120 REPORT OF THE CONFERENCE ON GENETICS, 
The gametic formula of the green pea would be (y+G) and the 
zygote green. It will be observed that both the yellow and the green 
peas are homozygous in G. 
The wild pea has yellow cotyledons, and on the view that its yellow 
is based on green, the evolution of the green pea from the yellow might 
be explained by the mutational absence of the yellow factor in a certain 
gamete. 
With regard to the pair of Mendelian contrasts, round and wrinkled 
seeds, the precise physiological nature of the irregular shrinking of 
wrinkled peas is hardly yet understood. Mr. R. P. Gregory found that 
in round peas the starch grains were large and oval in shape, while in 
wrinkled peas they were very small, rounded, and frequently compound.* 
Professor de Vries apparently regards the wrinkling of peas as similar 
in nature to the wrinkling of the sugar maize, z.e. due to lack of starch. 
He states that “the sugar is only accumulated as a result of an incapacity 
of changing it into starch.” f If this is so, then the Mendelian pair, round 
and wrinkled, might be regarded as presence and absence of roundness 
(starchiness) on a wrinkled basis (sugariness), presence being dominant 
over absence. Similarly, the Mendelian contrasting pair, tall and dwarf 
stems, might be regarded as presence and absence of tallness on a dwart 
basis, presence being dominant over absence. 
SwEET PERAs. 
In my experiments with Sweet Peas, four pairs of Mendelian 
characters have, so far, been met with, viz. coloured and white flowers, 
purple and red flowers, tall and dwarf habit, long and round pollen 
erains—the first-named of the pair being dominant over the other, which 
is recessive. 
With regard to the first two pairs, it has been clearly demonstrated 
by Mr. Bateson, Miss Saunders, and Mr. Punnett + that three pairs of 
unit-characters are concerned in the sap colours of sweet pea flowers, viz. 
presence (C) and absence (c) of first colour factor (probably a colour- 
forming stuff), presence (R) and absence (r) of second colour factor (prob- 
ably an enzyme), presence (B) and absence (b) of blueness. 
A coloured flower depends on the simultaneous presence in the zygote 
of the two colour factors C and R. I both the factors are present, the 
flower will be red; if either C or R is absent, the flower will be white. 
If the three factors C, R, and B are present, the flower will be purple; if 
B is absent, the flower will be red; if either C or R is absent, the flower 
will be white. From this it is evident that the two pairs of characters— 
coloured and white flowers, purple and red flowers, which in my experi- 
ments behaved as simple Mendelian pairs—are really due to three pairs 
of unit-characters, each pair being presence and absence of a certain unit, 
presence being dominant over absence. 
With regard to the remaining characters met with in my experiments, 
tall and dwarf habit might be regarded as presence and absence of 
* The New Phytologist, vol. ii. (1903), p. 226. 
t Species and Varieties (1904), pp. 283, 289. 
tf Report III. Lvol. Com. Roy. Soc. 1906, 
