Why does a person with normal chromosomes only have two alleles for any single gene trait?

A: A minimum of two alleles is required for the expression of monogenic traits.

B: When a dominant allele is paired with a recessive allele, only the dominant allele is expressed and the recessive allele is silent.

C: One allele for the monogenic trait is on the paternally derived chromosome and the other allele is on the maternally derived chromosome.

D: Expression of more than two alleles of any single gene trait results in enhanced expression of recessive alleles and suppressed expression of dominant alleles.