Foundations of the Mutation Concept. 13 



mutant was heterozygous for a single factor for red. Shull (1914a), 

 unfortunately, from a study of hybrids between rubricalyx and 

 (E. grandiflora which he mistook for pure rubricalyx, drew erroneous 

 conclusions regarding the origin and behaviour of this striking form. 

 Another important feature in its history, already mentioned, is the 

 fact that members of a family of rubricalyx which contains a single 

 factor for red, as shown by the 3 : 1 ratio, can give rise to individuals 

 which are heterozygous for two factors for red. How this may 

 come about has been discussed under the name duplicate 

 mutation. 1 



That two or even three factors for the same character may 

 exist in a given strain, was first shown by Nilsson-Ehle (1909) for 

 wheat, and has since been demonstrated in a number of other 

 forms. Nilsson-Ehle showed for example that while Extra 

 Squarehead wheat has only one factor for red kernels, Sammet and 

 Grenadier have three, and in a later paper (1911) that Swedish 

 Binkel has two. A list of such duplicate determiners has been 

 given by Shull (1914H). It includes glume colour, hairiness of glumes, 

 length of glumes, width of leaves, etc., in oats, and various 

 characters in beet, turnip, gourd, flax, tobacco, bean, pea, velvet 

 bean, wheat, and maize among plants ; also body weight in ducks, 

 fecundity in fowls, skin colour in man, ear length and body size in 

 rabbits, piebald coat colour in mice and the hooded pattern in rats. 

 Of course some of these cases are much better substantiated than 

 others, and the number of cases has since been considerably 

 increased. The Howards (1912) have demonstrated one of the 

 clearest instances of duplicate factors for red in wheat. 



Nilsson-Ehle did not consider the origin of this condition of 

 duplicate factors in wheat, but it appears probable that it has the 

 same explanation as the duplicate condition of the factor for red in 

 (E. rubricalyx. There are, as previously pointed out, 2 at least two 

 ways in which such a result is likely to have come about. 

 (1) Through a locus in a second chromosome having independently 

 undergone the same change or mutation. (2) An individual which 

 would normally be homozygous for one factor for red (RR') might, 

 by a mismating of the chromosome pairs in fertilization or after, 

 become heterozygous for two factors for red (RrRV). In this way 

 an individual giving a 15 : 1 family of offspring could arise from a 

 homozygous member of a 3 : 1 family. It is necessary to assume 



1 Gates 1915b. 

 * Gates 1915b. 



