Final answer:
The statement is true; a recessive allele is indeed masked by the presence of a dominant allele in a heterozygote, resulting in the expression of the dominant trait only.
Step-by-step explanation:
The statement that a recessive allele is one that is masked if a dominant allele is also present is true. Alleles can be either dominant or recessive. The dominant allele in a gene pair hides the effect of a recessive allele in a heterozygote individual (one with two different alleles for a trait). This means that only the dominant phenotype will be expressed. The recessive allele does not affect the phenotype unless the individual is homozygous for the recessive allele, meaning they have two copies of this allele.
For example, in the case of a Mendelian trait like pea plant flower color, purple is dominant to white. A plant with one purple and one white allele (Pp) will have purple flowers because the dominant 'P' allele masks the presence of the recessive 'p' allele. It is only when a plant has two recessive alleles (pp) that it will have white flowers. This pattern of inheritance is known as Mendel's law of dominance.