Hayes and Garber - Cucurbit Breeding

PLANT GENETICS 31
"3. The variability of the F 2 populations produced from such crosses
should be much greater than that of the F\ populations, and if a sufficient
number of individuals are produced the grand-parental types should be
recovered. The fulfillment of this condition comes about from the
general laws of segregation of factors in FI and their recombination in F 2 .
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4. In certain cases F 2 individuals should be produced showing a
greater or a less extreme development of the character complex than
either grandparent. This is merely the result of recombination of
modifiers, as was explained above.
"5. Individuals of different types from the F 2 generation should
produce populations differing in type. The idea on which this statement
is based is, of course, that all F 2 individuals are not alike in their
inherited constitution and therefore must breed differently.
"6. Individuals either of the same or of different types chosen from
the Fz generation should give F z populations differing in the amount
of their variability. This conclusion depends on the fact that some
individuals in the F 2
generation will be heterozygous for many factors
and some heterozygous for only a few factors."
From the standpoint of the student a hypothetical case may
be given to show how the factor hypothesis may be used to
explain the inheritance of quantitative characters. Given two
barley varieties as follows:
Variety 1, average length of internode of rachis 2.0 mm.
Variety 2, average length of internode of rachis 3.6 mm.
Suppose these varieties differ by two separately inherited factors,
A and B, each when homozygous causing a lengthening of the
internode by 0.8 mm.; when heterozygous by 0.4 mm.,
Variety 1 aabb Gamete ab
\r A. o A A r>T> n /ID^I Zygote AaBb
Variety 2 AABB Gamete AB
Combinations in F% would occur as follows :
F2 PLANTS
Fa BREEDING NATURE
1 AABB Would breed true far length of internode of 3.6 mm.
2 AaBB Would segregate from 3.6 mm. to 2.8 mm.
2 AABb Would segregate from 3.6 mm. to 2.8 mm.
4 AaBb Would segregate as F 2 .
1 AAbb Would breed true for length of internode of 2.8 mm.
2 Aabb Would segregate from 2.8 to 2.0 mm.
1 aaBB
2 aaBb
Would breed true for length of internode of 2.8 mm.
Would segregate from 2.8 to 2.0 mm.
1 aabb Would breed true for length of internode of 2.0 mm.
Probably few size characters are as simple in their inheritance as
this illustration. However, the factor notation assists in gaining