Pleiotropy and polygenic inheritance
In pleiotropy, one gene affects several traits, as in phenylketonuria. In polygenic inheritance, several genes add up to control one trait, like human skin colour.
Pleiotropy: one gene, many effects
A pleiotropic gene is a single gene that produces more than one phenotypic effect. Usually the gene controls one step of a metabolic pathway, and that pathway feeds several traits. In phenylketonuria, a mutation in the gene for the enzyme phenylalanine hydroxylase blocks the change of phenylalanine to tyrosine. The single faulty gene shows up as mental retardation and as reduced pigment in hair and skin.
Polygenic inheritance: many genes, one trait
Mendel's traits fell into clear types, like tall or dwarf. But many human traits show a smooth range: height and skin colour, for example. Such polygenic traits are usually controlled by three or more genes. Each dominant allele adds a small amount to the trait: the effect is additive. The environment also matters, for example sunlight for skin colour and diet for height. In a simple model, skin colour is controlled by genes A, B and C. Dominant alleles A, B and C give dark colour; a, b and c give light colour. The phenotype depends on how many dominant alleles a person has, not on which ones.
Skin colour with three genes
Two people, each AaBbCc (intermediate skin colour), have children. (a) What fraction of children will be AABBCC? (b) How many skin colour classes are possible, and in what ratio?
- Each gene pair (Aa × Aa) gives AA with probability 1/4.
- AABBCC needs AA, BB and CC together: 1/4 × 1/4 × 1/4 = 1/64.
- A child can carry 0, 1, 2, 3, 4, 5 or 6 dominant alleles.
- So there are 7 phenotype classes, from lightest (0) to darkest (6).
- Their frequencies are 1 : 6 : 15 : 20 : 15 : 6 : 1 out of 64; the middle class (3 dominant alleles) is the largest.
(a) 1/64. (b) 7 classes, in the ratio 1 : 6 : 15 : 20 : 15 : 6 : 1.
Pleiotropy vs polygenic inheritance
Pleio-tropy: one gene, many effects. Poly-genic: many genes, one effect.
Single-gene (Mendelian) traits vs polygenic traits
| Feature | Single-gene trait | Polygenic trait |
|---|---|---|
| Genes controlling it | One | Three or more |
| Kind of variation | Discrete classes (tall or dwarf) | Continuous range |
| Effect of environment | Usually small | Often large |
| Example | Pea stem height | Human skin colour, height |
Height is polygenic, so it also depends on childhood nutrition. Children in a family often grow taller than their grandparents when diets improve, though their genes have not changed.
2 marks: “What is pleiotropy? Give one example.” A single gene showing more than one phenotypic effect, usually by affecting a metabolic pathway. Example: phenylketonuria, where one faulty gene causes mental retardation and reduced hair and skin pigmentation.
3 marks: “Explain polygenic inheritance with the example of human skin colour. How is it different from pleiotropy?” Three genes A, B, C with additive effects; AABBCC darkest, aabbcc lightest, AaBbCc intermediate; phenotype depends on the number of dominant alleles and on environment. Pleiotropy is one gene with many effects; polygenic is many genes for one trait.
Pleiotropy: one gene plays many parts. Polygenic: many genes pool into one trait.