HEREDITY AND MUTATION AS CELL PHENOMENA 
The suggestion therefore occurs to one that it may be in the premeiotic 
rather than the meiotic divisions of the germ cells that the "crossing- 
over" of material from one chromosome to its mate occurs. Person- 
ally I have never seen such an intimate relation between the members 
of chromosome pairs as is to be observed in the somatic cells of Dro- 
sophila. The absence of crossing-over in the male is, however, a 
difficulty with any hypothesis yet proposed. 
Returning now to the subject of Oenothera, there is at least one 
mutation which is fundamentally chemical in nature. In the origin 
of Oe. ruhricalyx we see exhibited the type of change which must occur 
whenever a new monohybrid Mendelian character appears. In all 
such cases it is only necessary to assume that one chromosome, or a 
portion of one, underwent a change in its chemical nature. The 
meiotic mechanism performs the function of distributing this chromo- 
some and its descendants so that a Mendelian 3 : i ratio in the off- 
spring will result. 
There have been many suggestions as to why the new character is 
dominant in one case and recessive in another. Cases like that of 
Oe. ruhricalyx, in which the new character is dominant, are rare. 
It is possible that dominance may occur whenever the change results 
in the increased production of a substance, and recessiveness whenever 
there is loss of or reduction in the capacity for producing such a 
substance in the cell. This would apply to both dominant and re- 
cessive whites. Thus in the case of dominant White Leghorn fowls a 
substance is present in quantity which inhibits color production, 
while in recessive white flowers it is clear that the capacity* for pro- 
ducing color has in one way or another, been almost completely lost 
or suppressed. One may suppose that this color-inhibiting sub- 
stance is one of the substances produced by the cells in all Leghorn 
fow^ls, but that in the White Leghorn it has been very largely increased 
through a chemical modification on the part of a chromosome. 
In the present state of our knowledge it is impossible to determine 
the precise chemical nature of the change which produced Oe. ruhri- 
calyx from Oe. rubrinervis . The difference consists in an enormously 
increased capacity for anthocyanin formation in every cell of the 
10 For an explanation of later 15:1 ratios, see Gates, 1915, On successive dupli- 
cate mutations. Biol. Bull. 29: 204-220. 
" Of course this suppression may occur in various ways. Thus it has been 
suggested (Robertson, J. B. 1914. Bloodstock Breeders' Rev. Nos. i, pp. 16-31; 
2, pp. 91-107; Reviewed in Exp. Sta. Record 32: 361. 1915) that in the gray horse, 
