134 GENETICS 



organisms are crossed which differ from each other 

 with respect to two different unit characters, the situa- 

 tion becomes more complicated. 



Mendel solved the problem of dihybrids by crossing 

 wrinkled-green peas with smooth-yellow peas. He 

 found that smoothness S is dominant over wrinkled- 

 ness W and that yellow color Y is dominant over 

 green 0, or, as it would be stated according to the 

 presence and absence theory, smoothness is a positive 

 character which fills out the seed-coat to plumpness 

 while its absence leaves a wrinkled coat, and yellow- 

 ness 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)Y(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 *SF 

 and wrinkled-green WG, which are exactly like the 

 grandparental determiners from which 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 



