Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

Q: The majority of human genes have two or more possible alleles.

A: How many possible alleles do the majority of human genes have?
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Q: Very low density is the concentration of gas molecules in the mesosphere.

A: What is the concentration of gas molecules in the mesosphere?
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Q: Red blood cells are the most numerous blood cells.

A:
What are the most numerous blood cells?
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