Question

All organisms have even number of chromosomes." Why?

Answer 1

The reason why the majority of organisms have an even number of chromosomes is that chromosomes are in pairs. ... Another exception would be polyploidy, which occurs when organisms have more pairs of chromosomes than a diploid cell does.

Answer 2

The number is ‘even’ because one whole set is inherited from each of two parents. The number of distinct chromosomes is not necessarily even (humans have 23 pairs). But if you double that, it will always be even.

Of course, in evolution various exceptions occur. Some individuals have odd numbers of chromosomes because of breakage or inheritance anomalies; often these are harmful and/or incompatible with reproduction. But since different closely related species can have different numbers of chromosomes because of various rearrangements that occurred and must have been viable (or they wouldn’t be here today), some unevenness must occur from evolutionary time to time.

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