Q.Explain complementary gene interaction with the help of an example. OR What is the inducer in the lac operon? How does it ensure 'switching on' of genes?

Complementary gene interaction gives a modified dihybrid ratio of 9:7 because both dominant alleles are needed together to produce the trait.

Working

In a normal dihybrid cross with independent assortment and no gene interaction, selfing AaBb x AaBb gives the classic 9:3:3:1 ratio across four phenotypic classes. Complementary genes are a case of non-allelic (intergenic) gene interaction where two genes on different loci jointly control a SINGLE trait, and the dominant phenotype appears only when at least one dominant allele of EACH gene is present together (i.e., genotype A_B_).

All the other three classes -- A_bb, aaB_, and aabb -- fail to produce the dominant phenotype because one or both required dominant alleles are missing, and they are phenotypically indistinguishable from one another (all show the same recessive/alternate phenotype). So the classic 9:3:3:1 ratio collapses into just two classes:

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The masking of one gene's effect by a different (non-allelic) gene pair is epistasis, so the answer is (A).

Epistasis is a type of gene interaction in which one gene (the epistatic gene) suppresses or masks the phenotypic expression of another gene (the hypostatic gene) located at a different locus.

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Complementary gene interaction is a case of gene interaction in which two dominant genes at different loci must both be present to produce one phenotype; any combination lacking either dominant allele gives the alternative phenotype, converting the standard dihybrid 9:3:3:1 ratio into 9:7.

What is complementary gene interaction

Some traits (e.g. flower colour in sweet pea) depend on two genes, each with its own dominant and recessive allele, that act together (are 'complementary' to each other) to produce a single character. Neither dominant allele alone can produce the full/pigmented phenotype — both are required together.

Working out the ratio

Consider two genes, A/a and B/b, assorting independently. In a dihybrid cross AaBb × AaBb, the underlying genotypic classes are the usual 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb.

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Epistasis is the masking of one gene's phenotypic expression by a different, non-allelic gene, altering the expected Mendelian dihybrid ratio.

Definition and mechanism

In epistasis, two genes located at different loci influence the same character. One gene (epistatic) suppresses or masks the phenotypic expression of the other gene (hypostatic), so the phenotype expressed is that of the epistatic gene, regardless of which allele of the hypostatic gene is present.

Example

In summer squash, fruit colour is controlled by two genes: a dominant allele (W) for white colour is epistatic to a second gene (Y/y) that determines yellow versus green colour — so plants with at least one W allele are white irrespective of their Y/y genotype, and the yellow/green colour is seen only in ww plants.

Effect on ratio

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The correct matching is Dominant epistasis-12:3:1, Duplicate genes-15:1, Recessive epistasis-9:3:4, Complementary genes-9:7.

When two gene pairs interact non-additively to influence a single trait, the classic 9:3:3:1 dihybrid ratio is modified in characteristic, recognisable ways. In dominant epistasis, a dominant allele at one locus masks the expression of alleles at the second locus, merging two of the classes and giving a 12:3:1 ratio. In duplicate genes (duplicate dominant epistasis), either dominant allele alone (at either of two loci) is sufficient to produce the same dominant phenotype, so only a double-recessive genotype shows the alternative phenotype, giving a 15:1 ratio. In recessive epistasis, being homozygous recessive at one locus masks the phenotype normally determined by the second locus regardless of its genotype, producing a 9:3:4 ratio. In complementary gene action, two dominant genes from different loci must both be present together to produce a particular phenotype (such as flower colour in sweet …