ON SUCCESSIVE DUPLICATE MUTATIONS. 2O5 



for capsule form in Bursa bur sa- pastor is and, at the same time, 

 of the mutant B. Heegeri. That hypothesis will not, however, 

 apply to the probably more frequent cases in which duplicate 

 factors for a particular character are found without any other 

 mutation having taken place. An explanation will therefore 

 have to be found for the duplicate or triplicate condition in wheat 

 or in any other organisms in which it occurs. 



It is the purpose of the present paper to discuss more pre- 

 cisely the manner in which such monohybrid characters originate 

 and particularly the way in which they may afterward become 

 duplicate or triplicate. (Enothera rubricalyx affords a typical 

 case of a mutant originating as a monohybrid, probably through 

 a transformation in one chromosome or one pair of chromosomes. 1 

 I have pointed out elsewhere 2 that when the duplicate or tripli- 

 cate condition occurs it might be reasonably supposed to have 

 arisen through the same general change having taken place 

 independently in two or three different chromosomes of the 

 x series. 



In an original mutation of this kind the new character of 

 course forms a pair by contrast with the old unaltered character. 

 If a single chromosome has undergone this change and the new 

 condition is dominant, then a heterozygous mutant Aa will be 

 produced having the new character but splitting in its offspring 

 in a 3:1 ratio. This is the way in which (Enothera rubricalyx 

 originated from (E. rubrinervis, as I have shown elsewhere. 1 



If now in the mutant race one or both members of a second 

 pair, a' a', of chromosomes undergoes a corresponding change, 

 to A'a', or A' A' then we shall have duplicate factors AA' for 

 the same character, and in the offspring of such individuals 

 the new type and the original type would appear in the ratio of 

 15:1. A similar mutation in a third pair would give the triplicate 

 condition with a ratio of 63:1. 



It may be pointed out that this assumption of similar changes 

 in different members of the x series of chromosomes is by no 



1 See Gates, R. Ruggles, 1915. "On the Origin and Behaviour of Oenothera 

 rubricalyx," Journ. of Genetics, 4: 353360. 



2 Gates, "The Mutation Factor in Evolution," p. 317, Macmillan, London, 

 IQI5- 



