244 PRINCIPLES OF ANIMAL BIOLOGY 



generation) , and the second hybrid generation as F^ {second filial genera- 

 tion). Albinism disappears in tlie F] generation, a fact which is expressed 

 by saying that the black coat is dominant, or that albinism is recessive. 

 Its disappearance, however, does not indicate its absence, for albinism 

 reappears in the F2 generation. 



Explanation of the Experimental Results. — The explanation of these 

 results is fairly simple. The genes are in practice designated by letters. 

 When two characters are contrasted, both are commonly designated by 

 the initial letter of the dominant character. (Some geneticists, however, 

 employ the initial letter of the more recent character, whether it is domi- 

 nant or not.) The dominant character is represented by a capital letter, 

 the recessive character by a small letter. Thus, in the case of color in 

 the guinea-pig, when black is contrasted with albino, black may be repre- 

 sented by B, albino by h. 



The hereditary formulas may now be written for the guinea-pigs 

 whose color inheritq,nce was described. The formula of the Pi black pig 

 was BB, the formula being double because the animal had two parents. 

 In similar manner the formula of the albino parent is to be written hh. 

 When these animals produce germ cells, however, the two genes separate 

 from one another, clue to the separation of homologous chromosomes 

 in one of the maturation divisions as explained above, so that each germ 

 cell has only one gene. All the germ cells of the black parent have the for- 

 mula B, those of the albino parent the formula 6. 



When these germ cells unite in making the cross, the genes B and h 

 are brought together, and the formula of all the Fi individuals is Bh. 

 One gene B is capable, however, of producing ^nough black pigment to 

 render the Fi pigs indistinguishable from the Pi black. That is the 

 reason for the dominance of black over albinism. If one B could pro- 

 duce only a fraction of the amount of pigment produced by two P's, so 

 that the Pi pigs were lighter in color than their black parent, the char- 

 acter black would be only partially dominant over albinism. Cases of 

 partial dominance are not uncommon in other organisms. If one B 

 alone vv^ere incajmble of producing any pigment at all, all Pi's would then 

 be albinos, and albinism would be the dominant character, black pigment 

 would l>e recessive. In that case, also, three-fourths of the P2 would be 

 albino, one-fourth black, and one would probably have selected some 

 other symbols than B and h to represent the genes. 



When the Fx individuals, whose formulas are all Bh, produce germ 

 cells, *the phenomenon of segregation occurs just as in the germ cells of 

 their parents. This time, however, since the genes that segregate from 

 one another are unlike, some of the germ cells receive the one gene B, 

 others receive the one gene 6. The Pi pigs thus produce two kinds of 

 germ cells, which, if all cells survive, are present in equal numbers. Half 

 the eggs of each female contain B, half contain 5. Half the spermatozoa 



