42 The Third and Fourth Generation 



evident that one-third of the yellow peas of this 

 generation wUl produce yeUow-seeded plants 

 only, for they have two determiners in the 

 mother-cells that give rise to eggs and sperm, and 

 so these will each have the yellow determiner. 



The proportions of the resulting peas in a 

 cross between a smooth yellow pea and one 

 producing wrinkled green peas are obtained in a 

 similar way. The plants of the Fj generation 

 are represented by SY, and both smooth and' 

 yellow are dominant characters. The sperm 

 and eggs may (i) contain S and Y, (2) only S, 

 (3) only Y, (4) neither S nor Y. The possible 

 combinations are shown in the diagram on p. 43. 

 Nine combinations contain both S and Y, three 

 only S, three only Y, and one neither S nor Y. 

 Therefore in the F^ generation the proportion 

 must be the Mendelian ratio, 9-3-3-1. 



Mendel's law holds good with other forms, 

 both animal and plant, quite as well as with 

 peas. If, for instance, pure black guinea 

 pigs are crossed with whites, the children are 

 all black; the grandchildren are three-fourths 

 of them black, one-fourth white. The whites 



