Gene Action in Polygenic Inheritance

Correct Answer: Additive Gene Action

From the notes (gene action diagram): Polygenic inheritance is governed by Additive Gene Action (AGA). In additive gene action, each allele at multiple loci contributes a small, equal, and cumulative effect to the phenotype.

Additive Gene Action — Associated With

• Polygenic traits (multiple genes controlling one character)
• General Combining Ability (GCA)
• Resemblance between relatives
• Response to selection (heritability)

Non-Additive Gene Action (NAGA) — Associated With

• Allelic interactions: Dominance, Co-dominance, Incomplete dominance, Over-dominance
• Non-allelic interactions: Epistasis and all its forms
• Heterosis / Hybrid vigour
• Specific Combining Ability (SCA)

Additive Gene Action Example (from notes, Sample Q1)

If AA = 10 and aa = 0, then Aa = 5 for additive gene action (heterozygote exactly halfway between the two homozygotes).