1. The dihybrid F₂ ratio 9:7 represents
(1) Dominance
(2) Independent assortment
(3) Epistasis
(4) Co-dominance

Explanation of the Dihybrid F2 Ratio 9:7

The dihybrid F2 phenotypic ratio 9:7 is a classic example of epistasis in genetics, specifically a form called complementary gene interaction. In this scenario, two genes interact such that both dominant alleles from each gene are necessary to express a particular phenotype. If either gene lacks the dominant allele, the phenotype is altered, resulting in two phenotypic classes of offspring in the ratio 9 exhibiting one phenotype and 7 exhibiting the other, rather than the typical Mendelian 9:3:3:1 ratio.​

Option Explanations

• (1) Dominance: This refers to a relationship where one allele masks the expression of another at the same gene locus, usually producing a 3:1 ratio in monohybrid crosses. It does not explain the 9:7 ratio, which involves interaction between two genes rather than alleles of one gene.

• (2) Independent Assortment: This Mendelian law states genes for different traits segregate independently, generally producing a 9:3:3:1 ratio in dihybrid crosses. The 9:7 ratio deviates from this and indicates gene interaction rather than independent segregation.

• (3) Epistasis: A genetic phenomenon where alleles of one gene mask or modify the effect of alleles of another gene at a different locus. The 9:7 ratio arises in complementary gene interaction, a specific kind of epistasis where both genes’ dominant alleles are necessary for one phenotype; absence in either leads to the other phenotype.

• (4) Co-dominance: Both alleles of a gene are fully expressed in heterozygotes, leading to phenotypes displaying traits of both alleles simultaneously. This typically produces a 1:2:1 ratio, not a 9:7 ratio.

  Summary of the Genetic Ratios

  Genetics Concept	Phenotypic Ratio (F2)	Explanation
  Dominance	3:1	One allele masks the other at the same locus.
  Independent Assortment	9:3:3:1	Genes on different chromosomes assort independently.
  Epistasis	9:7	Interaction between two genes; both dominant alleles required for specific phenotype.
  Co-dominance	1:2:1	Both alleles expressed equally in heterozygotes.

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  The correct answer to the question is (3) Epistasis because the 9:7 dihybrid F2 ratio represents a complementary gene interaction where the presence of dominant alleles at both gene loci is essential to produce one phenotype; otherwise, the alternative phenotype appears.