522 EVOLUTION, HEREDITY, EUGENICS 



There may also be a masking of characters. If gray rabbits 

 are crossed with albinos, the first filial generation are all gray. The 

 second filial generation become 9 gray, plus 3 black, plus 4 albinos, 

 (i) Pigmentation is A and albinism is a.^^ (2) Gray is B and black is 

 b. When these are crossed we get a gray hybrid. 



The gray hybrids follow the rule for dihybrids, resulting in 9 

 AB (gray), plus 3 Ab (black), plus 3 aB, plus i ab (4 albinos). The 

 albinos are not all the same kind, however, for we have: i aBaB, 

 2 aBab and i abab. 



By crossing each type with a black we get aBaB X Ab — AB 

 all gray; aBab X Ab — i AB gray plus i Ab black; abab X Ab — 

 Ab all black. 



Trihybrids have three pairs of independently segregating char- 

 acters. 



RYT X WGD produces the hybrid RYT (WGD). 



We must eliminate RW, YG and TD as impossible of exhibition 

 (Second Law), and then we have the following combinations for 

 male and female germ cells: RYT RYD RGT RGD WYT WYD 

 WGT WGD. 



Placing the male germ cells in a horizontal row and the female 

 germ cells in a vertical row and superimposing the squares we may 

 secure 8X8 squares of possible matings or sixty-four matings. 



Results: 27 RYT plus 9 RYD plus 9 RGT plus 9 WYT plus 3 

 RGD plus 3 WYD plus 3 WGT plus i WGD. 



Mender s Third Law: The Law of Independent Assortment of 

 Different Allelomorphs. — " The relation of each pair of different 

 characters in hybrid union is independent of the other differences 

 in the two original parental stocks." 



This law becomes apparent when we try to follow the inheritance 

 of more than one character at the same time. Modern investigators 

 believe that the genes that show independent assortment are located 

 in different chromosomes (see page 500). 



Mendel's Fourth Law: The Law of Recombination. — Mendel 

 stated that " the constant characters which appear in the several 

 varieties of a group of plants may be obtained in all the associations 

 which are possible according to the mathematical laws of combina- 

 tion." We find that the genes occur in all possible combinations 

 according to the law of chance. 



1^ Where many factors are involved, it is desirable to use R for the dominant char- 

 acter and r for the recessive, as is done in the case of the discussion of rabbits herewith. 



