Polygenic Inheritance

• The inheritance of a trait that is controlled by two or more genes, where each gene contributes additively to the phenotype and the trait shows continuous variation rather than distinct categories, is called polygenic inheritance.
• When two or more genes control the traits having distinct alternate forms, it is called polygenic inheritance.

"Polygenic inheritance is defined as quantitative inheritance in which multiple independent genes have an additive (cumulative) effect on a single quantitative trait, resulting in continuous variation of the phenotype."

• Also called: Multiple gene inheritance, Multiple factor inheritance, or Quantitative inheritance​
• The individual genes involved are called polygenes

• Multiple genes involved: Three or more independent genes control a single trait​
• Additive allele effects: Each dominant allele adds an equal and cumulative increment; the effect of any single gene is too small to detect on its own​
• Continuous variation: Produces a spectrum of phenotypes, not distinct categories; phenotype distribution follows a bell-shaped (normal distribution) curve in the population​
• No simple dominance/recessiveness: There are contributing alleles (active) and non-contributing alleles (null), not classic dominant-recessive pairs​
• No epistasis: Unlike gene interaction, polygenes do not mask each other; all genes contribute equally​
• No linkage: Polygenes assort independently of each other (Law of Independent Assortment applies)​
• Environmental influence: Environmental conditions greatly modify phenotypic expression.
• Statistical analysis required: Individual phenotypes cannot be predicted precisely; population statistics (mean, variance, standard deviation) are needed

Genes involved: Three gene pairs - A/a, B/b, C/c (each located on different, unlinked chromosomes)

• Capital letters (A, B, C) = contributing alleles → darker skin
• Lowercase letters (a, b, c) = non-contributing alleles → lighter skin

Cross Analysis:

F₂ Genotypic Ratio (number of contributing alleles → phenotype):​

Around 60 genetic loci contribute to skin colour in humans - far more than the simplified 3-gene model​

• Approximately 400 genes are responsible for variation in height​
• Environmental factors such as diet, general health, and hormones greatly influence final height​
• Population distribution of height follows a bell-shaped normal distribution curve

• Determined by at least 16 genes in humans​
• At least 9 different eye colours are recognised​
• The two major genes are OCA2 and HERC2, both located on chromosome 15​
• Eye colour genes are X-linked in part

Kernel Colour in Wheat: Nilsson-Ehle's Experiment (Plant Example)

Scientist: Nils Herman Nilsson-Ehle (developed the "multiple factor" theory)​
Genes involved: Three independent gene pairs — A/a, B/b, C/c​

F₂ Kernel Colour Spectrum:​

F₂ Ratio = 63 Red (various shades) : 1 White

Polygenic inheritance in wheat kernel colour

The genetic control of colour in wheat kernels​

1. Polygenic traits are controlled by two or more independent genes, with each contributing allele adding an equal, additive increment to the final phenotype.
2. Instead of falling into distinct categories, these traits form a continuous spectrum that characteristically results in a bell-shaped, normal distribution curve within a population.
3. This type of inheritance lacks simple dominant-recessive relationships and gene masking (epistasis); alleles are simply contributing or non-contributing, and the genes assort independently.
4. The final physical expression of these traits is not strictly dictated by genetics, as it is heavily modified by environmental conditions, such as nutrition, affecting a person's height.
5. Prominent examples include human skin colour, human height (involving roughly 400 genes), human eye colour, and wheat kernel colour.