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.

Table of Contents

Definition

Characteristics

Polygenic Inheritance in Humans

Polygenic Inheritance in Plants

Effect of Environment

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; in fact, 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 a type of genetic inheritance whereby multiple genes are involved in determining a trait, as opposed to single gene inheritance, where only one gene is responsible for the trait.

“Polygenic inheritance is characterized as a form of quantitative inheritance in which multiple separate genes contribute to the expression of a single, quantifiable trait.”

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Polygenic inheritance is also known as multiple gene inheritance or multiple factor inheritance.

Polygenic Inheritance characteristics

Polygene refers to a gene that has a small effect on a phenotype when combined with other genes.

It is difficult to detect the effect of a single gene since it is too small.

Multiple genes have an equal effect

Each allele has a cumulative or additive effect

Polygenic inheritance differs from multiple alleles, as in the latter, three or more alleles are present in the same locus, while in the former, the trait is determined by the combined effect of multiple loci. An example of multiple alleles is the ABO blood group system, which is controlled by three alleles.

There is no epistasis present, meaning that the expression of an allele of a different locus is not being masked.

There is no linkage or dominance, rather there exist contributing and non-contributing alleles, referred to as active and null alleles respectively.

Polygenic inheritance is characterized by the continuous variation of the phenotype of a trait.

The complexity of the polygenic inheritance pattern makes it challenging to predict phenotype.

The statistical analysis can provide an estimation of population parameters.

Polygenic Inheritance Examples

Polygenic Inheritance in Humans

Humans possess many traits that are inherited polygenically, such as skin and hair colour, height, eye colour, the risk for diseases and resistance, intelligence, blood pressure, bipolar disorder, autism, longevity, etc.

Some of the traits include:

• Being organized
• Being proactive
• Being a good communicator
• Being a good listener
• Being dependable
• Being motivated
• Being detail-oriented

1. Skin Pigmentation: Inheritance of skin pigmentation is polygenic, with approximately 60 loci contributing to the inheritance of a single trait. For example, if we take a pair of alleles from three different, unlinked loci (A and a, B and b, C and c), where the capital letters represent the incompletely dominant allele for dark skin colour, the more capital letters present will result in darker skin colour, and the more small letters present will result in lighter skin colour. Parents with the genotype AABBCC and aabbcc will produce offspring of intermediate colour in the F1 generation, with the genotype AaBbCc. In the F2 generation, when 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.

Punnett Square Showing F2 Generation Offspring’s Continuous Variation

From dark to light→

1. Height: Approximately 400 genes are responsible for the phenotype, and the environment has a significant impact on how these genes are expressed.

2. Eye Colour: The colour of the eye is determined by a combination of multiple genes. At least 9 distinct eye colours are recognised in humans. There are two major eye colour genes as well as 14 more genes that influence the eye colour phenotype. A different number of alleles contribute to each colour, and these are found to be X-linked.

Polygenic Inheritance in Plants

Polygenic inheritance in plants can affect characteristics such as the color and shape of the stem, pollen, flower, yield, oil content, size of a seed, time to mature or flower, etc.

Some of the traits include:

• Being organized
• Being a good listener
• Being punctual
• Being reliable
• Being open to feedback

1. Kernel Colour of Wheat: The expression of kernel colour of wheat involves three independent pairs of alleles, which show independent assortment. When a dark red wheat kernel (AABBCC) is crossed with a white wheat kernel (aabbcc), the F1 generation has an intermediate red colour kernel (AaBbCc). When the F1 generation is crossbred, the F2 generation has 63 red kernel plants with 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 Shades of Red: 1 Shade of White

1. Length of the Corolla in Tobacco: The expression of phenotype for corolla length of tobacco is affected by approximately 5 genes. Polygenic inheritance contributes to the wide range of corolla lengths in tobacco.

Effect of Environment on Polygenic Inheritance

The genotype of an organism determines the potential range of a quantitative trait, however, the environment plays a major role in the actual expression of polygenes. Genes can be activated or deactivated depending on the environment, thus influencing the phenotype of the organism.

The range of phenotype possible under different environmental conditions from the same genotype is referred to as ’norm of reaction’. This norm of reaction can vary in breadth, with some genotypes having a very narrow range and others having a much broader range; for example, genotypes involved in human height typically have a wide norm of reaction.

Individuals may have genetic potential or vulnerability, but environmental conditions influence the expression of genotype. This is evident in the case of identical twins raised in two different environments. Human characters such as intelligence, depression, height, skin colour, and schizophrenia show the effect of the environment on gene expression, illustrating the importance of both nature and nurture in phenotypic expression.

Examples: This is an example.

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1. Height is greatly influenced by diet and general health.

2. The color of Hydrangeas, a shrub, depends on the level of aluminium present in the soil.

3. Effect of Temperature on the Skin of Himalayan Rabbits

Frequently Asked Questions

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Polygenic inheritance is a type of genetic inheritance where a trait is determined by the cumulative effect of two or more genes. This means that the trait is controlled by multiple alleles, instead of just one.

Polygenic inheritance refers to the inheritance of a trait governed by more than one gene. Generally, three or more genes govern the inheritance of polygenic traits. Multiple independent genes have an additive or similar effect on a single quantitative trait.

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Which human trait is an example of polygenic inheritance?

In polygenic inheritance, each allele contributes to the phenotype in a cumulative or additive way, resulting in a continuous range of variation, such as skin pigmentation, height, etc.

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Examples of polygenic traits include:

1. Height
2. Skin color
3. Eye color

Three examples of polygenic traits in humans are height, skin colour, and eye colour. These traits are governed by multiple genes.

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Yes, blood type is an example of polygenic inheritance.

A single gene can have multiple alleles, known as multiple allelism, which is exemplified by ABO blood type. This is different from polygenic inheritance, where a gene has multiple variants (alleles) located in the same locus and an individual can possess any two of these alleles. For example, Gene I has three alleles - IA, IB, and i - and the combination of two allele pairs determines the blood type.

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Yes, eye color is a polygenic inheritance trait.

Yes, eye colour is a polygenic trait. There are two major eye colour genes, OCA2 and HERC2, which are present on chromosome 15. Other than these, there are more than 13 genes identified that determine the expression of the phenotype (colour of eye). At least 9 colours of eye are recognised in humans.