PHENOMENA OF INHERITANCE 243 



Other Mendelian Ratios 



When a pure dominant is crossed with a 

 mixed dominant-recessive (Fig. 51 c) all the 

 offspring show the dominant character, though 

 one-half are pure dominants and the other half 

 dominant-recessives. Thus if a pure round- 

 seeded variety of pea is crossed with a hybrid 

 between a round-seeded and a wrinkled-seeded 

 one, all the progeny are round-seeded, though 

 one half of them carry the factor for wrinkled 

 seed; this may be graphically represented as 

 follows, R representing the factor for round 

 seed and W that for wrinkled seed : 



$ germ cells R \ ,R 



$ germ cells R y x W 

 Possible combinations 2 RR: 2 R (W). 



In subsequent generations the progeny of the 

 pure round (RR) breed true and produce 

 only round-seeded peas, whereas the progeny 

 of the hybrid round- wrinkled (RW) split 

 up into pure round, hybrid round-wrinkled, 

 and pure wrinkled in the regular Mendelian 

 ratio of 1 RR : 2R(W) : 1 WW (Fig. 52). 



