Which pair of chromosomes can contain two very different chromosomes and still be considered normal?  

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Humans contain 22 pairs of autosomes and the 23rd pair is called the sex chromosomes. It is possible to have two very different chromosomes when you are referring to the sex chromosomes. Since males are XY, these are two very different chromosomes in terms of shape and their size. However, although males are heterogametic for their sex chromosomes, this combination produces a child that develops male characteristics. A single gene called SRY on the Y-chromosome determines maleness, the production of the male hormone testosterone and development as a male child. Females are homogametic for their sex and their chromosomes are XX.

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