Lesson 6 of 11 · 7 min
Polygenic inheritance and pleiotropy
NCERT § "Polygenic Inheritance" and § "Pleiotropy"
Human height doesn't sort into just 'tall' and 'dwarf' the way Mendel's peas did — it spreads across a continuous range. Why don't traits like this follow a clean 3:1 pattern at all?
The lesson in notes
In short
Mendel's traits fall into distinct classes, but many traits, like human height or skin colour, vary continuously across a range. Usually three or more genes control such traits, which are called polygenic traits.
In polygenic inheritance each allele contributes a small additive effect, and the environment also influences the outcome.
In the NCERT model of human skin colour, three genes A, B and C control it, with dominant alleles adding dark pigment. AABBCC is darkest, aabbcc is lightest, and AaBbCc is intermediate.
Pleiotropy is when one gene produces more than one phenotypic effect. The usual basis is that the gene affects a metabolic pathway feeding several phenotypes.
Phenylketonuria is the NCERT example of pleiotropy: a mutation in the gene for phenylalanine hydroxylase leads to mental retardation as well as reduced hair and skin pigmentation.