Polygenic Inheritance: Characteristics, Examples, Practice Problems and FAQs

Most of you may be active in social media. Today's world gives us lots of opportunities to explore social media. In social media platforms like whatsapp and instagram you might have seen various emojis. You might have noticed that there are emojis for different skin tones like the ones given in the below image.

Fig: Some emojis with different skin tones in the social media

You all know that we all are not having the same skin tone. These skin tone emojis allow the users to feel more represented when expressing visual reactions. Some people have dark complexions and some people have fair complexions. But most of the people have a skin tone between the fair and dark colour. But can you tell me why the skin colours differ in people or what is the science behind this? Yes, you are correct. The skin colour is a polygenic trait which is controlled by two or more than two genes.

Similarly, you might have noticed that there are different hair colours in emojis? Yes, the hair colour is also a polygenic inheritance. Now let’s take a deep dive into the details of polygenic inheritance and understand the various examples related to it in this article.

Table of contents

• Polygenic inheritance
• Discovery of polygenic inheritance
• Characteristics of polygenic inheritance
• Examples of polygenic inheritance
• Effect on environment on polygenic inheritance
• Difference between monogenic and polygenic inheritance
• Practice problems
• FAQs

Polygenic inheritance

Polygenic inheritance is a type of inheritance in which a single trait is controlled by two or more pairs of genes. The dominant alleles of these genes have a cumulative effect. Each dominant allele expresses a different aspect of the trait, and the whole trait is only seen when all dominant alleles are present. It is also known as multiple factor inheritance. Examples include height and eye colour in human beings.

Quantitative inheritance

Polygenic inheritance is also known as quantitative inheritance as these traits can be measured in terms of quantity. The common examples of polygenic inheritance are height and skin pigmentation in humans. As you can see, there are many different forms of these two traits. One cannot categorise people in just two categories like tall and short for height and dark and light for skin pigmentation. One can see a continuous variation in these two traits because these are controlled by more than two genes.

Fig: Skin colour and height in human beings

Polygenes

The genes that are responsible for polygenic inheritance are called polygenes. The traits that are controlled by multiple genes are known as metric traits. This is because these types of traits are measured in terms of unit of size, height, weight or number.

Types of polygenes

Polygenes are of two types as follows:

Contributing alleles

These are the dominant polygenes that contribute to the expression of the trait. Examples include A, B and C genes in humans responsible for the dark skin tone.

Fig: Contributing alleles

Non-contributing alleles

These are the recessive polygenes. Examples include the a, b and c genes in humans responsible for the light skin tone.

Fig: Non-contributing alleles

Discovery of polygenic inheritance

Polygenic inheritance was first studied by J.Kolreuter in 1760 in tobacco and F. Galton (1883) in human beings. F. Galton predicted that in human beings characters like height, mental capabilities and skin colour are heritable and show a continuous range of variations. Nilson Ehle in 1908 obtained the first experimental proof of polygenic inheritance in wheat kernel colour.

Characteristics of polygenic inheritance

Major characteristics of the polygenic inheritance are listed below:

• Polygene is a type of gene that exerts a little effect on phenotype along with other genes.
• The effect of a single gene is so small that it is difficult to detect.
• All the multiple genes produce an equal effect.
• Each allele shows a cumulative or additive effect.
• The phenomenon of polygenic inheritance is different from multiple allelism. This is because in multiple allelism three or more alleles control a character and two alleles are present in an organism. The example of multiple allelism is ABO blood grouping.
• In polygenic inheritance, linkage and dominance does not occur.
• The alleles are classified into contributing and non-contributing alleles which are also known as active and null alleles, respectively.
• The continuous variation of a phenotype takes place here.
• The pattern of polygenic inheritance is complicated and the phenotype is difficult to predict.
• The statistical analysis can give a picture of the estimate of population parameters.

Examples of polygenic inheritance

Some of the common examples of quantitative inheritance are as follows:

• Kernel colour in wheat
• Cob length in maize
• Skin colour in human beings
• Height in human beings
• Eye colour in human beings
• Milk and meat yield in animals
• Length of the corolla in tobacco

Polygenic inheritance in humans

In humans, there are many traits that show quantitative inheritance. Some of the examples are skin colour, eye colour, height and the risk of diseases.

Skin pigmentation in humans

Human skin colour is a polygenic trait. It depends on the presence of melanin pigment. The amount of melanin produced depends on the three pairs of genes A, B and C. These genes are present on the different loci and each dominant gene is responsible for the synthesis of a fixed amount of melanin. The effect of all dominant genes is additive. Thus the amount of melanin produced is always proportional to the number of dominant alleles.

The homozygous dominant alleles are responsible for dark phenotype and the homozygous recessive alleles are responsible for light phenotype. When black or extremely dark (AABBCC) and white or albino (aabbcc) people marry, the offspring or individuals of the F₁ generation have a mulatto pigmentation (AaBbCc).

Fig: F1 generation in polygenic inheritance

When two people of mulatto or intermediate race marry, their children's skin colour will range from very dark or black to very light or white.

Fig: Polygenic inheritance of human skin colour

In the gametes, there are a total of eight allele combinations, resulting in 64 genotypes divided into seven phenotypes. The seven phenotypes are as follows:

• Very dark or pure black (1/64)
• Dark (6/64)
• Fairly dark (15/64)
• Intermediate (20/64)
• Fairly light (15/64)
• Light (6/64)
• Very light (1/64)

Fig: Allele combinations

Height in man

Height in man is also controlled by three pairs of genes located at different gene loci. The height develops by the addictive effect of the number of dominant alleles of these genes. People with all six dominant alleles are the tallest whereas people with any three dominant alleles have average height. People with all recessive alleles and no dominant alleles are the shortest.

Fig: Height in human beings

Eye colour in human beings

The colour of the eye is determined by polygenes. It is identified that a minimum of 9 colours of eye colour are recognised in humans. There are two major eye colour genes and 14 more genes that normally determine the expression of the phenotype in human beings. A different number of alleles contribute to each colour. The eye colour is mostly associated with the two adjacent genes on the chromosome 15. The OCA2 or Oculocutaneous albinism type 2 and HERC2 genes are mainly associated with the eye colour. HERC2 gene is a giant E3 ubiquitin protein ligase which is responsible for pigmentation.

Fig: Eye colour in human beings

Polygenic inheritance in plants

Polygenic inheritance is also seen in plants. Some of the examples of polygenic inheritance are listed below:

• Colour of the stem
• Shape of the stem
• Flowers
• Pollen grains
• Oil content
• Yield

Kernel colour in the wheat

The kernel colour of wheat is controlled by two pairs of alleles and they show independent assortment. The dominant genes are R₁ and R₂. The recessive alleles are r₁ and r₂. The homozygous dominant alleles are responsible for dark phenotype (dark red) and the homozygous recessive alleles are responsible for light phenotype (white). For example, the dark red wheat kernel (R₁R₁R₂R₂ ) is crossed with a white wheat kernel (r₁r₁r₂r₂). The F₁ generation obtained has an intermediate red colour kernel (R₁r₁R₂r₂). The F₁ generation is then self-crossed to obtain the F₂ generation. The F₂ generation produced 63 red kernel plants with different red shades and one white kernel plant.

The F₂ generation are shown below:

Fig: Kernel colour in the wheat

Length of the corolla in tobacco

More than four pairs of genes are identified to be involved in the expression of phenotype of corolla length of tobacco (Nicotiana). There is a wide variety in the length of the corolla in tobacco occurs due to polygenic inheritance.

Effect of environment on polygenic inheritance

The expression of genes involved in polygenic inheritance is influenced by environmental conditions. The genotype tries to set the range for a quantitative trait normally but it is the environmental conditions that decide the phenotype. This is due to the fact that the genes function differently in various environmental conditions. Environment regulates the activity of some genes and sets them on or off.

Norm of reaction

The range of phenotypes that are possible under the various environmental conditions from the same genotype is called the ‘norm of reaction’. The norm of the reaction may be narrow or broad. It depends on the genes. For example, the various genotypes produced in human height have a broad norm of reaction.

Example to show the effect of environment on polygenic inheritance

Consider the case of identical twins who are formed from the same zygote. If we raise them in two different environments then some of the genes show the genetic potential or vulnerability to express differently. Here the environmental conditions influence the expression of the genes. The common human characters like skin colour, intelligence, height etc., show the effect of the environment on gene expression. This may be due to the following reasons:

• The diet provided and the general health of the individual affect the height.
• The education provided and the social status affect intelligence.

Difference between monogenic and polygenic inheritance

The following are the major differences between the monogenic and polygenic inheritance:

Practice Problems

Q1. The phenomenon known as ___(A)___ occurs when a single gene impacts many phenotypes. The phenomenon known as __(B)__ occurs when more than two genes influence the same trait. Identify (A) and (B).

a. (A) polygenic inheritance, (B) pleiotropism
b. (A) pleiotropism, (B) polygenic inheritance
c. (A) multiple allelism, (B) pleiotropism
d. (A) pleiotropism, (B) multiple allelism

Solution: Pleiotropism is described as the phenomenon in which a single gene regulates many phenotypes. Examples include phenylketonuria in human beings. Therefore, (A) represents pleiotropism. Polygenic inheritance is a type of inheritance in which a single trait is controlled by two or more pairs of genes. The dominant alleles of these genes have a cumulative effect. Examples include inheritance of height in human beings. Therefore, (B) represents polygenic inheritance. Hence, the correct option is b.

Q2. Is the human blood group an example of polygenic inheritance?
Answer: Human blood group is an example of multiple allelism in which a single gene possesses different alleles. Out of these, a person contains any of the two alleles. It is different from polygenic inheritance. Here the gene I possess three alleles I^A, I^B and i. The combination of two alleles decides the type of blood group. For example, if a person has I^A and I^B , then he has AB blood group. Polygenic inheritance is a type of inheritance in which a single trait is controlled by two or more pairs of genes. The dominant alleles of these genes have a cumulative effect. Examples include inheritance of height or eye colour in human beings.

Q3. Is eye colour a polygenic inheritance? If yes, how?
Answer: Yes, eye colour in humans is a polygenic inheritance. It is identified that a minimum of 9 colours of eye colour are recognised in humans. There are two major eye colour genes and 14 more genes that normally determine the expression of the phenotype in human beings. A different number of alleles contribute to each colour. The eye colour is mostly associated with the two adjacent genes on the chromosome 15. The OCA2 or Oculocutaneous albinism type 2 and HERC2 genes are mainly associated with the eye colour. HERC2 gene is a giant E3 ubiquitin protein ligase which is responsible for pigmentation.

Q4. How does skin colour in humans inherited?
Answer: Human skin colour is a polygenic inheritance. It depends on the presence of melanin pigment. The amount of melanin produced depends on the three pairs of genes A, B and C. These genes are present on the different loci and each dominant gene is responsible for the synthesis of a fixed amount of melanin. The effect of all dominant genes is additive. Thus the amount of melanin produced is always proportional to the number of dominant alleles.

The homozygous dominant alleles are responsible for dark phenotype and the homozygous recessive alleles are responsible for light phenotype. There are seven phenotypes that exist in between these two. When black or extremely dark (AABBCC) and white or albino (aabbcc) people marry, the offspring or individuals of the F₁ generation have a mulatto pigmentation (AaBbCc).

FAQs

Q1. Why do polygenic traits have many phenotypes?
Answer: Polygenic inheritance has many phenotypes because of the combinations of multiple alleles. The environmental factors are also responsible for producing various types of phenotypes.

Q2. Is intelligence a polygenic trait?
Answer: Intelligence is a polygenic trait in which different genes exert extremely small influence. The different genes exert their own influence at different stages of development. It is also influenced by the environment. For example, the diet followed by the individual, education provided, social status etc.

Q3. What is the reason behind that if a person's face gets darker without sunlight?
Answer: Hyperpigmentation is the most common cause of skin darkening without exposure to the sun. This is a disorder in which the body produces extra melanin, the pigment responsible for skin colour. Hyperpigmentation causes dark spots and patches to appear on various sections of a person's body.

Q4. How does the environment affect polygenic inheritance?
Answer: Environmental factors have a significant impact on polygenic expression. The genotype sets a range for quantitative traits but the phenotype within those genetic boundaries is determined by environmental factors. Distinct environmental factors have different effects on genes. The environment controls how active some genes are and whether they are turned on or off. For example, the diet provided and the general health of the individual affect the height.

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