Tomato Flower Color Crosses
A grower studies flower color in a tomato variety. She crosses a purple-flowered plant with a white-flowered plant. All 48 offspring have purple flowers. She then crosses two of those purple offspring. That cross produces 72 purple-flowered and 24 white-flowered offspring. In a third cross, she crosses one of the original purple offspring with a white-flowered plant and gets 25 purple and 23 white offspring. All plants are grown in the same greenhouse with the same light, water, and soil. All seeds germinate, and every plant flowers, so the counts are complete. The grower wants to know which flower-color allele is dominant, whether the original purple parent was pure-breeding, and what to expect from another cross. She has limited greenhouse space, so she cannot grow thousands of plants.
1. Which flower-color allele is dominant? Use the first and second crosses to support your answer.
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Purple is dominant. In the first cross, a purple plant crossed with a white plant produces all 48 purple offspring. That is what happens when the purple parent is homozygous dominant (PP) and the white parent is homozygous recessive (pp): all offspring are Pp and show purple. In the second cross, two purple offspring from that first cross are crossed. If they are Pp, the expected offspring ratio is 3 purple : 1 white. The observed 72 purple : 24 white simplifies to exactly 3 : 1, which matches the model. The white trait reappears in the second generation even though the first generation showed no white, supporting purple as dominant and white as recessive.
2. Was the original purple parent pure-breeding? Explain using the third cross.
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Yes, the original purple parent was most likely pure-breeding, PP. The first cross already suggests this because all 48 offspring were purple; if the original purple parent had been Pp, about half the offspring would be expected to be white when crossed with pp. The third cross uses one of the original purple offspring, which should be Pp, crossed with a white plant, pp. A Pp x pp cross predicts 1 purple : 1 white. The observed 25 purple and 23 white is very close to 1 : 1. This confirms that the purple offspring carried a hidden white allele, while the original purple parent supplied only the purple allele.
3. Predict the expected result if two white-flowered offspring were crossed. Explain why.
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All offspring should be white. White flowers appear only when the plant has two recessive alleles, pp, because purple is dominant. A white plant therefore cannot pass on a purple allele. Crossing pp with pp gives pp offspring only. If the grower made this cross and found any purple offspring, the simple dominant/recessive model would be challenged, and other explanations such as a different gene or a mutation would need to be considered. With the data given, the prediction is 100% white, though small sample sizes can still show chance variation in other crosses.