How many recessive alleles are needed to show the recessive trait?

He is heterozygous dominant. The dominant trait masks the expression of the recessive trait.

The reason many harmful alleles are recessive is because the harmful alleles that were dominant stopped the carrier from reproducing so the allele was not carried on. A dominant trait is expressed if...

The recessive trait shows when the offspring has two recessive alleles and no dominant allele (aa instead of AA or Aa).

no

Depends on the characteristic. Assuming you're talking about a very simple monogenic characteristic (ie. a characteristic which is dependent on one gene only) your dominant allele will always trump...