HEREDITY AND VARIATION 



571 



always showed segregation, giving dominants and recessives in the 

 constant ratio of 3 : I. Since the pure dominants are only half as 

 numerous as the impure dominants, it follows that the impure domin- 

 ant on being self-fertilised, produces as offspring pure dominants, 

 impure dominants and recessives in the proportion of 1 : 2 : :. The 

 case of only one pair of characters has been considered here ; but 

 Mendel showed that the rule holds good for all the various pairs of 

 differentiating characters (seven in all) studied by him ; and since 

 his time his conclusions have been verified in numerous instances 

 both in plants and in animals. 



A general scheme may be constructed to show the result of crossing 

 individuals which each bear one of a pair of differentiating character- 

 If D represent the pure dominant ; if the impure dominant, which 

 cannot be distinguished from it by appearance, be represented by 

 (D) ; and if R represent the recessive, then the following will be the 

 scheme of inheritance (Fig. 435). 



D DIDJ [D] R D [D] ID] R R 



Fig. 435. 



Scheme of inheritance of Dominant. D, and Recessive. R. characters reM.ltmc 

 from thT casing of individuals which each bear pne of , ,,, fetttet 



characters, through three generations /„ /., f y (After Punni tt ) 



In any sexually reproduced organism the gametes form the link 

 between successive adult generations. The characters peculiar to 

 the adult must therefore be represented in their heritable constitution. 

 In a tall Pea some at least of the gametes, whether male or female, 



