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Exceptions to Mendel's Principles > Polygenic traits - Quantitative inheritance

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Estimated time: 14 minutes
CISCE: Class 12

Definition: Polygenic traits (Quantitative inheritance)

Polygenic traits are characters controlled by two or more independent genes, each having a small additive effect, resulting in continuous variation in the phenotype rather than distinct categories.

CISCE: Class 12

Introduction

  • Polygenic inheritance is the inheritance of a single trait controlled by two or more independent genes, where each gene contributes a small additive effect to the phenotype.
  • It is also called quantitative inheritance, multiple gene inheritance, or multiple factor inheritance.
  • Because many genes act together, the trait does not appear in only two clear forms; instead, it shows a gradual range of variation called continuous variation.
CISCE: Class 12

Main Features

  • More than one gene pair controls the same character.
  • Each contributing allele adds a small amount to the expression of the trait.
  • The combined effect of all such genes produces the final phenotype.
  • Polygenic traits are usually measurable, so they are called quantitative traits.
  • These traits often show a bell-shaped distribution in a population, with intermediate forms more common than extreme forms.
  • Environmental factors may also influence their expression.
CISCE: Class 12

Causes of Continuous Variation

  • In Mendelian inheritance, one major gene often produces distinct categories such as tall or dwarf.
  • In polygenic inheritance, many genes act together, so many different gene combinations are possible.
  • This produces a continuous series of phenotypes, such as different heights or skin shades, rather than sharply separated categories.
CISCE: Class 12

Examples of polygenic traits

  • Human height.
  • Skin colour in humans.
  • Eye colour and hair colour.
  • Body weight and risk of some diseases.
  • Milk production and egg production in animals.
  • Kernel colour in wheat and other yield-related traits in plants.
CISCE: Class 12

Human skin colour example

  • Human skin colour is a standard example of polygenic inheritance.
  • In the simplified model, three pairs of genes are considered, and each dominant allele contributes to darker pigmentation.
  • When a dark parent is crossed with a light parent, the F1 generation is usually intermediate in skin colour.
  • In the F2 generation, seven phenotypic classes may appear because different combinations of contributing alleles are produced.
  • The expected phenotypic distribution is 1:6:15:20:15:6:1, showing that intermediate phenotypes are most common, while the extreme phenotypes are least common.
CISCE: Class 12

Role of environment

  • Polygenic traits are often affected by both genes and environment.
  • For example, height depends on genes, but nutrition and health also influence how much height is actually attained.
  • Therefore, genes provide the potential range, while the environment affects the final expression.
CISCE: Class 12

Key Points: Polygenic Traits – Quantitative Inheritance

  • Polygenic inheritance occurs when a single physical trait is controlled by two or more independent genes working together.
  • Each contributing allele adds a small, measurable amount to the final physical appearance of the organism.
  • The combined genetic effect creates a continuous range of variation in a population rather than sharply separated categories.
  • Examples of these quantitative traits include human skin colour, human height, and kernel colour in wheat.
  • Environmental factors frequently interact with an organism's genetic potential to influence the final expression of the trait.
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