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 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.
Hence, out of four chromatids the two adjacent chromatids are recombinants and other two are original chromatids. Thus four types of gametes are obtained.
1. Crossing over leads to the production of new combination of genes and provides basis for obtaining new varieties of plants.
2. It plays an important role in the process of evolution.
3. The crossing over frequency helps in the construction of genetic maps of the chromosomes.
4. It gives us the evidence for linear arrangement of linked genes in a chromosome.
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.
When two genes are nearer, the probability of occurrence of crossing over between them is limited.
Let A, B, C, D and E be five knots on a string separated by the distances as shown. The probability of making a random cut between two knots is directly proportional to the distance between them. Every cut separates A from E, whereas 5/100th cut only separates C from D.If the knots or genes linearly arranged on a chromosome in randoms are the cross overs, then C and D remain linked, whereas A and E will not show linkage in this situation.
1. It is useful to determine the location, arrangement and linkage of genes in a chromosome.
2. It is useful to predict the results of dihybrid and trihybrid crosses.