DOMINANCE AND SEGREGATION ILLUSTRATED 341 



dominant type and a set which behave Hke 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 seeds, 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" 



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



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



Generation (Fi) Self-fertilised they yielded 



Second Filial (inbred) Yellows Yellows Greens 



Generation (F2) (pure type) (impure type) (pure type) 



' I I I I 



Third Filial (inbred) Yellows Yellows Yellows Greens Greens 

 Generation (F3) (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 



