Which of the following best describes a recessive allele?

• A. An allele that determines the phenotype in a heterozygous condition
• B. An allele that is expressed only in the presence of a dominant allele
• C. An allele that is always expressed regardless of other alleles
• D. An allele that is expressed only when two copies are present

Answer: (D)

Verified step by step guidance

1. Understand the concept of alleles: Alleles are different forms of a gene that can exist at a specific locus on a chromosome. They can be dominant or recessive.
2. Define a recessive allele: A recessive allele is one that is not expressed in the phenotype unless two copies are present, meaning the organism is homozygous recessive.
3. Contrast with dominant alleles: Dominant alleles are expressed in the phenotype even if only one copy is present, meaning the organism can be heterozygous.
4. Consider heterozygous vs. homozygous conditions: In a heterozygous condition, one dominant and one recessive allele are present, and the dominant allele determines the phenotype. In a homozygous recessive condition, two recessive alleles are present, and the recessive trait is expressed.
5. Apply this understanding to the options: The correct description of a recessive allele is that it is expressed only when two copies are present, which aligns with the definition of a recessive allele being expressed in a homozygous recessive condition.