Polygenic Inheritance
பல்லின மரபுவழி
Do not confuse Polygenic Inheritance with Pleiotropy. In Pleiotropy, one gene affects multiple traits. In Polygenic Inheritance, multiple genes dictate a single phenotype. This is the difference between a single switch and a complex control panel.
The idea
Polygenic traits are governed by three or more genes. Unlike Mendelian traits, these do not show distinct alternate forms but a continuous gradient. The phenotype is determined by the additive effect of each contributing allele. Human skin color is the classic NCERT example of this quantitative inheritance.
Why it matters
It allows for continuous variation in complex traits like height or skin color, which is essential for adaptation to environmental gradients.
NEET lens
NEET focuses on the 'additive effect' and 'quantitative' nature of these traits. Expect questions comparing Polygenic Inheritance with Pleiotropy. Remember: Polygenic = Many genes, one trait; Pleiotropy = One gene, many traits. Do not miss the skin color example.
Clinical significance
Polygenic inheritance is the basis for complex multifactorial conditions like hypertension, diabetes mellitus, and coronary heart disease. These are not caused by a single mutation but by the cumulative effect of multiple genetic loci interacting with environmental factors. Failure in this regulatory balance leads to chronic metabolic dysfunction.
Introduction
Polygenic inheritance is a form of inheritance where a phenotypic trait is controlled by multiple genes (polygenes), each contributing a small additive effect, resulting in a continuous range of phenotypes rather than discrete categories.
Core concept
Characteristics: controlled by many genes (polygenes). Each gene typically has two alleles. Additive effect: each 'contributing' allele adds a fixed, equal amount to the phenotype. Result: continuous variation (bell-shaped normal distribution in population). Examples in humans: height, skin colour, intelligence, weight, blood pressure. Classic example: skin colour (Davenport's study): multiple genes contribute melanin production. Each 'A' allele adds melanin; 'a' allele adds none. With 3 genes (AaBbCc × AaBbCc): 7 phenotypic classes (0 to 6 contributing alleles). With more genes: near-continuous distribution. Quantitative genetics: measures variance: total phenotypic variance (VP) = genetic variance (VG) + environmental variance (VE). Heritability (h²) = VG/VP: proportion of phenotypic variance due to genetics. Continuous traits affected by both genotype (multiple genes) and environment.
Go deeper
Polygenic traits vs qualitative traits: qualitative (Mendelian, discontinuous): distinct categories (tall/short), controlled by 1-2 genes, little environmental effect, clear Mendelian ratios. Quantitative (polygenic, continuous): range of values, many genes + environment, normal distribution, no discrete ratios. NEET context: skin colour in humans is polygenic — at least 6 major genes identified (including SLC24A5, MC1R, ASIP, OCA2, TYRP1, SLC45A2). Each contributes approximately additively.
From Class 10
Height in humans doesn't come in just tall or short — it varies continuously. That's because many genes each contribute a small effect to height, adding up to a continuous range of phenotypes. This is polygenic inheritance.