SOME COMPLICATIONS OF MENDELIAN INHERITANCE 307 



Other types of gene interaction — the 13:3, 15:1, and other ratios. 



Another way in which genes interact to produce a modified F 2 phenotypic 

 ratio is the inhibition by one gene of the phenotypic expression of another 

 gene, the first gene producing no visible effect by itself. An example is 

 found in poultry, where a gene A (dominant to a) for colored plumage 

 and an inhibiting gene B (dominant to b), if brought together in a typical 

 hybrid cross, result in a colorless Fi and an F 2 ratio of 13 colorless (white) 

 and 3 colored individuals. 



A dihybrid phenotypic ratio of 15:1 is produced by what are known 

 as duplicate genes. Here A is dominant to a, B is dominant to 6, and 

 either A or B alone or A and B together produce exactly the same effect. 

 Consequently AA Bb, AA BB, Aa BB, Aa Bb, AA bb, Aa bb, aa BB, and 

 aa Bb are indistinguishable in their phenotypic expression and only the 

 genotype aa bb out of 16 shows a visible difference. 



Modified trihybrid and tetrahybrid ratios are also known. Among 

 the former are such modifications of the typical 27:9:9:9:3:3:3:1 ratio 

 as 27:9:28, 63:1 and 27:37. Genotypically, such ratios are precisely 

 of the same sort as those of the typical trihybrid, but various types of gene 

 interaction make several of the groups phenotypically indistinguishable. 



THE MULTIPLE-FACTOR HYPOTHESIS 



One of the types of inheritance most difficult to understand is that 

 illustrated by skin color in the cross between the White and Negro races. 

 The color of the Fi mulatto is more or less intermediate between that 

 of the two parent races, but the F 2 generation fails to show any clear-cut 

 pattern of segregation. Instead, there results a widely variable F 2 , ranging 

 from very light-yellow individuals to others almost as dark as the Negro 

 grandparent. Similar cases are known in other animals and in plants, and 

 for a long time they were regarded as non-Mendelian. That is, since they 

 did not show a clear-cut segregation and recombination, it was assumed 

 that some other and unknown type of inheritance must be involved. 



The clue to an understanding of these crosses came from experiments 

 on the inheritance of seed color in wheat and of ear length in corn, which 

 furnish examples of this so-called " non-Mendelian," or "blended," 

 inheritance. In a cross between a certain variety of red-kerneled wheat 

 and a variety of white-kerneled wheat, the F x has pale red kernels; the 

 F 2 is markedly variable, ranging from very pale red to much darker than 

 the Fi, but with few if any kernels as white or as red as the par- 

 ent generation. 



It was finally seen, both by Nilsson-Ehle, who worked with wheat, 

 and by East, who demonstrated a similar condition in the inheritance 

 of ear length in corn, that such cases were probably Mendelian (involving 

 segregation and recombination of genes) but that the Mendelian pattern 



