Hayes and Garber - Cucurbit Breeding

INTRODUCTION 13
correlated may be recombined, a greater degree of segregation is
possible than when factor correlation is absolute.
of correlated
In general we may say that the number of groups
or partially linked factors is not greater than the number of
chromosome pairs. Whether the above explanation is correct,
partly so, or entirely wrong, it is a convenient theory with which
to account for a large body of facts.
It allows for classification
of facts in such a way that correct breeding methods may be used.
Mendel's law may then be summarized from the standpoint
of the plant breeder as follows :
L Plants breed true for certain characters when all factors
necessary for the development of the character are in a homozygous
condition. There is a relative stability of factors. Changes
in factors or " mutations" are far too infrequent to furnish a basis
for a system of breeding.
2. There is independent segregation of certain factors.
3. Partial coupling of certain determiners sometimes is found.
The degree of linkage in transmission is quite constant.
4. Perfect coupling of certain factors occurs, i.e., constant
association of characters in inheritance.
As a possible exception to the usual behavior we may mention
apparent segregation in the somatic cells of some hybrids. In
some forms these changes apparently occur frequently enough to
be of practical selective value.
We may summarize Mendel's law in another way by saying
that the first generation cross between stable forms may resemble
one parent in one character, the other parent in another character
All mem-
or may be intermediate in the character in question.
bers 1 of FI are of uniform habit. Segregation occurs in F 2 and
"segregation of potential characters in the germ cells of hybrids
and their chance recombination" (East and Hayes, 1911) may be
considered as a general law. In F 3 and later generations some
forms breed true, others segregate.
Homozygous forms may be obtained which contain the
desirable
characters of both parents. Such forms are as stable
as races which ha\e been bred by straight selection.
Hybridization as a Means of Producing Variations. A quite
recent explanation for the cause of germinal variation and
therefore the main cause of evolution is that of Lotsy (1916),
1 The meaning of F\, F^ etc., and other genetic terms not explained in the
text is given in the glossary.