Mendelism 263 



pigs x albino, smooth-haired; dark, curly x light, straight 

 hair in man, etc., there is only one out of sixteen offspring 

 in the second generation in which both of the recessive factors 

 come to expression. Thus in the case of black rough x white 

 smooth hair in guinea pigs, only one in sixteen second gen- 

 eration offspring will be white with smooth hair. This re- 

 sult follows as a mathematical necessity. of the chance com- 

 bination of two pairs of characters, just as the 3 :1 ratio re- 

 sults from the combination of one pair. Similarly if three 

 pairs of characters are involved in a cross, i.e., black, rough, 

 short and white, smooth, long hair in guinea pigs, there will 

 be only one out of sixty-four offspring in the second genera- 

 tion, which will show all three recessive characters. And if 

 four pairs are involved, only 1 in 256, etc. A graphical rep- 

 resentation of these results is given in the accompanying dia- 

 grams, which make sufficiently clear the chance combination 

 of characters in Mendelian inheritance. 



That two factors may be involved in the production of an 

 apparently simple character is conclusively shown in the case 

 of the sweet peas described by the English naturalist, Bate- 

 son. Bateson found that when two white peas were crossed 

 they produced colored offspring, which he interpreted as due 

 to the presence of two factors, one in each of the white par- 

 ents, which, uniting in the cross, produced a colored pea. 

 The results obtained by inbreeding these colored offspring, 

 details of which need not figure here, showed clearly that two 

 pairs of Mendelian factors were concerned. In the case of 

 the corn cited above a single factor for yellow produces the 

 same apparent result in the first generation, as do two fac- 

 tors, but in a variety of oats described by the Swedish breeder 

 Nilsson-Ehle, a different result is obtained. In this case a 

 variety of oats characterized by dark brown glumes or husks, 

 when crossed with a white-glumed variety produced in the 

 second generation nine plants with dark brown, six with light 

 brown, and one with white glumes. This result may be ex- 

 plained as due to the presence of two factors for brown in 

 the dark-glumed plants, one only in those with light brown 

 glumes and none in those with white glumes. It is obtained 

 in the same way as in the second diagram, two factors for 

 brown being substituted for black, rough. 



The theory that two or more factors may in some cases com- 

 bine to produce an apparently simple result is known as the 

 " multiple factor" hypothesis. In the case of lop-ear in rab- 

 bits and color in man, the results are readily explicable by 

 means of this hypothesis on the assumption (1) that several 

 factors are involved in the production of the character in 

 question, (2) that in order to produce the maximum result 



