Polygenic Trait: Definition, Types, and Examples
Polygenic Trait Definition
A polygenic trait is a characteristic, sometimes we call them phenotypes, that are affected by many, many different genes. A classic example of this would be height.
What is a Polygenic Trait?
A polygenic trait is one in which a number of non-allelic genes play a role. These sorts of genes are referred to as polygenes. They are a collection of genes that, when activated, express as a single unit. Each of these has an influence on the characteristics as a whole. Nonetheless, it is difficult to differentiate the influence of a single gene, especially when a polygenic trait comprises many genes.
The phenotypic character that results is frequently an intermediate of heritable characteristics, although traits at the extremes are less common than intermediates. As a result, the polygenic trait distribution may be represented as a bell-shaped pattern with continuous fluctuation.
The various trait outcomes can not be readily and straightforwardly divided into groups, such as white or black, because of the broad range of trait variances, but rather by a wide spectrum, such as light to dark.
Polygenic characteristics in humans include height, skin colour, hair colour, and eye colour. Polygenic diseases include type 2 diabetes, coronary heart disease, cancer, and arthritis. However, polygenes can be impacted by environmental variables, so these circumstances are not solely genetic.
Polygenic Trait Etymology
The phrase polygenic derives from the words poly, which means “many,” and genic, which means “of genes.”
Polygenic Trait vs Mendelian Inheritance
A polygenic trait is one that develops as a result of polygenic inheritance. A non-Mendelian inheritance is one that does not obey Mendelian rules. Gregor Mendel, an Austrian monk and botanist, proposed the Mendelian rules. From 1856 until 1863, his breeding efforts and research on garden pea plants went undetected.
They were only discovered in the early twentieth century. More than three decades later, separate investigations by Erich von Tschermark, Hugo de Vries, Carl Correns, and William Jasper Spillman confirmed Mendel’s results. Monogenic inheritance, in which only one pair of alleles or one gene is involved, is an example of Mendelian inheritance.
The phenotypic ratio of a test cross involving a single pair of alleles may be easily predicted using a Punnett square because it follows Mendelian rules such as the Law of Unit Characters, the Law of Segregation, and the Law of Independent Assortment.
However, additional research revealed that some modes of inheritance and the phenotypic ratio that resulted did not follow these principles. Polygenic inheritance is one of them. This is because this kind of inheritance is regulated by several genes (called polygenes), which can be found at various loci on different chromosomes and are not controlled by a single gene (or one pair of alleles).
These polygenes are more likely to be expressed together in order to create a certain phenotypic characteristic. A polygenic trait is a characteristic that is created by the expression of several genes. The Punnett square would indicate larger child differences from a test cross in polygenic inheritance, and hence would not be as clear as it is in monogenic inheritance.
Monogenic vs Polygenic Trait
As previously stated, a monogenic trait is one that is caused by the expression of a single gene or a pair of alleles. The seed colour of the garden pea would be an example of this in one of Mendel’s breeding experiments. The progeny of a cross between a true-breeding yellow-seeded pea and a true-breeding green-seeded pea generated exclusively yellow seeds.
The green seed characteristic resurfaced in the following generation (F generation), reappearing at a 3:1 ratio, meaning that for every three offspring generating yellow seeds, one produced green seeds. Mendel argued that the characteristic that was expressed (yellow) was dominant, while the one that was masked (green) was recessive, based on these results.
A monogenic characteristic, on the other hand, would be more forthright and may be divided into two groups, yellow or green in this example.
A polygenic trait is one that is caused by the expression of several genes. These genes seemed to have an additive influence on the offspring’s phenotypic.
Consider it a painting on an art canvas made by combining various colours and intensities. As a result, a polygenic character is a polygenic activity that occurs across time. When a single dominant allele is present, its expression is limited to a single unit rather than the entire characteristic.
As a result, a polygenic trait is quantitative as opposed to a qualitative monogenic trait. Because the polygenic characteristic is caused by many genes, the phenotypic ratio of the F generation would deviate from 3:1. Despite the fact that several genes influence a characteristic, their separate impacts are difficult to identify.
A polygenic trait’s sensitivity to environmental influences is another characteristic. Environmental factors, in addition to polygenes, may influence the polygenic characteristics.
Bell-shaped Distribution
When a monogene is expressed, it creates discontinuous variation; when a polygene is produced, it produces continuous variation. An intermediate type of polygenic characteristics is often exhibited. Only a few dominant and recessive types exist. As a result, frequencies and ratios are less effective in forecasting polygenic differences than they are in predicting monogenic differences.
Variations in polygenic inheritance are calculated and evaluated using variances, covariances, and averages. Polygenic character characteristics have a consistent continuous range of variation in their frequency (e.g. from the lowest to the highest, the thinnest to the thickest, the shortest to the longest, the brightest to the darkest, and so on.).
To put it another way, the range of variance is distributed in a bell-shaped pattern. As a result, polygenes open up a lot of options in terms of phenotypic characteristics.
Polygenic Trait Examples
A large number of human characteristics are polygenic. Height, skin colour, and eye colour are examples of polygenic characteristics expressed in humans. Polygenes have the benefit of generating a broader range of phenotypic and genotypic variability in the population.
Several genes control human height, resulting in a broad variety of heights in a community. A genome-wide association analysis discovered 697 genetic variations in 423 genomic loci that have a role in determining an adult human’s height. Non-genetic variables (such as diet) can impact the characteristics in addition to genetic predisposition.
As a result, predicting the height of the children based on the heights of the parents is difficult. Despite having a tall father, the kid may be short. Similarly, a tall child can be conceived by two short parents. In certain circumstances, the child’s height will fall somewhere between the parents’.
Skin colour, hair colour, and eye colour are all polygenic characteristics. These characteristics are determined by how much melanin is produced and deposited by the body. A person with many alleles related to melanin synthesis and deposition will have a dark complexion on their skin, hair, and eyes.
An individual with alleles for melanin synthesis and deposition in the skin but no alleles for melanin in the eyes, for example, will have a darker skin colour but a lighter eye colour. An individual with fewer alleles for melanin synthesis and deposition, on the other hand, will have a lighter complexion.
It’s important to distinguish between a polygenic trait and a codominant characteristic. Human blood type AB, for example, is not a polygenic characteristic. It’s more of a case of dominance. Blood type AB people have dominant alleles for A and B antigens on their red blood cells, which means they are expressed together.
Polygenic Trait Disorders
Polygenes produce a genetic condition known as polygenic disease. Type-2 diabetes is an example of a polygenic disorder that can progress to illness. Polygenes are thought to be the majority of the genes involved in this disease.
According to recent research, there are more than 36 genes involved in the increased risk of type 2 diabetes. The inheritance of the TCF7L2 allele, for example, can raise the risk of diabetes by 1.5 times.
Other medically significant polygenic diseases include hypertension, coronary heart disease, cancer, arthritis, and mental illness, in addition to type-2 diabetes.
These disorders, on the other hand, aren’t totally hereditary. There might also be environmental variables at play that enhance a person’s chances of contracting any of them. Nonetheless, because they are polygenes that operate cumulatively, inheriting many of the genes associated with them raises the risk.
Polygenic Trait 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.
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