Definition : The process of exchange of genetic material between two non-sister chromatids of homologous chromosomes is called Crossing over.
Introduction : If independent assortment takes place ( when genes are located on different chromosomes), we expect a test cross of ratio 1 : 1 : 1 : 1
Fig. : Showing Crossing over (after chromosome replication) at 4 Stranded Stage.
But in above case, very low recombinant classes appeared, it may be concluded that two genes are on the same chromosome and the appearance of recombination in low number resulted from Crossing Over.
Usually, chromosomes undergo breakage during meiosis. Thus, there is a mechanism by which group of genes on the same chromosomes, changes/exchanged with a similar group of genes on a homologous chromosome. Now, it is the distance between different linked genes in the chromosome, that determines, the percentage of crossing over. It, two genes are far apart on the same chromosome, there are more changes of crossing over/ recombination.
- From the figure, we can see that at a particular point only two of the four chromatids are involved in the exchange of their parts and to produce 50% recombinant gametes.
In such case, genes should be located such part, so as to allow crossing over during reductional division (in such conditions genes behave as if they are located on different chromosomes).
Mechanism :
During meiosis, in pachytene, crossing over takes place. At this stage, two non-sister chromatids of homologous chromosomes get connected at (with one another) a point called chiasmata (It is a point from where genetic exchange occurs). The genes which are located far off from each other in same chromosomes have more chances of crossing over. (While those genes which are closer to each other in same chromosome display LINKAGE)