Saving Seeds from the Biggest Tomatoes
A school gardener saves seeds from the largest tomatoes in the garden, which average 380 g. Those plants grew in compost-rich soil. The next year, the saved seeds are planted in ordinary soil, and the new plants produce tomatoes averaging 240 g. A neighbor plants seeds from the same original tomato variety, but from smaller 210 g tomatoes, in compost-rich soil. Those plants produce tomatoes averaging 350 g. The gardener is confused: the seeds from the biggest tomatoes produced smaller fruit than the neighbor’s seeds from smaller tomatoes. The garden also has a cherry tomato plant growing 15 m away, and some flowers may have been cross-pollinated by bees. The class must explain what happened and design a better seed-saving plan for the next growing season.
1. Explain why saving seeds from the largest tomatoes did not guarantee large tomatoes the next year. Use the distinction between inherited traits and environmental effects.
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Tomato size is influenced by both genes and environment. The 380 g tomatoes may have been large partly because the plants grew in compost-rich soil with more nutrients and water. Saving their seeds preserves inherited potential, but the next year’s ordinary soil may have limited growth, so the plants could not reach that potential. The neighbor’s result supports this: seeds from 210 g tomatoes grown in compost produced 350 g fruit, showing that soil conditions strongly affect size. Also, tomato size is not controlled by a single gene, so offspring vary. Finally, bees may have transferred pollen from the cherry tomato 15 m away. If that cross-pollination occurred, the saved seeds could carry genes for small fruit. The gardener’s mistake was treating one year’s largest fruit as proof of superior inherited genes without controlling the environment or pollination.
2. How could cross-pollination with the cherry tomato affect the next generation, and how would you test whether it occurred?
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If a bee moved pollen from the cherry tomato to a flower on the large tomato plant, the seeds in that fruit could be hybrids. The fruit itself would still be large because its tissue comes from the parent plant, but the embryos inside would carry genes from both parents. Those genes could produce smaller fruit, different color, or different flowering time in the next generation. To test whether cross-pollination occurred, the gardener could grow the saved seeds and look for offspring with cherry-tomato traits, such as very small fruit or a different leaf shape. A more reliable plan is to prevent cross-pollination in the first place: bag selected flowers before they open, pollinate them by hand, and keep the bags on until fruit forms. The gardener could also grow a control group of known self-pollinated seeds for comparison.
3. Design a multi-year seed-saving plan that would better test whether a tomato line has inherited large size. Include at least two controls or procedures.
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The gardener should first control pollination by bagging flowers on selected plants and hand-pollinating them, so the seeds are not crossed with the cherry tomato. Second, grow all test seeds in the same soil, watering schedule, and light conditions, and include a control group of seeds from the original variety. Third, measure many fruit from each plant rather than choosing only the largest, and record the average and range. Fourth, save seeds from several plants with large average fruit, not from one lucky fruit. Repeat this for at least three generations, because inherited potential is revealed by consistent performance across environments. If the selected line produces larger averages than the control in the same soil, there is stronger evidence that the trait is inherited. The plan separates genetic effects from environmental effects instead of confusing the two.