236 A TEXTBOOK OF GENERAL BOTANY 
Definition of terms. In discussing heredity there are a few 
technical terms which are very convenient and which are used 
in even the simplest discussions of the subject. A few of these 
are here defined. 
A gamete is a cell that fuses in sexual reproduction. A zygote 
is a cell formed as the result of sexual fusion. Two alternative 
characters, such as tallness and dwarfness, are known as adllelo- 
morphs. An individual is said to be homozygous for a certain 
character when it contains two factors for that character and 
does not contain the factor for the other member of the pair of 
allelomorphs. An individual is said to be heterozygous when it 
contains factors for both members of a pair of allelomorphs. 
When two plants are crossed, the parent plants are known as 
the P, generation, and the resulting offspring as the /, genera- 
tion; when this generation is self-fertilized or interbred, the 
offspring compose the F, generation; the subsequent genera- 
tion is known as the F, generation; ete. 
Dominance. Mendel crossed tall and dwarf varieties of garden 
peas and found that their offspring in the first generation were 
all tall. These may be called tall hybrids, as they were pro- 
duced by crossing tall and dwarf varieties. When these tall 
hybrids were self-fertilized, they gave rise to tall and dwarf 
plants in the ratio of three tall ones to one dwarf. When the 
resulting dwarf plants were self-pollinated, they always produced 
dwart offspring. One third of the tall ones contained two fac- 
tors for tallness and none for dwarfness, and when self-fertilized 
always gave rise to tall offspring only, while the other two thirds, 
which were tall hybrids, produced dwarf and tall, as did their — 
hybrid parents, in the ratio of one dwarf to three tall. These 
tall plants, like those of the preceding generation, consisted of 
one third which had no factor for dwarfness and always gave 
rise to tall plants when self-pollinated, and two thirds which 
were tall hybrids and again repeated the ratio of three tall to 
one dwarf. This relation is shown in Fig. 233. The inheritance 
of the factors for tallness and dwarfness after the crossing of 
tall and dwarf plants is shown by the letters under the words 
