Hayes and Garber - Cucurbit Breeding

PLANT GENETICS 21
in partial shade in the greenhouse under temperatures of 30 to
35C. only white flowers are produced. If those same plants
are brought into another greenhouse with temperatures of 15 to
20C. the flowers which then develop are the normal red color.
It is pointed out that what this red primula inherits is not a red
flower color but the ability to produce a certain flower color
under certain conditions of the environment. Non-inherited
variations have no value as a means of producing new varieties or
strains. Such variations are, however, of importance to the
breeder. For example, a small shriveled seed of wheat has the
same inherited characters as a large, plump seed of the same
pure line, nevertheless, the seedling produced by the shriveled
seed may get an unfavorable start. Familiar examples of nonheritable
variations are differences in height of plants, within a
variety, which are dependent on differences in food supply,
moisture, or sunlight.
Inherited variations may be placed in two classes: (1) mutations,
(2) new combinations.
Mutations are due to a sudden change in the hereditary factors
of an organism, or to the loss of a genetic factor.
In some cases
mutations result from abnormal chromosome behavior during
the process of cell division. Before we can discuss profitably
the reason why mutations occur it will be necessary to know
much more about the nature of hereditary factors than we now
do. Mutations are sometimes of much value to the breeder.
Examples of mutations of economic importance
will be found
under a discussion of the breeding of various crops. When a
desirable mutation occurs it can be utilized as a means of
producing a new race. As there is no known means of artificially
inducing mutations, the breeder can not depend on them as a
means of producing improved varieties.
New combinations result from crossing varieties which contain
different hereditary factors. The first generation
of a cross
between homozygous parents which differ in a certain character
may resemble either the one or the other parent or may be intermediated,
but all FI plants will be of like habit. F z plants, however,
are of different kinds, due to the segregation of hereditary
factors in the germ cells of the FI plants. New combinations of
factors may occur and thus new individuals may be produced
which have some of the characters of the one parent combined
with some characters of the other. In some cases characters