Polygenic Inheritance Definition

It is defined as a type of inheritance in which the expression of a gene is different from Mendelian Inheritance. The Mendelian inheritance expresses the trait of either dominant or recessive gene whereas the polygenic inheritance expresses the mixture or additive of traits displayed in parents. Therefore when the mixture of the traits present in parents is inherited into offspring, it is called a polygenic inheritance pattern.

What is Polygenic Inheritance?

A type of inheritance in which the trait is produced from the cumulative effects of many genes. Inheritance can be expressed mainly in two ways- monogenic inheritance and polygenic inheritance. When the traits result from the expression of only one trait, it is called monogenic inheritance.

The phenotypic ratio or Mendelian inheritance can predict the expression of monogenic inheritance but the polygenic inheritance is a non- Mendelian form. It is expressed together by multiple genes at different loci in the same trait.

For example- In monogenic expression, when heterozygotes having red and white progeny are crossed, the results will appear in the ratio of 3:1.

On the other hand in polygenic expression, the trait is controlled by two pairs of genes and the dominant allele of both loci are expressed in the flower. Here, the results produce red and white color in a ratio of 9:7.

The examples of polygenic inheritance inhuman include hair color, height, skin color, blood pressure, intelligence, and autism, etc. There are two types of alleles that control polygenic inheritance, namely:

1. Contributing alleles: These alleles contribute to continuous variation.

2. Non- contributing alleles: These alleles do not contribute to continuous variation.

They are also called effective and non-effective alleles.

Characteristics of Polygenic Inheritance

• A polygene is a gene that employs a minor effect on a phenotype.

• The effect of the gene remains undetected.

• An equal effect is exerted by numerous genes.

• The dominant gene does not mask the effect of the recessive gene.

• The genes may be either contributing and non-contributing, but there is not any dominant or recessive gene.

• Estimation of population parameters can be done by the statistical analysis of polygenic inheritance.

• The polygenic inheritance is different from multiple alleles. An example of multiple alleles is the human blood group system.

Analysis of Polygenic Traits

In 1918, Sir Ronald Aylmer Fisher describes the quantitative characters of Mendelian genetics. Polygenic inheritance is used to understand the differences between quantitative and qualitative inheritance.

1. The characters such as weight, length, width, height, duration are measured by polygenic traits.

2. The variations are expressed in terms of variance or co-variance.

3. The genetic interpretations of quantitative characters are studied in Quantitative genetics.

Polygenic Traits vs Oligogenic Traits

Polygenic Traits	Oligogenic Traits
The number of genes control the trait	Controlled by few genes
The expression is usually undetectable	The expression of an individual gene can be detected
Continuous variations	Discontinuous variations
Influenced by environmental factors	Not influenced by the environmental factors

Segmentation of Polygenic Variability

Segregation of the polygenic variation present in a genetic population is done by variance. There are three types of polygenic inheritance, namely:

1. Phenotypic: The variability can be observed and also include environmental variations. It is often considered total variability.

2. Genotypic: It is genetically inherited variability. The genotypic variability is not influenced by environmental factors. The components of genotypic variability are dominant, additive, or epistatic. Plant breeders use this variability for hybridization and are considered the most commonly exploited variability. The additive variance is produced due to the average effects of genes that are present on different loci. In other words, there is not any dominant allele in additive variance. For example- The offspring produced by a cross between AA and aa alleles show intermediate expressions. It is the most common variance among self-pollinating crops.

3. Environmental: The non-inheritable variations that occur due to environmental factors are called environmental variance. The variance depends upon the environmental conditions and cannot be controlled. It is measured in terms of error mean-variance.

Polygenic Inheritance Examples

i. Polygenic Inheritance in Humans

I. Skin Color and Pigmentation: Skin color is an example of polygenic inheritance. About 60 loci control the skin color in humans. For example- three pairs of alleles present at a locus are responsible for skin color. The alleles are represented as A and a, B and b, C and c. here, the capital letters represent the incomplete dominant alleles that are responsible for the dark color of skin. So, in the offspring, the dark skin color is indicated by a capital letter and the small letter indicates the lighter color of the skin.

Here, the progeny shows intermediate color in the F1 generation and the genotype would be AaBbCc. The F2 generation produces different skin colors in the ratio of 1:6:15:20:15:6:1. The skin color depends upon the amount of melanin in the skin, thus the higher amount of melanin results in darker skin, and a low or negligible amount of melanin results in lighter skin color.

II. Human Heights: Three genes control the height in humans. So, a person having all dominant alleles has a tall phenotype and a short person will have recessive alleles. The polygenic inheritance pattern for height has represented by a curve where the middle portion represents the population with average height.

III. Polygenic Inheritance of Eye Color: The eye color has controlled by two prominent genes and 14 additional genes in humans. A variety of eye colors has been produced due to different combinations of alleles. The eye color depends upon the melanin present in the iris. A high amount of melanin results in black and brown eyes while a low amount of melanin produces hazel or green eye color. The dominant allele increases the amount of melanin and produces black eyes. The combination of these dominant and recessive alleles results in different intermediate eye colors, e.g. light brown eyes.

ii. Polygenic Inheritance in Plants

The common polygenic traits in plants include flower, stem, pollen, yield, oil content, time of flowering, etc.

I. Kernel Color of the Wheat: Three pairs of alleles control the kernel color of wheat. Here, all dominant alleles result in dark red wheat kernel, while the recessive allele produces the white kernel color. The F2 generation produces kernels with different shades of red.

II. Length of the Corolla in Tobacco: There are 5 genes express for the corolla of tobacco. The length of the corolla also shows polygenic inheritance.

Effect of Environment on Polygenic Inheritance

The environmental factors highly influence polygenes. The phenotypic variance is controlled by various environmental factors. The norm of reaction is defined as the variation in the phenotypic expression which can be classified as

• Narrow norms of reaction

• Broad norms of reaction

All the phenotypic variances such as intelligence, depression, skin color, height are affected by the environment. Hereditary disorder phenylketonuria is an example of a polygenic character that is caused due to lack of enzyme phenylalanine.

Importance of Polygenic Inheritance

Polygenes or polygenic variance has great significance in evolution. The polygenesis results in the evolution of species that is called the theory of polygeny of evolution. The specific requirements or needs of particular species result in different variations that are responsible for adaptive changes in the species. Plant breeders utilize polygenes to improve the crops. The polygenes work on the principle of segregation and recombination of polygenic genes after interbreeding.

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Polygenic Inheritance Citations

• Polygenic risk scores: from research tools to clinical instruments. Genome Med . 2020 May 18;12(1):44.
• The personal and clinical utility of polygenic risk scores. Nat Rev Genet . 2018 Sep;19(9):581-590.
• An Expanded View of Complex Traits: From Polygenic to Omnigenic. Cell . 2017 Jun 15;169(7):1177-1186.