Hayes and Garber - Cucurbit Breeding

PLANT GENETICS 27
The diagram illustrates a change in linkage relations due to a
cross-over. C and W are located in the same chromosome of one
parent and c and w in homologous loci of a similar chromosome
of the other parent. If there was perfect linkage the only
gametes produced would be CW and cw. Owing to a cross-over,
however, Cw and cW are also obtained although less frequently
FIG. 7. Diagrammatic representation of crossing-over and results. At the
left, the two original chromosomes. In the middle, the twisted condition of
the chromosomes in synapsis and their subsequent separation. At the right,
the four types of chromosomes which result. (After Babcock and Clausen.)
than the combinations CW and cw. The following outline
expresses the result on a percentage basis:
CW 38.7 percent.; cw 38.7 per cent. cW 11.3 per cent.; Cw 11.3 percent.
non-cross-over gametes
cross-over gametes
Accepting the view that factors are located in particular places in
the chromosome, the value of the cross-over hypothesis in explaining
degrees of factor linkage becomes apparent. If certain combinations
of factors occur with less frequency than others, this
means that the breeder must grow a much larger population in the
segregating generations in order to obtain the combination desired
than when the factors are independently inherited.
Inheritance of Quantitative Characters. Many of the important
characters of economic plants are size or quantitative
characters, such as height of plants, size of seed, or relative date
of maturity. It was at first thought that these characters did
not follow Mendel's law. The discovery that color characters
were frequently due to the interaction of several inherited factors
led to the explanation of the inheritance of size characters by
similar means. Numerous controlled crosses have been studied.