4. In a hybridization experiment a plant shows phenotypic ratio of 15:1. How many genes control the trait for observed phenotypic ratio?
(1) One (2) Two
(3) Three (4) Polygene
Understanding the 15:1 Phenotypic Ratio in Hybridization
In a hybridization experiment where the phenotypic ratio of 15:1 is observed, this ratio is characteristic of a genetic phenomenon called duplicate gene action or duplicate dominant epistasis. This occurs when two genes control a single trait such that the presence of at least one dominant allele from either gene produces the dominant phenotype. Only the double homozygous recessive genotype (aabb) exhibits the recessive phenotype, resulting in 15 individuals with dominant phenotype to 1 with recessive phenotype in the F2 generation.
Explanation of Four Options
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(1) One gene: A single gene controlling a trait usually gives classic Mendelian ratios such as 3:1 in monohybrid crosses. A 15:1 ratio cannot be explained by one gene acting alone.
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(2) Two genes: The 15:1 ratio indicates two genes are involved, particularly showing duplicate gene action where either gene’s dominant allele can express the phenotype independently.
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(3) Three genes: Three genes controlling a trait would usually produce more complex ratios, often with many phenotypic classes and different ratios than 15:1.
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(4) Polygene: Polygenic inheritance involves many genes contributing additively to a trait, often resulting in continuous variation rather than discrete phenotypic categories like 15:1.
Genetic Basis of the 15:1 Ratio
The 15:1 ratio results from the interaction of two genes with duplicate functions. Both genes A and B contribute to the trait, and having at least one dominant allele in either gene is sufficient to express the dominant phenotype. Only when both genes are homozygous recessive (aabb) does the recessive phenotype show. This represents two genes controlling the trait redundantly, leading to the 15:1 ratio.
The correct answer to the question is (2) Two genes because the 15:1 phenotypic ratio indicates duplicate gene action by two genes controlling the trait in the hybridized plant.
