which determine 

 the inherited characters, (2) these unit factors must exist in pairs, and 

 (3) in the formation of gametes the members of these pairs separate from 

 each other, with the result that each gamete receives only one member 

 of the pair. The unit factor for green seed color is not affected by exist- 

 ing for a generation within a yellow seeded plant (e.g., the Fj individ- 

 uals). The two separate during gamete formation and, if a gamete 

 bearing this factor for green seed coat fertilizes another gamete with 

 this factor, the resulting seed has a green color. The generalization 

 known as Mendel's First Law, the Law of Segregation, may now be 

 stated as: Genes exist in pairs in individuals, and in the formation 

 of gametes each gene separates or segregates from the other member of 

 the pair and passes into a different gamete, so that each gamete has one, 

 and only one, of each kind of gene. 



In other experiments Mendel observed the inheritance of two pairs 

 of contrasting characters in a single cross. He mated a pure-breeding 

 strain with round, yellow seeds and one with wrinkled, green seeds. The 

 first filial generation all had round, yellow seeds, but when these were 

 self-fertilized he found in the Fo generation all four possible combina- 

 tions of seed color and shape. When he counted these he found 315 

 round, yellow seeds, 108 round, green seeds, 101 wrinkled, yellow seeds, 

 and 32 wrinkled, green seeds. There is a close approximation of a 3 : 1 

 ratio for seed color (416 yellow to 140 green) and for seed shape (423 

 round to 133 wrinkled). Thus the inheritance of seed color is inde- 

 pendent of the inheritance of seed shape; neither one affects the other. 

 When the two types of traits are considered together, it is clear that 

 there is a ratio of 9 with two dominant traits (yellow and round): 3 with 

 one dominant and one recessive (green and round): 3 with the other 

 dominant and recessive (yellow and wrinkled): 1 with the two recessive 

 traits (green and wrinkled). Mendel's Second Law, the Law of Inde- 



Table 10. AN ABSTRACT OF THE DATA OBIAINED BY MENDEL FROM 

 HIS BREEDING EXPERIMENTS WITH GARDEN PEAS 



FIRST 

 PARENTAL CHARACTERS SECOND GENERATION RATIOS 



GENERATION 



Yellow seeds X green seeds all yellow 6022 yellow: 2001 green 3.01 



Round seeds X wrinkled seeds all round 5474 round: 1850 wrinkled 2.96 



Green pods X yellow pods all green 428 green: 152 yellow 2.82 



Long stems X short stems all long 787 long: 277 short 2.84 



Axial flowers X terminal flowers all axial 651 axial: 207 terminal 3.14 



Inflated pods X constricted pods all inflated 882 inflated : 299 constricted 2.95 



Red flowers X white flowers all red 705 red: 224 white 3.15 



