26. In a plant r+ and a+ genes encode for a regulatory and a structural protein, respectively. These genes are responsible for blue color of flower. Mutation in either of the genes leads to white flowers, which is a recessive character. The two genes assort independently. When two homozygous white flowered plants are crossed. The F1 plants have blue colored flowers. If the F1 plant is backcrossed, the progeny will have plants with blue and white flowers in the ratio of:
(1) 9:7 (2) 1:1
(3) 3:1 (4) 1:0
In this plant flower color genetics problem, two genes r+ and a+ code for a regulatory and a structural protein, respectively, required to produce blue flower color. Mutation in either gene causes white flowers (recessive). The genes assort independently.
Genetic Explanation
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Both dominant alleles (R and A) are needed for blue color.
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Mutation (recessive alleles r or a) in either gene leads to loss of blue pigment, resulting in white flowers.
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This is a classic example of complementary gene interaction (duplicate recessive epistasis).
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When two homozygous white plants (rr aa) are crossed, their F1 will be heterozygous for both genes (Rr Aa) and thus blue-flowered.
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Backcross of F1 (Rr Aa) with homozygous recessive parent (rr aa) will produce:
| Genotype | Phenotype |
|---|---|
| Rr Aa, Rr aa, rr Aa | Blue flowers |
| rr aa | White flowers |
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Phenotypic ratio from backcross is 1 blue : 1 white.
Explanation of Options
(1) 9:7 — This ratio happens in F2 when two heterozygous F1 plants are crossed for complementary genes, not in backcross.
(2) 1:1 — Correct for a backcross of heterozygous F1 with recessive homozygous parent in complementary gene interaction.
(3) 3:1 — Classic Mendelian monohybrid dominant-recessive ratio, not relevant here with two genes interacting.
(4) 1:0 — All blue flowers, which contradicts the presence of recessive alleles.
Introduction:
Flower color inheritance in plants often involves complex gene interactions beyond simple dominance. Complementary gene interaction requires two genes working together to produce a phenotype. This article explains the genetics of blue flower color controlled by two genes and predicts the phenotypic ratios in backcrosses involving blue and white flower plants.
Detailed Explanation:
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Genes r+ and a+ encode proteins necessary for blue pigmentation.
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Mutations in either gene (recessive alleles r, a) cause white flowers, demonstrating complementary action.
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Crossing two white-flowered (rr aa) plants yields all blue-flowered F1 (Rr Aa) plants.
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Backcross of F1 with white plant (rr aa) produces a 1:1 ratio of blue to white flowers.
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This ratio reflects presence or absence of both dominant genes needed for pigment production.
Option Evaluation:
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Option (2) correctly identifies the expected 1:1 blue:white flower ratio in the backcross.
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Other options represent different genetic contexts or phenotypic ratios not applicable to this backcross.
Understanding complementary gene interaction and backcrossing principles helps predict outcomes in plant breeding and genetics research. This knowledge is vital for students preparing for genetics exams and researchers working with gene interactions in plants.
If you want, I can provide charts or Punnett square illustrations for deeper clarity.
