PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM
In pea plants (Pisum sativum), stem elongation depends on the gibberellin biosynthesis pathway. The Le locus encodes GA 3β-hydroxylase, an enzyme that catalyzes the conversion of GA20 to bioactive GA1 in shoot apical meristems. The dominant Le allele produces a functional enzyme with proper active-site geometry, allowing hydroxylation of GA20 at the C-3 position. Active gibberellin GA1 binds to the GID1 receptor in target cells, triggering ubiquitin-mediated degradation of DELLA repressor proteins via the 26S proteasome. This releases constraints on cell elongation in internodal regions, producing tall plants. The recessive le allele carries a missense mutation that disrupts the enzyme's catalytic pocket—le/le homozygotes accumulate inactive GA20 precursor but cannot generate GA1, resulting in constitutively short stature due to unrepressed DELLA signaling.
The phenotypic ratios observed in any population sample reflect the underlying distribution of Le and le alleles across individuals, shaped by meiotic segregation, parental genotypes, and stochastic sampling. During meiosis I, homologous chromosomes carrying Le or le alleles align at the metaphase plate, with spindle microtubules attached to kinetochore complexes. Segregation of homologues to opposite poles follows Mendel's First Law—each gamete receives exactly one allele. Independent assortment of the chromosome pair carrying the Le locus relative to other chromosome pairs during metaphase I generates gametic diversity. However, the phenotypic ratio observed in any finite sample is an empirical measurement, not a theoretical prediction—it simply reports the proportion of individuals displaying each trait.
PILLAR 2 — STEP-BY-STEP LOGIC
The question provides direct phenotypic counts from a population of ten pea plants: 3 plants exhibit the tall phenotype and 7 plants exhibit the short phenotype. To determine the correct ratio expression, we must identify what these numbers represent and how ratios are conventionally stated.
The data yields two values: 3 (tall individuals) and 7 (short individuals). Option B correctly expresses this observation as a 7:3 ratio of short to tall plants. The larger number (7) corresponds to the more frequent phenotype (short), and the smaller number (3) corresponds to the less frequent phenotype (tall). The colon notation separates these counts in the order specified by the wording—short to tall—matching the 7:3 format. This ratio is purely descriptive; it reports what was observed without invoking any expected Mendelian proportion or specific genetic mechanism.
Notably, this observation departs from classical Mendelian expectations. A cross between two heterozygotes (Le/le × Le/le) would theoretically produce a 3:1 dominant-to-recessive phenotypic ratio in the F2 generation. A test cross (Le/le × le/le) would yield approximately 1:1. The observed 7:3 ratio could reflect small sample effects, non-random mating, genetic drift in a limited population, or selection pressures favoring short-statured plants in this particular environment. The question asks only what ratio the data reflects—not what genetic cross produced it.
PILLAR 3 — DISTRACTOR ANALYSIS
Option A states a 3:1 ratio of tall to short plants. This distractor exploits automatic pattern recognition: students familiar with Mendel's classical monohybrid cross (heterozygote × heterozygote producing a 3:1 F2 phenotypic ratio) may reflexively select this familiar proportion without verifying it against the actual data. The fatal flaw is numerical inversion—the data contain 7 short and 3 tall plants, not three times as many tall as short. Selecting Option A reflects hasty retrieval of memorized ratios rather than careful reading of the stimulus values. The 3:1 ratio describes a theoretical expectation from a specific parental cross, not the observed 3 versus 7 count.
Option C states a 1:1 ratio of tall to short plants. This distractor targets students who associate equal phenotypic proportions with test crosses, where a heterozygous parent (Le/le) crossed with a homozygous recessive parent (le/le) produces 50% Le/le (tall) and 50% le/le (short) offspring. However, the observed data show 3 tall and 7 short—markedly unequal counts. A 1:1 ratio would require approximately 5 tall and 5 short in a sample of 10. Selecting Option C indicates confusion between expected theoretical ratios and actual empirical observations, or a failure to compute the proportion from the given numbers.
Option D describes the data as a clear example of incomplete dominance. This distractor preys on students who conflate any non-standard phenotypic ratio with non-Mendelian inheritance. Incomplete dominance produces heterozygotes with intermediate phenotypes—such as pink-flowered snapdragons from red × white crosses—because the single functional allele produces insufficient gene product (such as enzyme or pigment) for full phenotypic expression. Molecularly, this reflects dosage sensitivity at the transcriptional or translational level. The hallmark of incomplete dominance is three phenotypic classes in a 1:2:1 ratio. The pea plant data describe only two phenotypes (tall and short) with no intermediates, providing zero evidence for incomplete dominance. Selecting Option D reveals a fundamental misunderstanding of the molecular mechanism—namely, that incomplete dominance requires a quantitative gradient of gene expression producing intermediate phenotypes, not merely a skewed ratio of two discrete phenotypes.
A7:3 ratio of short to tall plants
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