Having a bent thumb is a recessive trait in humans. the trait is controlled by a single gene. how many recessive alleles must a person have to have a bent thumb?

The answer is; 2

This is because humans are diploid organisms- meaning they have two sets of chromosomes each carrying one of the sets of gene alleles. The expression of a  recessive allele is masked by the sister dominant allele, i.e if an individual has both of the alleles. However, for the recessive allele to be expressed, the individual must have both alleles being recessive.

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