Heredity and Environment 



Other Mendelian Ratios. When a pure dominant is crossed 

 with a hybrid dominant-recessive (Fig. 29 r) all of 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 repre- 

 senting the factor for round seed and W that for wrinkled seed : 



$ germ cells R^ .R 



1X1 



a germ cells R ^W 



Possible combinations 



p a 



2RR: 



Fi 



FIG. 29. DIAGRAM OF MENDELIAN INHERITANCE, in which the individual 

 is represented by the large circle, the germ cells by the small ones, domi- 

 nants being shaded and recessives white, a, Pure dominant X pure re- 

 cessive (yields) all dominant-recessives ; b f Dominant-recessive X domi- 

 nant-recessive = i pure dominant : 2 dominant-recessives : I pure recessive ; 

 c, Dominant-recessive X pure dominant = 2 pure dominant : 2 dominant- 

 recessive; d, Dominant-recessive X pure recessive = 2 dominant-reces- 

 sive : 2 pure recessive. 



