PHENOMENA OF INHERITANCE 237 



Other Mendelian Ratios 



When a pure dominant is crossed with a 

 mixed dominant-recessive all the offspring 

 show the dominant character, though one-half 

 are pure dominants and the other half domi- 

 nant-recessives. Thus if a pure round-seeded 

 variety of pea is crossed with a hybrid between 

 a round- 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: 



S germ cells R \ /R 



iX I 



? germ cells R ^ ^ 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 IRR :2R{W) : 1 WW (Fig. 52). 



