Final answer:
Mendel referred to the trait that is masked in the presence of a dominant allele as recessive, meaning it is only expressed when the individual has two copies of the recessive allele.
Step-by-step explanation:
Mendel called a trait that is masked recessive. A recessive trait is not expressed when the dominant allele is present. In genetics, traits are determined by alleles on chromosomes. An allele can be dominant or recessive. A dominant allele is expressed in both homozygous (AA) and heterozygous (Aa) genotypes and masks the expression of a recessive allele, which is only observed when present in a homozygous recessive (aa) state.
Mendel used pea plants in his studies, noting that traits like dwarfism were recessive because they were masked by the dominant trait of tallness unless both parents contributed the recessive allele. Hence, the recessive trait in the offspring would only be expressed in homozygous form.