In a test cross, a homozygous recessive pea plant with green seeds is mated with a yellow-seeded plant of unknown genotype. If all the progeny have yellow seeds, then the genotype of the yellow-seeded plant is
Question
In a test cross, a homozygous recessive pea plant with green seeds is mated with a yellow-seeded plant of unknown genotype. If all the progeny have yellow seeds, then the genotype of the yellow-seeded plant is
Solution
The genotype of the yellow-seeded plant is homozygous dominant.
Here's the step-by-step explanation:
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In pea plants, seed color is determined by a single gene with two alleles: the yellow-seed allele (Y) is dominant, and the green-seed allele (y) is recessive.
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A homozygous recessive plant will have two green-seed alleles (yy), and will therefore have green seeds.
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If this green-seeded plant is crossed with a yellow-seeded plant of unknown genotype, and all the progeny have yellow seeds, this tells us that the yellow-seeded plant must have been homozygous dominant (YY).
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This is because, in order for all the progeny to have yellow seeds, they must all receive at least one yellow-seed allele. If the yellow-seeded plant were heterozygous (Yy), then approximately half the progeny would receive the recessive green-seed allele and would have green seeds.
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Therefore, the only way for all the progeny to have yellow seeds is if the yellow-seeded plant is homozygous dominant (YY), so that all progeny receive a dominant yellow-seed allele.
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