Answer:
One allele is dominant while other serves as recessive allele.
Step-by-step explanation:
Two alleles of a trait exhibit dominant-recessive behavior. The allele that can express itself in both homozygous and heterozygous condition is called "dominant" allele. On the other hand, the allele which is expressed only in homozygous genotype is termed as a recessive allele.
Example: T and t are the alleles for plant height. Both "TT" and "Tt" genotype give "tall" plants while "tt" genotype gives dwarf plants.
Since the presence of allele "t" in the genotype "Tt" is masked by "T" allele, the allele "T" is dominant over "t" allele.