Recent Advances in the Study of Heredity. i 77 
O y O G 
o Y 
1 
o 
o Y 
1 
o 
1 
o Y 
i 
1 
o 
o Y 
o 
6° 
o Y x 6 
A. B. 
Cross between yellow and extracted Cross between yellow and green, 
green in F 5 . both of pure race. 
Our interest centres in the possible difference between the 
two F 2 generations produced by crosses A and B. For we know, 
or expect, yellow to be dominant over green in a cross of type B; 
a fortiori, on the contributional view should we expect Y to dominate 
over the G used in Cross A. 
The Mendelian expectation is that the proportions of yellows 
and greens will be the same in the two crosses. The expectation 
based on the theory of ancestral contributions is that the great 
weight of yellow ancestry, as it may be expressed, behind the green 
in cross A will diminish the relative number of greens in the F 2 
from that cross. 
The result of the experiment 1 is entirely in accord with the 
Mendelian expectation, which proves that the Mendelian inter¬ 
pretation may be the correct one, but that the contributional 
explanation is incorrect. The actual numbers obtained in this F 2 
were 105,045 yellows and 34,792 greens, which gives a ratio of 
75T2% yellows and 24-88% greens. 
All that the experiment just described proves is that the somatic 
characters of the ancestors behind the parental generation are 
not effective in determining the proportions of the yellows and 
greens in the F 2 generation in question ; it does not prove, though 
it strongly suggests, that the characters of the actual forms crossed 
play no such part. The experiment is incomplete, inasmuch as both 
are crosses between yellows and greens—the difference between the 
two crosses lying only in their ancestry. In my next Lecture 
I shall describe an experiment which proves the correctness 
1 For details of this experiment see Proc. Roy. Soc. B, Vol. 81, 
p. 61, 1909. 
