What is Polygenic Inheritance?
Polygenic inheritance occurs when a single trait is controlled by many genes at different loci, each contributing a small additive effect. This produces continuous variation — like height or skin color — instead of discrete Mendelian categories.
Polygenic inheritance is the control of one trait by two or more genes, whose combined additive effects create a continuous range of phenotypes that follows a bell-shaped distribution.
Step-by-step worked examples
Human skin color is controlled by roughly 3-6 genes with additive effects. If a trait were controlled by just 3 genes (6 alleles total), how many phenotype classes (levels of darkness) would be possible?
With n gene pairs (2n alleles), the number of phenotype classes = 2n + 1 Here n = 3, so classes = 2(3) + 1 = 7 So 7 distinct shades are possible, from lightest to darkest
Two people, each heterozygous at three independently assorting genes for a polygenic trait (AaBbCc × AaBbCc), have a child. What fraction of offspring is expected to have the extreme phenotype (all 6 dominant alleles, AABBCC)?
Each gene pair independently gives 1/4 chance of the homozygous dominant genotype (AA, BB, or CC) Combined probability = 1/4 × 1/4 × 1/4 = 1/64 So about 1/64 of offspring show the extreme phenotype
A population's height data forms a bell-shaped (normal) distribution rather than distinct short/tall categories. What does this indicate about the trait?
A continuous, bell-shaped distribution suggests many genes contribute additively Each gene adds a small amount to the trait, plus environmental influence This is the signature of polygenic inheritance, not single-gene inheritance
Flashcards
Quick quiz
Q1.Polygenic inheritance produces which kind of phenotype distribution?
Q2.Which of these is a classic example of a polygenic trait in humans?
Q3.For a trait controlled by 3 gene pairs (AaBbCc × AaBbCc), how many phenotype classes are possible?
Q4.What makes polygenic inheritance different from a simple dominant/recessive trait?
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Common mistakes
Thinking polygenic traits follow the same 3:1 or 9:3:3:1 ratios as simple Mendelian traits. — Correct: Polygenic traits produce a continuous bell curve, not discrete Mendelian ratios.
Assuming a trait with continuous variation must be entirely environmental. — Correct: Continuous variation often comes from many genes acting additively, though environment can also contribute.
Believing only one gene controls height or skin color. — Correct: These traits are controlled by many genes (polygenic), each adding a small effect.
Confusing polygenic inheritance (one trait, many genes) with pleiotropy (one gene, many traits). — Correct: Polygenic = many genes affect one trait; pleiotropy = one gene affects many traits — they are opposites in structure.
FAQ
What is polygenic inheritance?
It's when a single trait, like height or skin color, is controlled by the additive effects of multiple genes rather than just one.
What is the formula for polygenic inheritance phenotype classes?
The number of phenotype classes equals 2n + 1, where n is the number of heterozygous gene pairs involved.
What are examples of polygenic inheritance?
Human height, skin color, and eye color are classic examples of polygenic traits.
How do you calculate polygenic inheritance outcomes?
Treat each gene pair as an independent Mendelian pair, then combine their probabilities; the more genes involved, the more phenotype classes and the smoother the distribution.