Foundations of the Mutation Concept. 16 



However, the recent breeding .experiments with Drosophila 

 have disclosed cases (Bridges 1917) of duplication in which the 

 genetic behaviour is as though a group of genes from the middle of 

 one X-chromosome has become attached to the end of the other 

 X-chromosome in a female. Another exceptional case, discovered 

 by Bridges and reported by Morgan (1919), is explained on the 

 assumption that a piece from the second chromosome has become 

 attached to the middle of the third chromosome. In the resulting 

 race, when these chromosomes separate and recomhine in reduction 

 and fertilization, zygotes which receive the deficient second 

 chromosome fail to develop unless they also receive the third 

 chromosome with the additional (duplicate) piece. We are 

 therefore at liberty to suppose that a redistribution of certain 

 chromatin elements, rather than a fresh transformation of a new 

 determiner has taken place in certain instances. But such cases 

 will usually involve a group of factors simultaneously rather than 

 a single one. 



Recessive Mendeliau Factor Mutations. 



If we turn now to (Enothera gigas nanella as the type of a 

 recessive Mendelian factor mutation, the manner of its origin 

 seems clear from its hereditary behaviour. De Vries {1915) has 

 brought together the evidence concerning its behaviour. (E. gigas 

 produces this dwarf in 1 2% of its offspring, i.e., as mutations. 

 But certain individuals of gigas are known, from observations of 

 Schouten, Gates and de Vries, to produce as many as 18% of dwarfs. 

 Such individuals are evidently heterozygous, arising from the union 

 of a normal gigas germ cell with one which has mutated so as to 

 carry the dwarf factor instead of that for tallness. Theoretically 

 they should give 25% of dwarf offspring but the number is reduced 

 by their lesser viability. This phenomenon of producing in the 

 offspring a large number of a new form has been called by Bartlett 

 (1915) mass mutation. In such a case it is obvious that the original 

 change or pre-mutation, an invisible change in a germ cell, must 

 have occurred in a plant at least two generations earlier than that in 

 which the mass mutation exhibits itself. The 1 or 2 per cent of 

 mutants arising from other gigas plants are the result of the chance 

 meeting of two germ cells both of which have mutated so as to carry 

 the dwarf instead of the tall condition. This interpretation is clearly 

 substantiated by the fact that (E. gigas nanella when crossed with 

 CE. gigas behaves as a simple Mendelian recessive. It has 28 

 chromosomes, like the parent form. 



