Duplicate gene and its parental copy can subfunctionalize so that each carries out only some of the roles that the ancestral gene performed. 

Ex- a duplicate of the β-hemoglobin locus is expressed in the fetus. 

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In instructive interaction, 

a signal from the inducing cell is necessary for initiating new gene expression in the responding cell.

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Instructive interaction

It occur where the responding cell has a choice of fates and will follow one developmental pathway following induction and an alternative pathway in absence of the inductive signal. 

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In Permissive interaction

The responding tissue has already been specified and needs only an environment that allows the expression of these traits. 

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Epithelial-mesenchymal interactions

the interactions of sheets of epithelial cells with adjacent mesenchymal cells.

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Properties of epithelial-mesenchymal interactions:

Regional specificity of induction

Genetic specificity of induction

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Regional specificity of induction:

mesenchyme plays an instructive role, calling into play different sets of genes in the responding epithelial cells.