Based on the data, which of the following would be the most likely outcome of crossing a plant with genotype RR (tall) with a plant with genotype rr (short)?

Core Concept

PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM

Step-by-Step Analysis

The inheritance pattern described in this cross hinges on the molecular relationship between dominant and recessive alleles at a single gene locus. In this system, the R allele encodes a functional version of a protein—often an enzyme such as gibberellin 20-oxidase in the gibberellin biosynthesis pathway—that catalyzes reactions necessary for longitudinal stem cell elongation in the plant's internodes. The r allele carries a missense or nonsense mutation in the coding sequence, producing either a nonfunctional enzyme or a truncated polypeptide degraded by proteasomal pathways. When only nonfunctional enzyme is present (the rr homozygous condition), gibberellin precursors are not converted into active gibberellin hormones at sufficient concentrations, and stem elongation fails, yielding the short phenotype.

Why Other Options Are Wrong

In a heterozygous Rr individual, the single functional R allele produces enough functional enzyme to maintain adequate flux through the gibberellin biosynthesis pathway. This phenomenon, termed haplosufficiency, arises because gene expression from one functional allele yields sufficient functional protein concentration for the phenotype. The molecular geometry of the enzyme's active site, its binding affinity for substrate (cofactors such as Fe²⁺ and 2-oxoglutarate), and the kinetic parameters (Vmax and Km) are preserved even with just one functional copy. Therefore, Rr plants achieve the same tall phenotype as RR homozygotes because the rate-limiting enzymatic step is satisfied.

PILLAR 2 — STEP-BY-STEP LOGIC

The parental cross is RR × rr. During meiosis in the RR parent, homologous chromosomes—both carrying the R allele at this locus—segregate into separate gametes during Anaphase I. Every gamete produced by this parent receives one chromosome bearing the R allele. Similarly, the rr parent's meiosis produces gametes that each carry only the r allele. Fertilization between an R-bearing gamete and an r-bearing gamete produces zygotes that are uniformly Rr. Because R is completely dominant over r (due to haplosufficiency explained above), every offspring expresses the tall phenotype. A Punnett square analysis confirms that 100% of the offspring are Rr and 100% are tall. No genetic variation exists among offspring at this locus because both parents are homozygous, eliminating allelic segregation diversity in gametes.

PILLAR 3 — DISTRACTOR ANALYSIS

Option B (all offspring short) would require the r allele to mask R, which contradicts the molecular mechanism: the functional enzyme encoded by R is present in every Rr cell and drives gibberellin biosynthesis. This distractor exploits confusion about which allele is dominant and may trap students who assume the lowercase letter r must be dominant simply because it appears in the cross.

Option C (50% tall, 50% short) describes the expected outcome of an F₂ cross (Rr × Rr) or a testcross (Rr × rr), not the P generation cross between two homozygous parents. This option reflects a misunderstanding of segregation ratios: heterozygous parents produce R and r gametes in equal proportions, but homozygous parents produce only one gamete type. Students selecting this option conflate the F₁ cross with the F₂ cross.

Option D (a combination of tall and short traits in each individual) describes incomplete dominance or codominance, patterns in which the heterozygous phenotype is intermediate (e.g., pink snapdragon flowers from red × white crosses, where neither allele produces sufficient functional pigment enzyme for full coloration) or both alleles are expressed simultaneously (e.g., IAIB blood type producing both A and B glycosyltransferase enzymes). Here, complete dominance governs the trait; the functional R allele alone is sufficient for full tall phenotype expression.