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B B
b b
L L l l
B B b b
L l L l
B B b b
L l L l
Fig. 3.4 Crossing over
Crossing over
heterozygous blue long (BbLl) of F 1 hybrid and
double recessive red round (bbll) did not result
in ratio 1:1:1:1 but gave unexpected phenotype
frequency as shown below.
Here, blue round and red long are parental
forms and show greater frequency 88 per cent.
Blue long and red round are recombinant forms
and show lesser frequency 12 per cent. The
dihybrid test cross ratio obtained is 1:7:7:1 and
not 1:1:1:1. This indicates that the genes do
not independently assort. From the above
testcross, it is clear that if dominant alleles or
recessive alleles are present in the different
plants, they tend to remain separate resulting
in increased parental forms. This aspect is called
repulsion.
Coupling and repulsion offered explanation
for higher frequency of parental forms. They
are two aspects of a single phenomenon called
linkage. The genes that are carried on the same
chromosome will not assort independently
because of their tendency to remain linked
together. This is called linkage. The genes
located on the same chromosomes that are
inherited together are known as linked genes.
They tried to reconfirm the law of independent
assortment. But they could not get expected
result because the genes are linked.
The process, which produces recombination of genes by interchanging
the corresponding segments between nonsister chromatids of homologous
chromosomes, is called crossing over. A crossing over between linked
genes allows their recombination during meiosis.
Crossing over takes place in pachytene stage of prophase I of meiosis.
In pachytene stage, the bivalent chromosome becomes tetrad i.e. with
four chromatids. The adjacent nonsister chromatids are joined together at certain
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