Polygenic Inheritance: Definition, Characteristics, Examples & Affecting Factors

Polygenic Inheritance: Have you ever heard someone say ‘You are just like your father’ or ‘She is just like her great grandmother’? Yes, we inherit a few or more traits from our parents, grandparents or great grandparents. These traits are generally controlled by genes. And, when the inheritance of such traits are controlled by three or more genes, it is called polygenic inheritance.

Polygenic Inheritance is produced by the cumulative effect of many genes. It is a form of non-Mendelian inheritance which deviates from the typical phenotypic Mendelian ratio. Polygenic Inheritance is found in both animals and plants. No two people can be exactly the same; even the twins have minute differences in them. Features or traits such as height, skin pigmentation, eye colour, kernel colour of wheat, etc., are some of the examples of polygenic inheritance.

What is Polygenic Inheritance?

Polygenic Inheritance is defined as the quantitative inheritance of multiple independent genes having an additive or cumulative effect on a single quantitative trait. It is also known as multiple gene inheritance or multiple factor inheritance. Joseph Gottlieb Koleuter is known as the Father of Polygenic Inheritance. Thus, polygenic inheritance refers to the inheritance of the phenotypic trait (like height, colour, intelligence, etc.) that is not a single gene trait but a cumulative expression of multiple genes, which is different from the monogenic inheritance, in which the trait results from the expression of one gene only. Monogenic inheritance follows the Mendelian laws of inheritance.

Polygenic Inheritance is controlled by two types of alleles, namely:
1. Contributing (or effective) Alleles: These are those alleles that contribute to continuous variation.
2. Non-contributing (or non-effective) Alleles: These are those alleles that do not contribute to continuous variation.

Polygenic Inheritance Characteristics

Following are the characteristics of polygenic inheritance:

1. The genes of polygenic inheritance known as polygenes have a minor effect on a phenotype along with the other genes.
2. The effect of individual genes is too minor and remains undetected.
3. A number of genes exert an equal effect.
4. Each individual allele contributes to the phenotype in an additive or cumulative manner.
5. In polygenic inheritance, the expression of one gene is not masked by the presence of the other genes, i.e., epistasis is not involved.
6. It is different from multiple allelism because, in multiple allelism, three or more alleles are present on the same locus, out of which only two alleles are present in an organism.
7. Unlike Mendelian inheritance, polygenic inheritance does not have any genes called dominant or recessive.
8. In polygenic inheritance, continuous variation is observed in the phenotype of a trait.
9. The pattern of polygenic inheritance is difficult to predict and highly complex.
10. The statistical analysis of polygenic inheritance helps to provide an estimate of population parameters.

Polygenic Inheritance Examples

Polygenic Inheritance occurs in both animals and plants.

Polygenic Inheritance in Humans

There are many examples of polygenic inheritance in humans like skin and hair colour, height, eye colour, the risk for diseases and resistance, intelligence, blood pressure, autism, longevity, etc. Let us know in detail about a few examples:

A. Skin Colour and Pigmentation

1. The skin colour and pigmentation are controlled by multiple genes, which are present at around 60 loci.
2. To understand the pattern of inheritance of skin colour, let us consider three pairs of different alleles present at different loci, which are represented as A and a, B and b, C and c.
3. The alleles responsible for dark skin colour are represented by capital letters. The greater the number of capital letters in the genetic pattern, the darker is the skin colour and vice versa.
4. When the parents having the genotype AABBCC and aabbcc are crossed, then all the offspring produced in the F1 generation have intermediate colour, i.e., with a genotype AaBbCc.
5. Then, when there was a cross between the two triple heterozygotic parents in the F1 generation, i.e., AaBbCc × AaBbCc, then the offsprings obtained in the F2 generation have varying skin colours ranging from very dark to very light with a phenotypic ratio of 1:6:15:20:15:6:1.
6. The skin colour of an individual is due to the presence of a pigment called melanin.
7. A darker skin-coloured individual with all the dominant alleles, ‘AABBCC,’ would have the highest amount of melanin in the skin, and a lighter skin-coloured individual with the genotype ‘aabbcc’ would have the least or negligible amount of melanin in the skin.

Fig: Polygenic Inheritance of the skin colour by using Punnett square in the F₂ generation

B. Human Height

1. It is a polygenic trait that is controlled by three genes having six alleles. So, a tall person would have all dominant alleles, whereas a short person would have all recessive alleles.
2. There are around 400 genes responsible for the different phenotypes of human height, which are greatly influenced by the environment.

C. Human Eye Colour

1. Nine eye colours are recognized in humans who follow a polygenic inheritance pattern.
2. This trait is controlled by two major genes and fourteen additional genes linked to the X-chromosome.
3. Different combinations of these alleles result in a variety of eye colours.
4. The eye colour is due to the presence of melanin in the front portion of the iris.

Polygenic Inheritance in Plants

Examples of this type of inheritance include the shape and colour of the flowers, stem, pollen, size of a seed, yield, oil content, time to mature or reach flowering, etc.

A. Kernel Color of the Wheat

1. It is an example of polygenic inheritance in plants which is controlled by the expression of three independently assorted pairs of alleles.
2. When all the alleles are dominant, i.e., AABBCC, the colour of the wheat kernel is dark red, and when all the alleles are recessive, i.e., aabbcc, the colour of the wheat kernel is white.
3. If a dark red wheat kernel is crossed with a white wheat kernel, all the F1 offspring obtained have an intermediate red colour kernel, i.e., with a genotype AaBbCc.
4. But when the F₁ plants are crossbred further, then in the F₂ generation, there would be 1 white kernel plant while 63 red kernel plants with varying shades would be produced.

Fig: Polygenic Inheritance of wheat kernel colour by using Punnett square in the F₂ generation

B. Length of the Corolla in the Maize

1. It is also another example, whose expression is controlled by 5 genes.
2. There are a number of varieties in the length of the wheat due to polygenic inheritance.

Effect of Environment on Polygenic Inheritance

Nature plays a very important role in shaping the characteristics of an individual. Internal or inherent features are also affected by the environment. Usually, the genotype sets the range for a quantitative trait while the phenotype results from the varying environmental factors. One of the examples where we can find the impact of the environment on the genes is in the case of twins. If both individuals are brought up in different environmental conditions, then both individuals may have different characteristics and may act differently. Another example of how the environment can affect the phenotype is a hereditary disorder called phenylketonuria (PKU). People who are homozygous for this disease lack the activity of an enzyme that breaks down the amino acid phenylalanine, which is very toxic for our bodies. PKU can be treated in a very simple way by giving the affected babies and children a diet low in phenylalanine. If this diet is followed strictly from a very young age, then the person would have few or even no symptoms of the disorder. Thus, it is evident that the environment has an effect on polygenic traits.

Summary

Polygenic inheritance is defined as the quantitative inheritance of multiple independent genes having an additive or cumulative effect on a single quantitative trait. It is also known as multiple gene inheritance or multiple factor inheritance. It is greatly influenced by environmental factors. It is different from multiple alleles because, in multiple alleles, three or more alleles are present on the same locus, out of which only two alleles are present in an organism. There are many examples of polygenic inheritance in humans like skin and hair colour, height, eye colour, the risk for diseases and resistance, intelligence, blood pressure, autism, longevity, etc. Examples of this type of inheritance include the shape and colour of the flowers, stem, pollen, size of a seed, yield, oil content, time to mature or reach flowering, etc.

FAQs on Polygenic Inheritance

Q.1. What is meant by polygenic inheritance?
Ans: Polygenic inheritance is defined as the quantitative inheritance of multiple independent genes having an additive or cumulative effect on a single quantitative trait. It is also known as multiple gene inheritance or multiple factor inheritance.

Q.2. What are the characteristics of polygenic inheritance?
Ans: The characteristics of polygenic inheritance are as follows:
1. Polygenes are the genes that have a minor effect on a phenotype along with the other genes.
2. Individual gene effect is too minor which remains undetected.
3. A number of genes exert an equal effect.
4. Each individual allele contributes to the phenotype in an additive or cumulative manner.
5. The expression of one gene is not masked by the presence of the other genes, i.e., epistasis is not involved.

Q.3. Which human trait is an example of polygenic inheritance?
Ans: There are many examples of polygenic inheritance in humans like skin and hair colour, height, eye colour, the risk for diseases and resistance, intelligence, blood pressure, autism, longevity, etc.

Q.4. Is blood type an example of polygenic inheritance?
Ans: No, blood type is not an example of polygenic inheritance, rather it is an example of multiple allelism where a single gene has three different alleles in the same locus but an individual contains any of the two alleles.

Q.5. Who is known as the father of polygenic inheritance?
Ans: Joseph Gottlieb Koleuter is known as the Father of Polygenic Inheritance.

Learn About Mendel’s Law Of Inheritance Here

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