THE ORIGIN OF VARIATION 23 



Haldane (1932, p. 37 and foil.) has distinguished six modes 

 of genetic representation which are tabulated below, though 

 it is by no means clear that all are found among animals. 



(1) Characters determined by extra nuclear factors (plas- 

 mons). Haldane thinks that some of Goldschmidt's results 

 (1923) on sexuality in moths illustrate this (cf. also Boycott, 

 Diver and others (1930) ; Toyama (191 2) on heredity of 

 voltinism in silkworms) . 



(2) Characters determined by a single gene. 



(3) » » » several genes. 



(4) ,, ,, ,, genes which undergo re- 



arrangement (but not alter- 

 ation in number and 

 quality). 



(5) „ „ ,, genes some (but not all) of 



which are represented more 

 or less than twice in aber- 

 rant types of individual, e.g. 

 non-disjunction. 



(6) „ „ j, genes the total diploid num- 



ber of which is increased 

 by one or more whole sets 

 (polyploidy). 



Before proceeding to discuss these various modes of genetic 

 representation we ought to remind the reader that the term 

 ' mutation ' is applied either in a narrow sense to changes in 

 a single gene or to the various phenomena of chromosomal 

 abnormality and other variations dependent on variation in 

 the genetic basis of characters. It seems clear that in Haldane's 

 list the differences enumerated under 2, 3 and 4 are chiefly 

 related to differences in the distribution of characters and 

 to recombination. Differences in sex and fertility are also 

 associated with 4 (attachment of X to Y chromosome). 



Morphological change seems to be associated with 5 in 

 plants, and Haldane states (I.e. p. 52) that the presence of an 

 extra chromosome generally produces a very unhealthy type 

 (cf. production of intersexes possessing the second and third 

 chromosomes in triplicate and the X in duplicate in Drosophila 

 (Morgan, Bridges and Sturtevant, 1925, p. 156) ). It is not 

 clear if any morphological changes are associated with this 



