400 



SCIENCE. 



[N. S. Vol. XVIII. No. 456. 



This case has been tested for peas and 

 for mice and found to be substantially as 

 stated. 



We have thus far considered only cases 

 of cross-breeding between parents differ- 

 ing in a single character. We have seen 

 that in such cases, no new forms, except 

 the unstable hybrid form, are produced. 

 But when the parent forms crossed differ 

 in two or more characters, there will be 

 produced in the second and later hybrid 

 generations individuals possessing new 

 combinations of the characters found in 

 the parents; indeed, all possible combina- 

 tions of those characters will be formed, 

 and in the proportions demanded by 

 chance. Thus when parents are crossed 

 which differ in two respects, A and B, 

 let us designate the dominant phase of 

 these characters by A, B, the recessive 

 phase by a, b. The immediate offspring 

 resulting from the cross will all be alike, 

 AB(ab),* but the second and later genera- 

 tions of hybrids will contain the stable, i. 

 e., pure classes, AB, Ab, aB, ab, in addi- 

 tion to other (unstable or still hybrid) 

 forms, namely, AB(ab), AB(b), A(a)B, 

 A(a)b and aB(b). In every sixteen 

 second-generation offspring there will be, 

 on the average, one representing each of 

 the stable combinations. Two of the stable 

 combinations will be identical with the 

 parent forms, the other two will be new. 

 The remaining twelve individuals will be 

 hybrid in one or both characters. 



An illustration may help to make this 

 case clear. Among domesticated guinea- 

 pigs, as among mice and rabbits, albinism 

 is recessive with respect to pigmented coat. 

 Further, there occur among guinea-pigs in- 

 dividuals known as ' Abyssinians, ' whose 



* This is Mendel's use of lower-case letters to 

 designate recessive characters, with which I have 

 combined the use of a parenthesis when a charac- 

 ter by nature recessive is not visible in the indi- 

 vidual. 



coat presents a curious rough appearance, 

 for the reason that the hair stands out 

 stiffly from the body in a number of 'cow- 

 licks ' or rosettes. In crosses the Abyssinian 

 or rough coat regularly dominates over 

 the normal or smooth coat. Now let us 

 consider what happens when a cross is 

 made involving both these pairs of Men- 

 delian characters, albinism vs. pigmented 

 coat, and smooth vs. rough coat. If a 

 white Abyssinian is bred to a pigmented 

 smooth guinea-pig, the young are without 

 exception pigmented and rough, these 

 being the dominant members of the two 

 pairs of characters. But the young of 

 course contain in a latent condition the 

 two recessive characters, white coat and 

 smooth coat, which fact may be indicated 

 by designating them as already suggested, 

 AB(ab) [A, a referring to the rough or 

 smooth character of the coat and B, b to 

 its color]. 



These primary hybrids, if bred inter se, 

 will produce young of four different sorts, 

 viz., rough pigmented, rough white, 

 smooth pigmented and smooth white. A 

 certain number of the animals of each 

 sort will breed true, i. e., will produce only 

 their own sort when mated to animals like 

 themselves. Theoretically there should be 

 one pwre individual of each of the four 

 sorts in a total of sixteen young. The 

 four pure individuals answer to the classes 

 AB, Ab, aB, ab already mentioned. 



But, besides these pure individuals, there 

 will occur in three of the four classes 

 impure or hybrid individuals, which will 

 transmit to some of their young the domi- 

 nant character or characters which they 

 themselves possess, but to others of their 

 young the corresponding recessive char- 

 acter or characters. Only the class of 

 smooth white animals (of which there 

 should be one in sixteen young) contains 

 none but pure individuals, for they bear 



