Which of the following best describes a dominant allele?

• A. An allele that is only expressed in homozygous individuals
• B. An allele that masks the effect of a recessive allele
• C. An allele that does not affect the phenotype
• D. An allele that is always expressed

Answer: (B)

A dominant allele is characterized by its ability to mask the expression of a recessive allele when both are present in a heterozygous individual. This means that if an individual carries one dominant allele and one recessive allele for a particular trait, the traits associated with the dominant allele will be expressed in the phenotype.

For example, in the case of flower color, if purple color (dominant) is represented by "P" and white color (recessive) by "p," a plant with the genotype "Pp" will display purple flowers because the presence of "P" masks the effect of the "p" allele. This is a fundamental concept in Mendelian genetics, highlighting how dominant alleles influence phenotype regardless of the presence of a recessive counterpart.

While some may think a dominant allele is always expressed, as suggested by one of the choices, this can lead to confusion without considering the context of genotype. Thus, the key takeaway is that dominant alleles are defined specifically by their ability to override the effects of recessive alleles.