230 Mendel's Laws of Alternative Inheritance in Peas 
258 plants were grown, which produced 8023 seeds ; and these seeds had the 
characters indicated by the law of segregation. The combinations of pure 
ancestral character, which are here possible, are four ; since the gamete of either 
sex may conceivably be capable of transmitting either " yellowness " or " green- 
ness " of cotyledon ; Mendel's assumption is that every gamete inherits only one 
of these " alternative " characters, half the gametes of either sex inheriting each 
character, or the power of transmitting it, so that there may conceivably be the 
following combinations : 
$ yellow X yellow 
$ yellow X green (/ 
$ green x yellow J* 
? green x green (f. 
But by the law of dominance, if unlike gametes unite, the resultant seed has 
the dominant character, while if similar gametes unite the resultant seed has 
naturally the character transmitted by both its constituent gametes. It follows 
that three of the above combinations give rise to seeds with yellow cotyledons, 
and only one to green seeds ; and if each combination occurs equally often, the 
chance that a given hybrid seed of the second generation will be yellow is |, the 
chance that it will be green is |. 
Now, of the 8023 seeds of Mendel's second generation, 6022 were yellow and 
2001 green. Seeds of intermediate colour did not occur. The ratio between 
either of these numbers and the number of seeds observed is an excellent 
approximation to that required by Mendel's law of segregation*. The plants 
of the first hybrid generation invariably bore seeds of both colours, and as a 
rule seeds of both colours were associated in the same pod. Pods containing 
only yellow seeds did occur, pods with green seeds only did not. This is all in 
accordance with the law of segregation; for the number of peas per pod being 
from six to nine, the chance of getting a pod with yellow seeds only is from 
(f)" for pods with six seeds to (f)' for pods with nine seeds, or from 0'18 
to 0"075 ; so that about 18 per cent, of smaller, and about 8 per cent, of larger 
pods, should contain only yellow seeds ; but the chance of a pod containing even 
six green and no yellow seeds is only (J)" = ^j-^-^-^. Assuming that there were some 
1500 pods on the plants, it is clear that the absence of pods without yellow seeds 
is in good accord with Mendel's law of dominance. 
The third hybrid generation must, if the foregoing statements are true, be 
heterogeneous, for the hybrid seeds of the second generation are said to be of 
three kinds ; those formed by the union of two gametes each transmitting only 
dominant characters, those formed from gametes each transmitting only recessive 
characters, and those formed by the union of dissimilar gametes. Therefore the 
offspring of these seeds should also be of three kinds ; in the case before us, one 
kind, produced from yellow seeds, should give rise to yellow-seeded plants, like 
* See below, Table I. p. 233. 
