Why is the pea wrinkle-seed allele a recessive allele?
a. It 'recedes' in the F2 generation when homozygous parents are crossed.
b. The trait associated with the allele is not exhibited in heterozygotes.
c. Individuals with the allele have lower fitness than that of individuals with the dominant allele.
d. The allele is less common than the dominant allele. (The wrinkled allele is a rare mutant.)

Verified step by step guidance

1. Understand the concept of dominant and recessive alleles: In genetics, a dominant allele is one that expresses its trait even when only one copy is present (heterozygous condition), while a recessive allele only expresses its trait when two copies are present (homozygous condition).
2. Analyze the options given: The question provides four options, each suggesting a reason why the pea wrinkle-seed allele is recessive.
3. Evaluate option b: 'The trait associated with the allele is not exhibited in heterozygotes.' This option aligns with the definition of a recessive allele, as the trait (wrinkled seeds) would not be visible in heterozygous individuals (those with one dominant and one recessive allele).
4. Consider the other options: Option a refers to the behavior of alleles in the F2 generation, which is more about Mendelian inheritance patterns rather than the definition of recessiveness. Option c discusses fitness, which is not directly related to whether an allele is recessive. Option d mentions allele frequency, which does not determine dominance or recessiveness.
5. Conclude that option b is the most accurate explanation for why the pea wrinkle-seed allele is considered recessive, as it directly relates to the expression of the trait in heterozygotes.

Key Concepts

Recessive Allele

A recessive allele is one that does not produce a phenotype when paired with a dominant allele in a heterozygous organism. It requires two copies (homozygous) to express the trait. In the context of pea plants, the wrinkle-seed allele is recessive because its trait is not visible in heterozygotes, who display the dominant round-seed phenotype.

Heterozygote

A heterozygote is an organism that has two different alleles for a particular gene, one dominant and one recessive. In pea plants, heterozygotes for the seed shape gene have one allele for round seeds and one for wrinkled seeds, but only the round seed phenotype is expressed due to the dominance of the round-seed allele.

Dominant Allele

A dominant allele is one that expresses its phenotype even when only one copy is present in a heterozygous organism. In the case of pea plants, the round-seed allele is dominant over the wrinkle-seed allele, meaning that peas with at least one round-seed allele will exhibit the round-seed phenotype, masking the presence of the recessive wrinkle-seed allele.