£69 TH age SMIDI DI HUNG 
which each character of the offspring is determined. What this some- 
thing is, whether a material particle, a definite chemical substance, or 
some special arrangement of the molecules, we have no idea. For want 
of a better name it is usual to call it a “* factor.” Thus we should say that 
each ovule and each pollen grain of the green pea contains a “ factor ” 
for green colour ; each ovule and each pollen grain of the pure yellow pea 
contains a “‘ factor” for yellow colour. When he had to deal with two 
alternative characters in a plant, such as green and yellow colour, Mendel 
assumed that any particular gamete was able to contain the factor for 
only one of these characters. In the same gamete the two characters are 
mutually exclusive. Each gamete must be pure for one or other of the 
factors. In the zygote, the individual produced by the union of two 
gametes, on the other hand, the factors for the two characters can both 
have place. 
Let us see how this conception can be applied to the case of the yellow 
and green peas. In Diagram 3 the factor for yellow colour is represented 
YELLOW AND GREEN PEAS. 
DOMINANT CROSSED WITH RECESSIVE 
YELLOW Ty CREEN 
Parent Zycortes 
FEMALE MALE 
P Ganmetes ovutes (¥) < G) POLLEN 
F | Zycotes 
F'| Gametes OVULES We: a 
G)% : 
POLLEN 
F 2 Zycotes & 
IpuRE YELLOW 2YELLOW HYBRIDS | PURE GREEN 
DIAGRAM 3. 
by ¥, that for green colour by G. The pure yellow peas never produce 
anything but yellow ; we may therefore represent their constitution by 
yp one factor having been derived from each parent. Similarly, pure 
G 
sreen will be G|: These will produce gametes 
Y male, Y female, in the one case, 
G male, G female, in the other, and no other kinds are possible. 
If we cross the two, the only possible combination is E , which will 
represent the constitution of the Ist Hybrid generation. What Mendel’s 
