A recessive allele is expressed in ..........a. heterozygous condition only. b. $F_1$ generation. c. homozygous condition only. d. both homozygous and heterozygous conditions.

Sure, here are the step-by-step solutions: 1. Define the term "recessive factor or allele": The factor of an allelic or allelomorphic pair, which is unable to express its effect in the presence of its contrasting factor in a heterozygote, is called recessive factor or allele. 2. Give an example: For example, in a hybrid tall pea plant with the genotype Tt, the allele "t" is recessive. It is unable to express its effect in the presence of the dominant allele "T". 3. Explain when the effect of the recessive factor becomes known: The effect of the recessive factor becomes known only when it is present in the pure or homozygous state. For example, in a dwarf pea plant with the genotype tt, both alleles are recessive and the plant expresses the recessive trait of being dwarf. 4. Summarize: To summarize, a recessive factor or allele is one that is masked by a dominant factor or allele in a heterozygous individual, but expresses its effect when present in a homozygous state.