108 GENETICS 



or, as it would be stated according to the presence or 

 absence theory, smoothness is a positive character 

 which fills out the seed-coat to plumpness while its 

 absence leaves a wrinkled coat, and yellowness is a 

 positive character due to a fading of the green which 

 causes the yellow to be apparent. In the absence of 

 this green-fading factor, or determiner, the green of 

 course appears. 



If smooth-yellow, SY, and wrinkled-green, WG, are 

 crossed, all the offspring are smooth-yellow, but 

 they carry concealed the recessive determiners for 

 wrinkledness and greenness according to the formula 

 S(W)lf(G). When the determiners of these cross- 

 breds segregate out during the maturation of the germ- 

 cells, they may recombine so as to form four possible 

 double gametes, namely, smooth-yellow, 5F, and 

 wrinkled-green, WG, which are exactly like the grand- 

 parental determiners from w_hich they arose, and in 

 addition, two entirely new combinations, smooth-green, 

 SG, and wrinkled-yellow, WY. 



Since the male and the female cross-breds are each 

 furnished with these four possible gametic combina- 

 tions, the possible number of zygotes formed by their 

 union will be sixteen (4X4=16). That is, the mono- 

 hybrid proportion of 3 to 1 in dihybrid combinations 

 is squared, (3+l) 2 =16. 



It of course does not follow that the offspring in 

 dihybrid crosses will always be sixteen in number, or 

 that they will always conform strictly to the theoreti- 

 cal expectation of (3+1 ) 2 . The offspring obtained 

 undoubtedly obey the laws of chance, but the greater 



