Pea-plant offspring ratio and onion root tip mitosis
In a genetics unit, students cross a tall pea plant with smooth seeds and a short pea plant with wrinkled seeds. All F1 plants are tall with smooth seeds. The F1 plants are self-pollinated, and 320 F2 seeds are counted: 238 smooth and 82 wrinkled. The seeds come from the same pod and are grown under the same conditions. A separate group observes onion root tip cells under a microscope and counts 100 cells: 62 in interphase, 20 in prophase, 8 in metaphase, 6 in anaphase, and 4 in telophase. One student says the 238:82 ratio is 'wrong' because it is not exactly 3:1, and another says the root tip is a good place to see mitosis because the cells are large. The teacher asks the groups to evaluate both claims using the data.
1. Which seed shape is dominant? Give the genotypes of the original parents, the F1 plants, and the F2 plants. Explain how the 238:82 data support your answer.
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Smooth seed shape is dominant. Let S be the smooth allele and s the wrinkled allele. The original cross is SS × ss, so all F1 plants are Ss and show the dominant smooth phenotype. Self-pollination of Ss × Ss gives an expected F2 ratio of 1 SS : 2 Ss : 1 ss, which is 3 smooth : 1 wrinkled. In 320 offspring, the expected numbers are 240 smooth and 80 wrinkled. The observed numbers are 238 smooth and 82 wrinkled, which are very close to the 3:1 expectation. The wrinkled phenotype reappears in F2 because the recessive allele was hidden in the heterozygous F1 plants.
2. Evaluate the claim that the 238:82 ratio is 'wrong.' Include a calculation and a reason for the deviation.
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The ratio is not wrong. The expected numbers for a 3:1 ratio in 320 offspring are 3/4 × 320 = 240 smooth and 1/4 × 320 = 80 wrinkled. The observed numbers differ from these expectations by only 2 in each class. Offspring genotypes are determined by random segregation and fertilization, so small samples naturally deviate from exact expected ratios. A deviation of 2 out of 320 is easily explained by chance. The claim would be stronger only if the sample were much larger and still showed a large, consistent deviation. The data therefore support a single-gene 3:1 model rather than contradict it.
3. Identify the stage in which chromosomes line up at the cell equator and the stage in which sister chromatids separate. Explain why onion root tips are suitable for observing mitosis, using the cell counts.
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Chromosomes line up at the cell equator during metaphase; sister chromatids separate during anaphase. The counts show 8 metaphase cells and 6 anaphase cells out of 100, so these stages are present and can be found. Onion root tips are suitable because the tip contains a meristem, a region of actively dividing cells. Mitosis occurs frequently there, so many cells are in prophase, metaphase, anaphase, or telophase rather than all being in interphase. The claim that the cells are large is not the main reason; root tip cells are not especially large. The important point is the high frequency of cell division in the meristem.