MENDELISM 



137 



The four albinos are not all of the same constitution, 

 as will be seen at once if we set out the case in a Punnett's 

 square, taking the same two pairs of allelomorphs, Aa 

 and B6 : 



FIG. 59. PUNNETT'S SQUARE FOR RABBITS. 



The shaded squares denote the greys, the black ones the 

 blacks, and the white ones the albinos. It will be seen 

 from the above diagram that there are three different 

 kinds of albinos viz., one aBaB, two aBab, and one abab. 

 On crossing each type with a black, e.g., the difference of 

 their constitution becomes at once apparent, for we get : 



aoBx A&. . = AB. . (all grey). 



aBabxAb.. = i AB.. (grey) +i Ab.. (black). 



abab xAb.. = Ab.. (all black). 



The next case, that of the sweet-pea, is still somewhat 

 more involved. Here we have two white peas giving, on 

 crossing, a purple dihybrid. This dihybrid, on inbreed- 

 ing, produces a progeny of 9 purples + 7 whites. The 

 explanation is similar to that of the previous case ; but 

 while in the last case of the rabbits the presence of one 

 factor only, that of pigmentation, was essential for the 

 reproduction of coloured rabbits, here two factors are 



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