Wednesday, September 30, 2009

Linkage and Crossing Over (Short Note)

According to Conservative estimate there are thousands of genes in four pairs of chromosomes of Drosophila. It means that each chromosome contains large number of genes, similar is the situation in all organisms. The chromosomes behave as single units. All the genes in a given chromosome tend to remain together during inheritance. This tendency of genes in a chromosome to remain together is called ‘Linkage’. This linkage is not absolute and the genes do not remain locked up in the same chromosome for ever. Otherwise the inheritance of traits would also have remained constant. During meiosis the homologous chromosomes come together and form pairs, a process called ‘Synapsis’. Soon after they sometimes exchange segments mutually, a process is called ‘Crossing over’. This exchange occurs ranelonly along the length of chromosome. After separation the chromosomes carry some genes that were earlier located in different member of their pair of homologous chromosome. Such exchanges of chromosomal segments or crossing over may occur at more than one point in homologous chromosomes in single meiotic division.

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