3. In a genetic test 9: 7 ratio in F2 generation represents
(1) Epitasis
(2) Co-dominance
(3) Incomplete dominance
(4) Complete dominance
Explanation
In a typical dihybrid cross with independent genes and no interaction, the F₂ phenotypic ratio is 9:3:3:1. When the observed ratio is modified to 9:7, it indicates that the two genes are interacting so that:
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Only genotypes with at least one dominant allele at both loci (A_B_) show one phenotype (the “9” part).
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All other genotypes (A_bb, aaB_, aabb) show the second phenotype (the “7” part).
This is called complementary gene action, a specific case of epistasis.
Option-wise explanation
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Epistasis – correct
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Epistasis is interaction between non-allelic genes where one gene’s alleles mask or modify the effect of another gene.
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Complementary epistasis (both dominant alleles needed) produces the classic 9:7 F₂ ratio.
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Co-dominance
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Both alleles in a heterozygote are fully expressed (e.g., AB blood group).
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It usually involves a single gene and does not give a 9:7 ratio; instead, it affects monohybrid ratios (like 1:2:1).
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Incomplete dominance
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The heterozygote shows an intermediate phenotype (blending).
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Again, this is a single-gene interaction and gives a 1:2:1 F₂ ratio, not 9:7.
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Complete dominance
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Classic Mendelian dominance with a 3:1 (monohybrid) or 9:3:3:1 (dihybrid) F₂ ratio when no gene interaction is present.
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It does not explain modification to 9:7.
Therefore, a 9:7 ratio in the F₂ generation represents epistasis with complementary gene interaction.
