which part of sex chromosomes does not show crossing over
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Since there is only one Y chromosome in males (none in females), it is kind of a moot point. There is no sister chromatid to recombine with. Even if there was recombination between the 2 Ys in a tetraploid cell, after DNA replication, recombination would not change the Y because they are exactly the same. So any recombination at that point would not alter the chromosome at all. This is one reason why the Y can be used to trace the ancestral origin of males. Also, there are actually very few genes on Y.
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Crossing over is defined as the phenomenon of recombination of the genes occurring due to interchange of chromosomal segments at the time of pairing.
In an experiment performed on male drosophila some chromosome doesn't show crossing over. No crossing over is observed in male drosophila because they possess unusual prophase I of meiosis in which no synaptonemal complexes and chiasmata develops in the chromosome.
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