Hayes and Garber - Cucurbit Breeding

MODE OF REPRODUCTION IN RELATION TO BREEDING 49
cross between different varieties a large percentage of the plants
are homozygous and will breed true for their characters if
selffertilization
is continued. The number of different biotypes
which can be isolated from a cross depends upon the number of
allelomorphic pairs of factors involved and their linkage relations.
Formerly the heterozygous condition was believed to carry
with it an increased developmental stimulus. It was also believed
that this stimulus was greater when the mate to an allelomorphic
100*
Percentage of Heterozygous
Individuals in each Selfed
Generation when the Number
of Allelomorphs Concerned
Are: 1,5,10,15.
12345
Segregating Generations
FIG. 12. The percentage of heterozygous individuals and the percentage of
heterozygous allelomorphic pairs in the whole population in each generation
of self-fertilization. (After Jones.)
pair was lacking than when both were present.
The physiological
cause of this growth stimulus was not known although it
was recognized that "the greater the degree of heterozygosis the
greater is the vigor of the resulting plant" (East and Hayes, 1912).
A considerable number of studies showed that the rapidity and
amount of cell division was increased.
A Mendelian explanation of this growth stimulus which is so
frequently found in crosses, has been advanced.
Jones (1918) has
explained the vigor of F\ which has been called heterosis on the