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Why chromosomes differ in length ? Why one arm is shorter and one in longer (except metacentric). What is the reason for this design ?
Answer 1:

It's happenstance, not design. Chromosomes are the length they are because of evolutionary accident. In some cases there are important adaptive reasons for why chromosomes would be of a certain length (for example, on the mammalian Y chromosome, there are really only a handful of genes that are important, because females don't have that chromosome at all, which means that the Y is really short).


Answer 2:

Chromosomes just happen to be different in number, size, and lengths of the arms. Life started from single-celled organisms like bacteria that just have a circle of double-stranded DNA and no chromosomes. Then each species of life changed in different ways to get all the different forms we see today.



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