Polygenic Inheritance: Definition, Overview, Meaning, Examples, Plants and Humans

Polygenic inheritance, in simple language, means a character or phenotypic trait that is regulated by more than one gene. It refers to the type of quantitative inheritance wherein two or more independent genes, altogether, affect a single phenotypic trait. Therefore, polygenic inheritance is a phenotypic trait that is not a single-gene trait but a cumulative expression of multiple genes.

This Story also Contains

1. What is Polygenic Inheritance?
2. Mechanism of Polygenic Inheritance
3. Examples of Polygenic Inheritance
4. Polygenic Inheritance and Evolution
5. Recommended video for "Polygenic Inheritance"
6. MCQs on Polygenic Inheritance

Polygenic inheritance is different from Mendelian inheritance in terms of pattern and expression. In Mendelian inheritance, monogenic expression of a gene is seen, where the trait is either expressed or masked based on it being dominant or recessive. In polygenic inheritance, on the other hand, the phenotypic trait does not display complete dominance in the expression of the trait in an offspring, but is a mixture or additive of traits seen in parents. Polygenic inheritance is an important topic in biology subject.

What is Polygenic Inheritance?

The polygenic pattern of inheritance can also be described as a system whereby several genes, each influencing a given characteristic. Polygenic traits are inherited somewhat differently from simple Mendelian genetics. It is rather the multiple genes located on different chromosomes that affect the trait. This leads to what is termed a ‘smooth spectrum’ of phenotypes, which makes it difficult to have distinct classes.

For instance, height, skin colour, and even eye colour are controlled by many genes or polygenic. Height is polygenic because genes responsible for height and its determinant factors are several, where skin and eye colour depend on the effect of several genes that produce a range of colours. Likewise, in plants and animals, any character, from grain shade in wheat to coat shade in dogs, also shows polygenic transmission.

Mechanism of Polygenic Inheritance

Polygenic inheritance is the type of inheritance that occurs by gene interaction, between two or more genes, but is only responsible for a single characteristic. The gene has multiple alleles that jointly affect the result in the phenotype of an individual. Usually, these genes are on separate chromosomes and participate in the development of the trait.

How do Multiple Genes Contribute to a Single Trait?

The effects of genes in polygenic models are small and cumulative; that is, the effects of genes on the phenotype are small, and they increase as a whole. For example, the height of people is determined by many genes, whereas none of the genes is completely responsible for height. The overall expression of all these genes adds up to produce the final phenotype.

Interaction of Alleles from Different Genes

As such, alleles of different genes combine to form several phenotypes. A gene can exist in more than one form, and the different forms, when interacting with each other, may produce variation in the trait. For instance, variations within the genes controlling skin colour result in variations in skin shade, colour or tone.

Additive Effects of Alleles

Additivity implies that the relative impact of the individual alleles is summative to produce the phenotype of an organism. Therefore, while dominant alleles could contribute a quantity to the trait, recessive alleles contributed lesser quantities. When the dominant alleles are more numerous, then the organism is characterised by a more intensified feature. For example, in human height, the dominant alleles contained in a person mean that they are most likely to be taller; the more alleles that point towards a certain height an individual has, the taller he or she is likely to be.

Examples of Polygenic Inheritance

Polygenic inheritance is seen in many traits such as human height, coat colour of dogs, and grain shade in wheat. Some examples are discussed below-

Human Height

The human height is a good example of an inherited trait that is polygenic. This is because it depends on the summed-up impact of several genes, all of which make relatively small contributions towards height. Some of the key genes that are affected are the ones that are related to growth hormones and bone formation. These genes can also be variable and do not directly cause one to fit into either tall or short, but are rather on a continuum.

Skin Colour

The shade of your skin is defined by the activity of several genes that regulate the production of melanin. The colour of skin depends on the pigment called melanin, whose synthesis is controlled by genes such as MC1R, SLC24A5, and SLC45A2. The phenotypes of the skin colour arise from the delayed association of the alleles of this gene.

Eye Colour

Most people believe that eye colour is determined by a single gene, but there is more than one gene associated with eye colour. The prime genes are OCA2 and HERC2, the genes control the amount and type of pigments present in the iris of the eye. This interaction between these genes defines one’s eye colour, which can be brown, blue, green, hazel or any other colour with different shades and different intensities.

Weight and Body Shape

Fatness and body mass are determined by the various genes that affect metabolic rate, body fat pattern and hunger. Specific genes including FTO, and MC4R are associated with obesity and the distribution of body fat. Thus, the genetic influences are further mitigated by other factors such as diet and exercise in determining a person’s weight and physique.

Polygenic Inheritance and Evolution

Polygenic inheritance has an impact on evolution as well. They influence natural selection and adaptation as well. It is described in detail below-

Contribution to Evolutionary Processes

When it comes to the evolution of populations, polygenic inheritance plays an important role as it leads to variability within the population. There is a continuous modification of traits resulting from multiple gene combinations, which consequently offer an array of phenotypes. Such diversity is beneficial for the populations to be able to cope with the new conditions and selective pressures as they change in future.

Natural Selection and Polygenic Traits

The impact of natural selection on a population pertains to polygenic traits, where certain individuals inherit good alleles for those particular traits. Polygenic traits are linked with many genes; hence, the selection pressure causes variations in the combination of alleles within a population. For instance, if a given height or skin colour relieves a person from high risks of being attacked by predators or getting sunburnt, among others, this characteristic is passed on to the next generations.

Adaptive Significance of Polygenic Traits

Polygenic characters usually have adaptive value because characters that are developed in response to variation can be changed gradually in small steps, which is most appropriate to finely tune the response to the environment. For instance, skin colouration enables people to cope with UV radiation at different levels in different geographical locations, and body sizes and shapes also contribute to our survival in various climatic conditions. Such changes are beneficial in increasing the advantage of an organism’s fitness in certain habitats and the overall genetic stock of the population.

Recommended video for "Polygenic Inheritance"

MCQs on Polygenic Inheritance

Q1. What is the pattern of inheritance for a polygenic trait?

Option 1: X-linked recessive inheritance pattern

Option 2: Mendelian inheritance pattern

Option 3: Non-Mendelian inheritance pattern

Option 4: Autosomal dominant pattern

Correct answer: 3) Non-Mendelian Inheritance Pattern.

Explanation:

Polygenic traits are affected by several genes, which are known as polygenes, with every gene having a small effect on the resultant expression of the trait, and that leads to a continuous variation in characters. Examples of such human traits are height, skin colour, and eye colour. As the pattern of inheritance is due to more than one gene and does not follow the simple dominant-recessive relationship of Mendelian characteristics, it is known as non-Mendelian inheritance. Environmental factors may also change the expression of these characteristics.

Hence, the correct answer is option 3) Non-Mendelian Inheritance Pattern.

Q2. How does the number of alleles in a genotype affect skin color in humans?

Option 1: The number of dominant alleles determines skin color.

Option 2: The number of recessive alleles determines skin color.

Option 3: The number of each type of alleles in the genotype determines the darkness or lightness of the skin.

Option 4: The number of alleles in a genotype has no effect on skin color.

Correct answer: 3) The number of each type of allele in the genotype determines the darkness or lightness of the skin.

Explanation:

The text gives an example of how the number of each type of alleles in the genotype determines the darkness or lightness of the skin in humans with the dominant forms A, B, and C responsible for dark skin color and the recessive forms a, b and c for light skin color.

Hence, the correct answer is option 3) The number of each type of allele in the genotype determines the darkness or lightness of the skin.

Q3. Statement i): Polygenic inheritance is the same as multiple allele systems.

Statement ii): Environmental factors have a greater influence in determining a trait by polygenes.

Option 1: Statement (i) is correct and statement (ii) is incorrect

Option 2: Statement (i) is incorrect and statement (ii) is correct

Option 3: Both statements are wrong

Option 4: Both statements are correct

Correct answer: 2) Statement (i) is incorrect and statement (ii) is correct.

Explanation:

Explanation of statement i) :

Statement i) is incorrect because inheritance and multiple alleles are different from each other. Multiple alleles are Genes with three or more alleles. Multiple alleles involve the existence of multiple alleles. Its example is human ABO blood group system. In Polygenic inheritance, one character is controlled by two or more genes. here genes are large in quantity but small in effect. Examples of polygenic inheritance in humans are height and skin colour.

Explanation of statement ii) :

Statement ii) is correct because environmental factors do not affect whether a trait is determined by multiple alleles. But Environmental factors have a greater influence in determining a trait by polygenes. So statement ii) is correct.

Hence, the correct answer is option 2) Statement (i) is incorrect and statement (ii) is correct.

Also Read:

• Practice MCQs on Polygenic Inheritance
• Essential Genes
• Complementary Genes