What is Polygenic Inheritance?

Many traits and phenotypic characters present in plants and animals such as height, skin pigmentation, hair and eye colour, milk and egg production are inherited through many alleles present in different loci. This is known as polygenic inheritance.

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If we take an example of height or skin pigmentation in humans, we find many different forms of the two traits. We can’t categorise people in just two categories like ‘tall’ and ‘short’ for height or ‘dark’ and ‘light’ for the skin colour. We find continuous variation for both these traits because these traits are controlled by multiple genes. There are as many as 400 genes that control the trait of height and are responsible for variation in height present in the population.

Polygenic Inheritance Definition

Polygenic inheritance is defined as quantitative inheritance, where multiple independent genes have an additive or similar effect on a single quantitative trait. Polygenic inheritance is also known as multiple gene inheritance or multiple factor inheritance.

Polygenic Inheritance characteristics

  • Polygene refers to a gene that exerts a slight effect on a phenotype along with other genes
  • Effect of a single gene is too small so it is difficult to detect
  • Multiple genes produce an equal effect
  • Each allele have a cumulative or additive effect
  • Polygenic inheritance differs from multiple alleles, as in multiple alleles, three or more alleles are present in the same locus of which any two alleles are present in an organism, e.g. ABO blood group system, which is controlled by three alleles
  • There is no epistasis involved, i.e. masking of the expression of an allele of the different locus
  • There is no linkage or dominance, rather there exist contributing and non-contributing alleles, which are known as active or null alleles respectively
  • Polygenic inheritance is characterised by the continuous variation of the phenotype of a trait
  • The polygenic inheritance pattern is complex. It is difficult to predict phenotype
  • The statistical analysis can give the estimate of population parameters

Polygenic Inheritance Examples

Polygenic Inheritance in Humans

There are many traits in humans, which show polygenic inheritance, e.g. skin and hair colour, height, eye colour, the risk for diseases and resistance, intelligence, blood pressure, bipolar disorder, autism, longevity, etc.

Brief description of some of the traits:

  1. Skin pigmentation: inheritance of skin pigmentation is polygenic inheritance. Around 60 loci contribute to the inheritance of a single trait. If we take an example of a pair of alleles of three different and unlinked loci as A and a, B and b, C and c. The capital letters represent the incompletely dominant allele for dark skin colour. The more capital letters show skin colour towards the darker range and small letters towards the lighter colour of the skin. Parents having genotype AABBCC and aabbcc will produce offsprings of intermediate colour in the F1 generation, i.e. AaBbCc genotype. In the F2 generation of two triple heterozygotes (AaBbCc x AaBbCc) mate, they will give rise to varying phenotypes ranging from very dark to very light in the ratio 1:6:15:20:15:6:1.

Polygenic inheritance of skin pigmentation

Punnett square showing F2 generation offsprings continuous variation

From light to dark→

  1. Height: There are around 400 genes responsible for the phenotype and environment greatly influences the expression of genes.
  2. Eye colour: The colour of the eye is determined by polygenes. At least 9 colours of eye colour are recognised in humans. There are two major eye colour genes and 14 more genes that determine the expression of the phenotype. A different number of alleles contribute to each colour. These are found to be X-linked.

Polygenic Inheritance in Plants

Polygenic inheritance in plants includes the colour and shape of the stem, pollen, flower, yield, oil content, size of a seed, time to mature or flower, etc.

Brief description of some of the traits:

  1. Kernel colour of the wheat: The three independent pairs of alleles are involved in the expression of kernel colour of wheat. They show independent assortment. When dark red wheat kernel (AABBCC) is crossed with the white wheat kernel (aabbcc) the F1 generation has an intermediate red colour kernel (AaBbCc). When F1 generation is crossbred, F2 generation has 63 red kernel plants having different shades of red and 1 white kernel.

F2

1

6

15

20

15

6

1

Dark

red

Moderate Red

Red

Intermediate red

Light red

Very light red

White

63 Red (many shades):1 (white)

  1. Length of the corolla in tobacco: There are around 5 genes involved in the expression of phenotype for corolla length of tobacco. There is a wide variety in the length of the corolla in tobacco due to polygenic inheritance.

Effect of environment on Polygenic Inheritance

The expression of polygenes is greatly influenced by environmental conditions. The genotype sets the range for a quantitative trait but the environmental conditions decide the phenotype within its genetic limits. Genes function differently in different environmental conditions. Environment regulates the activity of certain genes and sets them on or off.

The range of phenotype possible under the different environmental conditions from the same genotype is termed as ‘norm of reaction’. The norm of the reaction is narrow for certain genotypes and broad for some genotypes, e.g. genotypes involved in human height have a very broad norm of reaction.

Identical twins raised in two different environments show that individuals may have genetic potential or vulnerability but environmental conditions influence the expression of genotype. Human characters such as intelligence, depression, height, skin colour, schizophrenia show the effect of the environment on gene expression. Phenotypic expression is dependent on both nature and nurture.

Examples:

  1. Diet and general health greatly influences height
  2. Flower colour of a shrub, Hydrangeas, depends on the level of aluminium present in the soil
  3. Effect of temperature on the skin of Himalayan rabbits.

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