Hayes and Garber - Cucurbit Breeding

50 BREEDING CROP PLANTS
basis of dominance and linkage.
In comparing crosses with their
parents it is quite common to find that the F\ generation has a
higher value for nearly every growth character than has the average
of the parents. Modern geneticists recognize that each
character is due to the interaction of many inherited factors. If
each growth factor gives as great an effect when heterozygous as
when homozygous or proves partially dominant when heterozygous,
it would be easy to explain heterosis by the actual physiologicial
growth development which is a part of the normal
expression of a particular inherited factor. This explanation
was formerly advanced to account for heterosis but was considered
unreliable, as it was difficult to account for the almost
universal decrease in vigor when such plants as maize were
selfed. This can be explained by the facts of linkage, as it is
possible to have a greater number of different growth factors
present in a heterozygous than in a homozygous individual.
The explanation has much in its favor.