When one allele is dominant to another, what is the most common biological mechanism for this dominance?

A. Dominant alleles often encode functional products, whereas recessive alleles often do not encode functional products

B. Dominant alleles subdue or repress recessive alleles

C. Cellular machinery recognizes which allele is dominant and only reads the

instructions from that one

D. A dominant allele on one chromosome causes the other, homologous

chromosome (containing the recessive allele) to be completely inactivated

A. Dominant alleles often encode functional products, whereas recessive alleles often do not encode functional products

Explanation:

One gene can have it's variant forms called allele but in one individual only two allele form of a gene can be present. These two allele form can be the same (homozygous) or different (heterozygous). A dominant allele often encode a function protein of product while recessive allele often code for non - functional product.

Therefore when the dominant allele is present in the chromosome then function products are formed which can be seen in the phenotype and even a single dominant allele produce the functional product like in heterozygous condition so phenotype in heterozygous and homozygous dominant condition appears same. So the correct answer is A.