54. GENETIC STUDIES ON A CAVY SPECIES CROSS. 



self-fertilization proved that the F2 individuals were of the formula 

 demanded by such an explanation, viz : 



r 1 BiBiBjBa bred true. 



„^^ 2BibiB2B2 " " 



^ ^^^n 2 B:BiB,b3 " " 



I 4 BibiB2b2 gave 15 black, 1 white. 



3 Bib2 ( ^ BiBib2b2 bred true. 



I 2 Bibib2b2 gave 3 black, 1 white. 



3 biB2 1 ^ bibiB2B2 bred true. 



I 2 bibiB2b2 gave 3 black, 1 white. 



1 bib2 1 bibib2b2 bred true. 



Bi was not allelomorphic to B2, but each was allelomorphic to its own 

 absence; both Bi and B2 caused development of black in the glume 

 independently. 



Carrying out similar work on other characters, Nillson-Ehle found 

 that the presence of red in the pericarp, presence of brown in the ears, 

 presence of ligules, internodal length, rust resistance, and the like were 

 due to more ''present mutually independent, separable factors than 

 might be concluded from external appearances." In any such case 

 involving n allelomorphic pairs, the ultimate recessive would appear 

 in 1 out of 4'' individuals. In a trihybrid or tetrahybrid cross, the 

 ratios would be 63 : 1 and 255 : 1 respectively — subject, of course, to 

 the law of error. It is true that the dominant classes may often show 

 whether they contain a smaller or larger quota of the dominant factors. 

 Environmental conditions may also prevent the complete somatic 

 development of the characters which a plant may transmit to its 

 progeny. 



Emerson (1910) gave a short, concise interpretation, in Mendelian 

 terms, of the inheritance of shape and size in three species of plants. 

 His data (on size and shape of the fruits in gourds and summer squashes, 

 size and shape of bean seeds, and size of seeds and height of the stalk 

 in corn) show a blend in the Fi generation and a marked increase of 

 variability in the F2 generation over the parents or Fi generation. The 

 difference between the Fi and F2 plants is great enough to leave no 

 doubt that this increased variability has been delayed until the second 

 generation after the cross. Shull (1910) reports a similar increase in 

 the F2 generation in the variability of the number of rows per ear in corn. 



East and Hayes (1911) likewise demonstrated that yellow in the 

 endosperm of maize may be due to two factors, Yi and Y2, each allelo- 

 morphic to its own absence. Hence, they obtained in a cross between 

 a homozygous yellow race (Y1Y1Y2Y2) with a white race (yiyiy2y2) a 

 ratio of 15 yellows : 1 white. In crossing types of maize, differential 

 characters in the number of rows per ear, length of plant, length of ear, 

 and weight of seed were studied. By crossing the dominant with the 

 recessive type of each character, an increased coefficient of variability 



