2oo POPULAR SCIENCE MONTHLY. 



From the facts that cross-bred animals form sexual elements (or 

 gametes) of two sorts, and that the two sorts are equally numerous, it 

 follows that among their offspring dominant and recessive individuals 

 will occur in definite proportions. It has been found by experiment 

 that when two cross-bred (or hybrid) dominant animals are mated to- 

 gether, the offspring consist of a mixture of dominant with recessive 

 individuals in approximately the proportions, three dominant to one 

 recessive. Further, when a hybrid dominant is mated with a recessive 

 animal, half the offspring are hybrid dominants, half recessives. These 

 proportions of necessity result, provided neither sort of gamete has 

 greater affinity for one kind than for the other. For consider all the 

 possible unions between two sets of gametes each D and R (the case in 

 which two hybrid dominants are bred together) : 



D and R 



D and R 



DD+ 2D{E) + BR 

 ~ 3~ 1 



They are 1 DD, 2 DR and 1 RR, or three unions involving the D 

 character to one involving the R character only; hence three dominant 

 individuals will be produced to one recessive. Further, one of the 

 three dominants (DD) will be pure, while the other two are hybrid in 

 character. Recessive individuals are, however, necessarily pure and 

 breed true inter se. Thus, smooth-coated guinea-pigs produce only 

 smooth-coated offspring, albinos only albino offspring, and long-haired 

 ones only long-haired offspring, when mated to animals like themselves. 

 The reason for this will be clear if we return to the illustration with 

 the glass plates. Pairs of transparent plates can be separated only 

 into transparent pieces, and these can be recombined only into trans- 

 parent pairs. 



With the dominant offspring of hybrids, however, the case is dif- 

 ferent. Only pure dominants (DD) will breed true when mated inter 

 se; hybrid dominants (DR) will continue to give a mixed progeny. 

 For pairs of opaque plates, when separated, can be recombined only 

 into opaque pairs; but pairs composed each of an opaque and a trans- 

 parent plate, when separated, may be recombined either into opaque 

 pairs or into transparent pairs, the chance frequencies of the two sorts 

 of combination being as three to one. 



Accordingly, any pair of recessive individuals may form the begin- 

 ning of a pure race of recessives, but in starting a pure race of domi- 

 nants we must test each animal used, to make sure that it does not 



