15. Two plants with white flowers are crossed. White flowers arise due to recessive mutation. All F1 progeny have red flowers. When the F1 plants are selfed, both red and white flowered progeny are observed. In what ratio will red-flowered plants and white-flowered plants occur?
(1) 1:1 (2) 3:1
(3) 9:7 (4) 15:1
Explanation of the Red and White Flower Ratio in Pea Plants
When two pure white flower plants are crossed, and the white color is due to a recessive mutation, all F1 progeny having red flowers indicates that red is the dominant flower color. Upon selfing the F1 plants, which are heterozygous for the flower color gene, the F2 generation shows both red and white flowers. According to Mendelian inheritance of a single gene with two alleles, the phenotypic ratio observed will be 3 red : 1 white. This is because plants with either homozygous dominant or heterozygous genotype display red flowers, while only homozygous recessive shows white.
Explanation of Each Option
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(1) 1:1 ratio: This ratio is typical in a test cross or when parents have heterozygous and homozygous recessive genotypes, not in this F1 self-cross.
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(2) 3:1 ratio: Correct for a monohybrid cross with simple dominance, showing three red-flowered plants for every one white-flowered plant in the F2 generation.
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(3) 9:7 ratio: This is typical of complementary gene action involving two genes, which doesn’t fit the current monogenic inheritance case.
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(4) 15:1 ratio: Corresponds to duplicate gene action with two genes, not relevant for this single gene cross.
Genetic Background of the 3:1 Ratio
Red flower color is dominant (R) over white (r). Crossing two white flowered pure lines (rr) produces F1 plants with genotype Rr (heterozygous), all red-flowered because of dominant R allele. Selfing F1 (Rr × Rr) yields:
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RR (homozygous dominant) – Red flowers
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Rr (heterozygous) – Red flowers
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rr (homozygous recessive) – White flowers
Thus, the F2 generation exhibits red:white flowers in a 3:1 ratio.
The answer to the question is (2) 3:1 because selfing the F1 heterozygous plants results in three red-flowered offspring for every one white-flowered offspring, consistent with classical Mendelian monohybrid inheritance for dominant and recessive alleles.
