Referring to an allele whose phenotypic effect is observed only in homozygous individuals.

recessive

Explanation:

Alleles are the alternating forms of a gene and may be dominant or recessive. A recessive allele is the one that is not expressed in the presence of the dominant allele. Therefore, the expression of the recessive alleles occurs only in homozygous recessive genotype.

The recessive allele code for a non-functional or less functional protein which is not observed in the presence of fully functional protein encoded by a dominant allele in a heterozygous state. However, in homozygous recessive genotype, when the dominant allele is not present to code the functional protein, the non-functional proteins are apparent and the recessive allele is said to be expressed.

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