MENDEL'S LAW 93 



short-haired individuals in F2 to be three times as numerous 

 as the long-haired ones, and colored ones to be three times 

 as numerous as albinos. Further, individuals which are both 

 short-haired and colored should be 3 X 3 or nine times as 

 numerous as those which are neither short-haired nor colored. 

 The expected proportions of the four classes of F2 offspring 

 are accordingly nine short colored : three long colored : three 

 short albino : one long albino, a proportion which is closely 

 approximated in actual experience. 



The Mendelian theory of independent unit-characters ac- 

 counts for this result fully. No other hypothesis has as yet 

 been suggested which can account for it. Suppose that each 

 independent unit has a different material 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 re- 

 combinations arise as shown in Fig. 39. The composition of 

 the gametes furnished by the parents is shown in the first line 

 of the figure; that of an Fi zygote, in the second line; 

 that of the gametes formed by Fi individuals in the third 

 line. S meets s and C meets c in fertilization to form an Fi 

 individual duplex and also heterozygous as regards hair- 

 length and hair-color, but these units segregate again as the 

 gametes of the Fi individuals are formed, and it is a matter 

 of chance whether or not they are associated as originally, 

 S with C and s with c, or in a new relationship, s with C and 

 S with c. Hence we expect the Fi individuals to form four 

 kinds of gametes all equally numerous: SC, sc, sC, and Sc. 

 By chance unions of these in pairs nine kinds of combinations 

 become possible, and their chance frequencies will be as 

 follows : 



Short Colored Long Colored Short Albino Long Albino 



1 SSCC 1 ssCC 1 SScc 1 sscc 



2 SSCc 2 ssCc 2 Sscc 

 2 SsCC 



4 SsCc 



9 3 3 1 



Four of these combinations, including nine individuals, will 

 show the two dominant characters, short and colored; two 



