70 GENETICS IN RELATION TO AGRICULTURE 
experiments it so happened that one member of each of the seven pairs 
of characters displayed complete dominance. 
The explanation for the appearance of the recessive character in 
the second generation and in subsequent generations rests on the fact 
1 ff & 
T Pollen 
Grains 
Ovules 
t Tit t\t 
TALL HYBRID DWARF 
Fia. 34.—Diagram showing factor history 
in a cross between tall and dwarf peas. 
that the contrasted characters are 
represented in the germ cells by units 
or factors. The factor for tallness 
may be represented by T and the 
factor for the contrasted character 
dwarfness by t. The relations which 
exist when plants bearing these dif- 
ferent factors are crossed are shown 
in Fig. 34. In the tall race of plants 
the gametes all bear the factor 7, 
so that since any individual of this 
race arises from the union of two 
germ cells its genetic constitution 
with respect to this particular factor 
is TT. Similarly the genetic con- 
stitution of any plant of the dwarf 
race is represented by tt and it pro- 
duces germ cells each of which bears 
the factor t. When tall and dwarf 
plants are crossed, the hybrid receives 
a factor JT from one germ cell and a 
factor ¢ from the other, so that the 
tall hybrids which are produced are of 
the genetic constitution Tt. 
In the production of germ cells 
and in the union of these germ cells 
to produce the individuals of the 
second generation is seen the opera- 
tion of Mendelian principles. The 
contrasted units J’ and ¢ separate in 
the germ cells of the offspring so that 
a particular germ cell receives only one of these factors, either 7 or f. 
Half the germ cells consequently bear the factor JT and half bear the 
factor t, and this is true of both pollen grains and ovules. 
When a tall hybrid plant is self-fertilized, therefore: 
_a T ovule may be fertilized by a T pollen grain producing a TT plant, tall, 
a T ovule may be fertilized by a é pollen grain producing a T¢ plant, tall hybrid, 
at ovule may be fertilized by a T pollen grain producing a Tt plant, tall hybrid, 
at ovule may be fertilized by a ¢ pollen grain producing a tt plant, dwarf. 
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