GENETICS 



245 



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of each male contain B, half contain h. These formulas are sho^nff in 

 the first five lines of the accompanying figure (Fig. 193). 



When these two kinds of eggs are fertilized by the two kinds of sperms, 

 there are four possible combinations, which according to the theory of 

 chance should be equally numerous. These four combinations (naming 

 the contribution of the egg first, the sperm second, in each formula) are 

 BB, Bb, bB, and bb. Since, however, in a fertilized egg it is immaterial 

 whether a given gene came from the egg or the sperm, the combinations 

 Bb and bB are identical, so that there are really but three combinations 

 instead of four. This duplicated combination, which is written Bb in 

 Fig. 193, is twice as common as either of the other two. Since one ^ is ( 

 capable of producing enough pigment to make a fully black animal, all* 

 F2 animals having the formula BB or Bb are black. Three-fourths of 

 the F2 generation are therefore black; one-fourth (bb) are albino. How- 

 ever, as pointed out above, the F2 blacks are of two kinds, practically 



Fig. 193. — Diagram showing the genetic formulas of all individuals and their germ 

 cells, as far as the F2 generation, involved in the cross between a pure black and an 

 albino guinea-pig. 



alike in appearance, but differing in their hereditary behavior. One- 

 third of these blacks are homozygous {BB) like the black parent, two- 

 thirds of them are heterozygous {Bb) like the Fx individuals. 



The F3 Generation. — If two individuals of the Fi generation be mated 

 together, their offspring constitute an Fz generation. Not all Fz famihes 

 are alike, however, for there are three kinds of F^ individuals from which 

 to select parents. Thus there are six possible mntin^ s J ^ptwppn F2 indi- 

 viduals. With the results of four of these we have already become 

 familiar in the matings previously described. For example, one might 

 mate two Fi individuals each of which was of the composition BB\ in 

 this case, of course, all the Fz individuals would be homozygous black s. 

 Or the two Ft parents might both have the formula b\, in which c§ise 

 the Fz offspring would all be albinos. Or one of the Fi parents might 

 be homozygous black {BB) , the other albino (bb) ; in this case the Fz 

 generation would be all black, but heterozygous (Bb), just like the Fi 



