ON SUCCESSIVE DUPLICATE MUTATIONS. 2I9 



found, moreover, that the same strain may be in one case monom- 

 erous and in another case dimerous for this character; and that 

 while, for example, Grenadier wheat possessed three independent 

 units for red, Extra-Squarehead possessed only one. The origin 

 of the original "factor "for red may be accounted for in the wheats 

 as in CEnothera riibricalyx, through the chemical transformation 

 of one chromosome or a pair of homologous chromosomes. The 

 duplicate condition for the character R may have arisen (i) 

 through a chemical mutation in a second pair of chromosomes, (2) 

 through a re-mating of the chromosomes (RR) forming a homo- 

 zygous pair. The latter method is for various reasons the more 

 probable. 



Although the original CEnothera ruhricalyx wis a monohybrid 

 and continued so for at least two generations, yet in subsequent 

 generations involved in crosses with CE. grandiflora, 15:1 or 

 di-hybrid ratios were derived from the offspring of members of 

 3:1 families. This can best be accounted for by supposing that 

 in a plant (RR) homozygous for one factor, a re-grouping of the 

 chromosome pairs occurred. This re-grouping involves merely 

 an exchange of mates on the part of the chromosomes RR so 

 that they now belong to different pairs. The formula for the 

 plant may now be written RrR'r', i. e., the plant is heterozygous 

 for two independent units for red and its offspring will give a 

 15:1 ratio. 



The second mutation, producing the duplicate condition for 

 R, is thus probably a purely mechanical process, while the origi- 

 nal mutation which produced the "factor" R is a chemical 

 change of wholly different nature. It is possible that in some 

 cases the duplicate and triplicate conditions also arise through 

 the chemical transformation of additional chromosomes. 



When a 15:1 family arises from a 3:1 or 5:1 family, as has 

 happened several times in CE. ruhricalyx hybrids, it is necessary 

 to assume that the regrouping or remating of chromosome pairs 

 which led from the monohybrid to the dihybrid condition, took 

 place at fertilization, or at any rate early in the ontogeny, and 

 is then handed down to the germ cells by mitosis. The chro- 

 mosomes are known to be paired in the somatic divisions, and 

 it seems probable that the manner of pairing set up in fer- 



