HEREDITY 



and dark should be 3 X 3 or 9 times as nu- 

 merous as those which are not. The ex- 

 pected proportions of the four classes of F 2 

 offspring are accordingly 9:3:3:1, a propor- 

 tion which is closely approximated in actual 

 experience. The Mendelian theory of inde- 

 pendent unit-characters accounts for this re- 

 sult fully. No other hypothesis has as yet been 

 suggested which can account for it. 



Suppose that each unit has a different mate- 

 rial basis in the gamete. Let us represent the 

 material basis of hair-length by a circle, that 

 of hair-color by a square, then combinations 

 and recombinations arise as shown in Fig. 30. 

 The composition of the gametes furnished by 

 the parents is shown in the first line of the 

 figure; that of an F individual (or zygote), 

 in the second line; that of the gametes 

 formed by the F individual in the third line. 

 L meets S and W meets D in fertilization to 

 form an F x individual double and also hetero- 

 zygous as regards hair length and hair color, 

 but these units segregate again as the gametes 

 of the Fj individuals are formed, and it is a mat- 

 ter of chance whether or not they are associated 



44 



