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Polygenic Inheritance Definition

• Polygenic inheritance is a genetic concept where multiple genes, rather than a single gene, collectively influence a particular trait or characteristic in an organism. This means that a specific phenotype, or observable trait, is the result of the combined effects of several independent genes. Unlike Mendelian Inheritance, where traits are determined by dominant or recessive genes, polygenic inheritance involves the additive influence of genes from both parents. In this type of inheritance, the expression of a trait in offspring is a blend of the traits exhibited by their parents.
• Examples of polygenic inheritance in humans include traits like hair color, height, skin color, blood pressure, intelligence, autism susceptibility, and longevity. These traits don't fall into simple categories like "tall" or "short" or "light" or "dark" but instead show a range of continuous variations. Polygenic inheritance involves two types of alleles or genes: contributing alleles, which actively contribute to the variation in the trait, and non-contributing alleles, which do not play a significant role in determining the trait's variation.

Characteristics of Polygenic Inheritance

Polygenic inheritance exhibits distinct characteristics:

• Minor Individual Gene Effects: Each gene involved in polygenic inheritance exerts a subtle influence on the overall trait.
• Additive Contribution: Multiple genes collectively contribute to the trait in an additive manner, with their effects cumulatively shaping the phenotype.
• No Dominance: Unlike Mendelian inheritance, where dominant and recessive genes can mask each other's effects, polygenic inheritance does not involve such masking.
• Continuous Variation: Traits subject to polygenic inheritance display a continuous spectrum of variation, with no clear-cut categories.
• Complexity: Predicting polygenic inheritance patterns is challenging due to their complexity, often requiring statistical analysis.
• Normal Distribution: Many polygenic traits follow a normal distribution curve, with most individuals falling around the mean.

Analysis of Polygenic Traits

The analysis of polygenic traits is an important field in genetics, and it was greatly advanced by the pioneering work of Sir Ronald Aylmer (R.A.) Fisher in 1918. Polygenic inheritance, being a form of quantitative inheritance, requires distinct approaches compared to qualitative inheritance, where traits are typically categorized into discrete groups.

Here are some key points regarding the analysis of polygenic traits:

• Nature of Polygenic Traits: Polygenic traits are those that are measured on a continuous scale, such as weight, height, length, width, duration, and many others. These traits do not fall into simple categories like color or shape, making them challenging to analyze using classical Mendelian genetics.
• Continuous Variation: In polygenic inheritance, individuals cannot be easily classified into distinct groups. Instead, these traits exhibit continuous variation, where there is a wide range of possible values within a population.
• Population Analysis: To study polygenic inheritance, researchers often analyze entire populations rather than looking at individual genotypes. This involves collecting data from a large number of individuals within a population and calculating mean values for the trait of interest.
• Variation Measurement: Variance and covariance are commonly used statistical measures in the analysis of polygenic traits. Variance quantifies the degree of variation within the population, while covariance assesses the degree to which two traits are associated with each other.
• Quantitative Genetics: The field that specializes in the genetic interpretation of quantitative characters, such as those involved in polygenic inheritance, is known as quantitative genetics or biometrical genetics. It involves the application of statistical methods to study the inheritance of complex traits.

Polygenic Traits vs. Oligogenic Traits

Segmentation of Polygenic Variability

Polygenic variability can be categorized into three types:

• Phenotypic Variability: This encompasses observable variation, including both genetic and environmental influences, expressed as phenotypic variance.
• Genotypic Variability: This is the genetically inherited variability unaffected by environmental factors, measured as genotypic variance.
• Environmental Variability: Non-heritable variations arising from environmental factors, quantified as error mean-variance.

Polygenic Inheritance Examples

Polygenic Inheritance in Humans

A. Skin color and pigmentation

The color of the skin is polygenic inheritance. It is controlled by around 60 loci. To understand the inheritance pattern of skin color let us consider an example of pair of three different alleles present at unlinked loci represented as A and a, B and b, C and c. The alleles responsible for dark color skin are represented by letters where capital letters represent those that are incompletely dominant alleles. Accordingly, the greater number of “capital letters” in the genetic pattern indicates dark skin color whereas the presence of a greater number of “small letters” represents the lighter color of the skin.
The progeny of the parents with genotype AABBCC and aabbcc will have intermediate color in the F1 generation, i.e. genotype would be AaBbCc. Further, in the F2 generation of two triple heterozygotes parents — AaBbCc x AaBbCc — will produce varying skin colors ranging from very dark to very light, the ratio of which would range in 1:6:15:20:15:6:1 (see Figure 1). The skin color of an individual is due to the presence of melanin in the skin. A dark skin color (with all dominant alleles, AABBCC) would have the highest amount of melanin in the skin. Conversely, a light skin color (aabbcc) would have the least or a negligible amount of melanin in the skin.

B. Human height
Human height is a polygenic trait that is controlled by three genes that have six alleles. So, a tall person would have all dominant alleles whereas a short person will have the most number of recessive alleles. Like all the polygenic inheritance patterns, human height inheritance also follows a normal distribution curve wherein the extreme ends of the curve represent either extremely short or tall people, while the middle portion of the curve represents the population with average height (see Figure 2).

C. Polygenic inheritance of the human eye color

The eye color follows a polygenic inheritance pattern. In humans, 9 eye colors are recognized. Phenotypic expression of eye color is controlled by two major genes and 14 additional genes, which are linked to X chromosomes. Different combinations of these alleles result in a variety of eye colors. The eye color is due to the presence of melanin in the front portion of the iris.
Black and brown eye color has a high amount of melanin in comparison to the hazel or green eye color while the complete absence of the melanin results in blue eye color. Dominant allele (BBGG) contributes to the melanin synthesis in the iris that results in black eye color while the combination of all recessive alleles (bbgg) results in blue eye color. The rest of all the eye colors are the combination of these dominant and recessive alleles. See below:

• BBGG results in Black eyes
• BBGg or BbGG results in Dark Brown eyes
• BbGg or BBgg or bbGG results in Light Brown eyes
• Bbgg or bbGg results in Green eyes
• Bbgg results in Blue eyes

Polygenic Inheritance in Plants

• Kernel Color of Wheat: Wheat kernel color exhibits polygenic inheritance, resulting in a range of shades depending on the combination of alleles present.
• Length of the Corolla in Tobacco: The length of the tobacco plant's corolla is influenced by five genes, demonstrating polygenic inheritance in the plant kingdom.

Effect of Environment on Polygenic Inheritance

• Polygenes are highly influenced by environmental factors. Basically, the genotype of an individual sets the range of the quantitative trait while the phenotype of the trait is eventually an outcome of the environmental factors. Different environmental conditions regulate the gene function resulting in varying gene function. Accordingly, under different environmental conditions, the gene function may switch OFF or switch ON. This variation in the phenotypic expression of the same genotype under different environmental conditions is known as the ‘norm of reaction’.
  Depending on the genotype involved, the norm of reaction can be classified as:
  • Narrow norm of reaction, e.g., human height
  • Broad norm of reaction
• Nature and nurture both affect phenotypic expression. Intelligence, depression, height, skin color, schizophrenia are some of the human characters that are affected by the environment.
• A medical example of the effect of environmental factors on polygenic inheritance is the hereditary disorder phenylketonuria. The individual with homozygous alleles for the PKU disease lacks the enzyme that breaks down the amino acid, phenylalanine. In such patients, phenylalanine is retained in the body resulting in toxic buildup. This toxic build-up eventually causes intellectual disability, seizures, and mood disorders. The treatment for this disorder is very simple, by keeping the patients on a specific diet. The dietary change helps to reduce or even alleviate the phenylketonuria disorder. Due to this, many countries screen newborns for their genetic constitution to identify phenylketonuria disorder.

Why is Polygenic Inheritance Important?

Polygenes play a pivotal role in population variance and contribute to species' evolution. Understanding polygenic inheritance helps unravel the intricate mechanisms behind complex traits. This knowledge has practical applications in fields like medicine, where screening for polygenic disorders, such as phenylketonuria, can help identify individuals at risk.

In conclusion, polygenic inheritance offers a captivating perspective on the intricate genetic determinants of multifactorial traits. It emphasizes the importance of considering multiple genes and their collective influence in comprehending the complexities of heredity and evolution.