BOTANY Higher Secondary Second Year - Textbooks Online

points called chiasmata. Crossing over occurs between the nonsister
chromatids of paired chromosomes in the region of chiasma. At each
chiasma, the two nonsister chromatids break, exchange their segments
and rejoin resulting the crossing over.
5 M
A ←⎯⎯⎯ ←⎯⎯⎯ 50 M ⎯⎯⎯→
⎯⎯⎯→B←
25 M →C←→D← 20 M →E
Fig. 3.5 Illustration to explain the probability of crossing over
Hence, out of four chromatids the two adjacent chromatids are
recombinants and other two are original chromatids. Thus four types of
gametes are obtained.
Significance of crossing over
Crossing over leads to the production of new combination of genes
and provides basis for obtaining new varieties of plants.
It plays an important role in the process of evolution.
The crossing over frequency helps in the construction of genetic maps
of the chromosomes.
It gives us the evidence for linear arrangement of linked genes in a
chromosome.
Gene mapping
Genes are arranged linearly in a chromosome. The point in a
chromosome where the gene is located is called locus. The diagrammatic
representation of location and arrangement of genes and relative distance
between linked genes of a chromosome is called linkage or genetic map.
The unit of genetic map is Morgan or centimorgan. When the
percentage of crossing over between two linked genes is 1 per cent, then
the map distance between the linked genes is one morgan.
There is a greater probability of occurrence of crossing over, when the
two genes are farther apart in a chromatid. The probability of crossing over
between two genes is directly proportional to the distance between them.
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