The chromatids in which crossing over has occurred, have new combinations of genes and are called cross overs. According to its occurrence in the germinal or somatic cells following two types of crossing over have been recognised.
A. Germinal or meiotic crossing over: Commonly crossing over occurs only in the germinal cells of reproductive organs during the process of gametogenesis which includes meiosis. This type of crossing over is called germinal or meiotic crossing over. It is universal in its occurrence and has great genetic significance.
B. Somatic or mitotic crossing over: Sometimes crossing over may occur during mitosis of somatic cells. This type of crossing over occurs in rare cases, has no genetic significance and is caned somatic or mitotic crossing over. It has been observed in body cells of Drosophila by Stern and in the fungus Aspergillus nidulans by Pontecorvo.
By somatic crossing over two chromosomes with unequal chromatids are formed. In the following division, in 50% of cases two different alleles go to the, same daughter nucleus and the original situation is restored. In the other 50%, identical alleles go to the same pole, a situation comparable to anaphase II segregation with double reduction. Thus, in one cell two recessive alleles are present and recognizable, marking the event of crossing over. |
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