What is Polygenic Inheritance?

Polygenic inheritance is a type of inheritance in which multiple genes control the phenotype of an organism. The phenotypes or traits can be height, skin color, the color of the eyes, etc. This type of inheritance is also known as quantitative inheritance or multifactorial inheritance. Such traits are known as polygenic traits. It is different from mendelian genetic inheritance in a number of genes controlling the traits. Polygenic inheritance can be seen in animals as well as plants. Every gene that controls traits is not necessarily equal. However, environmental factors also play an important role in polygenic inheritance. Polygenic inheritance has important implications for our understanding of complex diseases and traits, as well as for conservation and agricultural breeding.

What is Polygenic inheritance?

Polygenic inheritance is a type of inheritance in which a single phenotypic trait (height, color of the eye, skin color, etc.) is governed by more than one pair of genes. Such traits are called polygenic traits. Let’s understand polygenic inheritance in humans as well as plants.

Characteristics of Polygenic Inheritance

1. A single gene affects multiple traits. 
2. Alleles of genes produce a cumulative or additive effect on phenotypes.
3. Each allele has a cumulative or additive effect
4. Polygenic inheritance differs from epistasis in a number of genes. In epistasis, more than one genes produce an effect whereas, in polygenic inheritance, a single gene produces the effect.
5. It shows the continuous variation of traits.
6. Being complex, it is difficult to predict the phenotype of an organism. 
7. Being large complex data, statistical methods can give the best output.

Examples of Polygenic Inheritance

Polygenic inheritance in humans

In humans, there are so many examples of polygenic inheritance. Like skin or hair color, height, and eye pigmentation.

Skin Pigmentation in Human

Skin color is controlled by three pairs of genes, Aa, Bb, and Cc, located in different chromosomes. A very dark person will have all six dominant genes (AA, BB, and CC). A person with very light skin color has all the six recessive genes (aa, bb, cc). Suppose a man with a homozygous condition for dark skin (AABBCC) marries a woman who is homozygous for light skin color (Aabbcc), their progeny in the F1 generation will have an intermediate color (AaBbCc) in the skin that differs from that of both parents. This is called mulatto color.

The crossing between two F1 individuals results in a highly variable F2 progeny in which a few individuals resemble one grandparent, a few resemble the other grandparent, and the remaining range between the two. The skin color ranges from pure black (AABBCC) in 1/64 individuals very dark brown in 6/64 individuals, dark brown in 15/64 individuals, mulatto (or intermediate) (AaBbCc) in 20/64 individuals, light brown in 15/64 individuals, very light brown in 6/64 individuals and pure white (aabbcc) in 1/64 individuals.

Polygenic Inheritance in Plants

Kernel Color of the Wheat

The kernel color of wheat is controlled by three pairs of alleles, R1R1R2R2, and r1r1r2r2. The dark red dominant wheat kernel is represented by AABBCC and the white recessive color is represented by R1r1R2r2. The offspring produced in the F1 generation will be red in color with genotype AaBbCc. Self-breeding of F1 plants will produce 64 offsprings, of which 63 are red kernel plants with varying shades and 1 is white.

A total of 64 offspring were formed out of which, 1 is dark red, 6 are moderate red, 15 are red, 20 are intermediate red, 15 are light red, 6 are very light red, and 1 is white.

How Does Polygenic Inheritance Work?

Polygenic inheritance works by combining the effects of several individual genes. Every gene involved in a given trait contributes to the trait, but their contribution is not necessarily equal. In addition to the genes that contribute to a trait, environmental factors also play an important role. Different environmental factors can influence the expression of a trait and can even influence the expression of genes that contribute to the trait. The interaction of genes and environmental factors makes it difficult to predict the outcome of a particular trait. A geneticist studying a trait might be able to identify the genes that are likely to be involved and the likely contribution of each gene, but it remains difficult to predict the exact outcome of a trait.

Importance of Polygenic Inheritance

The idea of polygenic inheritance, commonly referred to as quantitative inheritance, is crucial to understanding how qualities that are influenced by numerous genes are passed down across generations. Height, weight, skin tone, and other characteristics can be influenced by polygenic inheritance. These qualities can be found in plants, animals, and people, and they frequently have a greater influence on an organism's phenotype than single-gene features. There are numerous useful applications for understanding how polygenic inheritance functions. For instance, it can shed light on the breeding of crops and the transmission of diseases from one generation to the next. Understanding the inheritance of complex qualities like intelligence or conduct has also been done using polygenic inheritance.

Effect of environment on Polygenic Inheritance

The effects of polygenic inheritance on organism evolution and environmental adaptation are significant. The progressive transmission of genes from one generation to the next under this type of inheritance will cause a trait to gradually change over time. Even though they happen gradually, these modifications might eventually produce noticeable variations in an organism's phenotype. If one group of creatures diverges from another through time, this may result in the emergence of new species. A population's features may abruptly change due to sudden bursts of mutations caused by polygenic inheritance. Depending on the surrounding environment, this may be advantageous or detrimental. A population may exhibit deleterious features as a result of sudden mutations, or advantageous traits may emerge quickly.

Effect of environment on polygenic inheritance

Environment plays a significant role in determining the phenotype of an organism. For example- the amount of aluminum present in soil can determine the color of shrubs, flowers, etc. Similarly, the skin of rabbits is greatly influenced by temperature changes in Himalayan regions.

It has been observed that different genes work under different environmental conditions. The effect can be broad or narrow. For example-gene responsible for height have a broader range of phenotypes. Similarly, twins living in different environments may have changes in their genes. Likewise, other factors such as intelligence, skin color, etc. genes are also affected by changes in the environment.

The progressive transmission of genes from one generation to the next under this type of inheritance will cause a trait to gradually change over time. Even though they happen gradually, these modifications might eventually produce noticeable variations in an organism's phenotype. If one group of creatures diverges from another through time, this may result in the emergence of new species. A population's features may abruptly change due to sudden bursts of mutations caused by polygenic inheritance. Depending on the surrounding environment, this may be advantageous or detrimental. A population may exhibit deleterious features as a result of sudden mutations, or advantageous traits may emerge quickly.

Conclusion

Genetics' understanding of polygenic inheritance, which describes the inheritance of traits influenced by several genes, is crucial. Understanding polygenic inheritance has many practical applications, from helping researchers create novel crops through agricultural breeding to comprehending the inheritance of complex features. In addition, polygenic inheritance is crucial to evolution since it promotes the formation of new species as well as slow but steady changes in an organism's phenotype through time. It has also been linked to abrupt bursts of mutations that can alter a population in both positive and negative ways. In general, scientists need to understand polygenic inheritance, particularly those working in the disciplines of genetics, agriculture, and conservation. The ramifications of this type of inheritance are enormous and can aid in our understanding of how complex diseases originate as well as how animals and plants evolve.