How many copies of the recessive allele are needed to express an autosomal recessive trait?

• A. One
• B. Two
• C. Three
• D. None

Answer: (B)

To express an autosomal recessive trait, an individual must possess two copies of the recessive allele. This is because by definition, an autosomal recessive trait occurs only when both alleles at a gene locus are recessive. In genetic terms, an allele that is dominant will mask the presence of a recessive allele.

In the case of an autosomal recessive trait, even if one dominant allele is present at the locus, the presence of the recessive trait will not be observed phenotypically. Therefore, individuals must inherit one copy of the recessive allele from each parent, resulting in the homozygous recessive condition (often denoted as "aa"), for the trait to manifest.

Interest in understanding these inheritance patterns showcases the principles of Mendelian genetics, where phenotypic traits are determined by the genotype—an essential aspect to grasp when exploring genetic inheritance.