THE RED SUNFLOWER 57 



orange background, it was naturally suggested that if 

 the color could be put on the "primrose" rays, an en- 

 tirely new effect would be the result. How might this 

 be done ? Homozygous (pure-bred) red was crossed 

 with primrose, and to save a year the progeny were 

 grown in the greenhouse during the winter. They were 

 very ordinary heterozygous reds. The cross had been 

 as follows, using for orange and o for primrose (no- 

 orange), R for red and r for no-red : 



RROO X rroo = RrOo (gametes RO X ro give RrOo 



zygote) 



RROO is the same as RR given above. So long as all 

 the plants had the orange background, it was not neces- 

 sary to insert it in the formula.* The RrOo plants are 

 red on orange, because red is dominant over no-red, and 

 orange over no-orange (primrose). In former times 

 breeders had sometimes made first crosses as just de- 

 scribed, and failing to get the desired result had neg- 

 lected to continue the work. Thanks to Mendel, it was 

 possible to see ahead in this case. The apparently or- 

 dinary reds had one property which no reds had ever 

 had before, they were heterozygous for orange. It was 

 only necessary to cross them together. In this cross 

 the "red" factors and the "orange" ones combined 

 independently of each other. The reds, as explained 

 above, gave RR, Rr, rR, and rr, which is three reds to 

 one plain orange. The orange, on the same principle, 

 gave 00, Oo, oO, and oo, which is three orange to one 

 primrose. But as these combinations were independ- 

 ent, each one was as likely to occur with one as another 

 of the other group. The theoretical expectation, follow- 

 ing the so-called law of chance, is that RR, for example, 

 in each four times will happen to occur once with 00, 



