DOMINANCE AND SEGREGATION ILLUSTRATED 341 



dominant type and a set which behave like the first generation 

 of hybrids i.e. they go on splitting up into dominant- like forms 

 and pure recessives. These two sets occur in the proportions 

 of i : 2. 



A Case of Peas. Let us consider a concrete case. Peas with 

 rounded seeds were crossed with peas having angular wrinkled 

 seeds. In the offspring the character of roundness was dominant ; 

 the angular wrinkled character had disappeared or receded. It 

 was not lost, as the next generation showed. 



The hybrid offspring, all with rounded sesds, were allowed 

 to self-fertilise. In their progeny roundish seeds and angular 

 wrinkled seeds occurred in the proportions of 3 : i. Here were 

 the recessives again, and when they were allowed to self -fertilise 

 they produced pure recessives only, with angular wrinkled seeds. 



The dominants, however, were not all pure dominants, for 

 when they were allowed to self-fertilise they produced one-third 

 pure dominants and two-thirds " impure " dominants, the latter 

 being distinguished by the fact that in their offspring recessives 

 reappeared in the proportion of one recessive to three dominants. 



The outstanding facts, taking the case of yellow-seeded and 

 green-seeded peas, may be thus summarised : 



Parental Yellow-seeded "pure" Green-seeded "pure 1 



Generation (Pi) plant (dominant) plant (recessive) 



I I 



First Filial (Hybrid) All the offspring were yellow-seeded. 



Generation (Fi) Self-fertilised they yielded 



I 



I .1 I 



Second Filial (inbred) Yellows Yellows Greens 



Generation (Fa) (pure type) (impure type) (pure type) 



I I 



Third Filial (inbred) Yellows Yellows Yellows Greens Greens 

 Generation (F$) (pure type) (pure) (impure) (pure) (pure type) 



Thus intercrossing of forms with contrasted characters 

 results not in transitional blends, but in the dominance of one 



