Polygenic Inheritance- Definition, Characteristics, Examples

Polygenic inheritance is defined as some phenotypic character determined by the additive effect of more than one gene on a single character.

  • Polygenic inheritance is commonly known as quantitative inheritance and multiple gene inheritance.
  • Height, skin pigmentation, eye color, hair color, and milk and egg production are phenotypic characteristics present in plants and animals. Many traits and phenotypic characters are inherited by many alleles present in different loci, known as polygenic inheritance.
  • It is the type of inheritance controlled by more than one gene where the dominant alleles have a commutative effect, with each dominant allele expressing a part or unit of the trait.
  • The combined effect of many genes shows a significant effect, but a single gene has little effect on the phenotype.
  • The full trait is shown if all dominant alleles are present in it.
  • The Father of phenotypic inheritance is Koleronter, and Nilson-Ehle and East discover polygenic inheritance in kernel color in wheat.
  • A gene that exerts a bit of effect on a phenotype along with another gene refers to polygene.

Two types of genes and alleles are involved in polygenic inheritance controlled:

  1. Contributing alleles: that contribute to continuous variation.
  2. Non-contributing alleles: alleles or genes which do not contribute to continuous variation.

Characteristics of polygenic inheritance

  • Each gene has a too-small effect, which is why it is difficult to detect, but their combined effect is significant and easy to detect.
  • Each allele has an additive and cumulative effect.
  • The equal effect is produced by multiple genes.  
  • One phenotypic character or particular trait is controlled by more than one gene. All such genes are non-allelic.
  • It is difficult to predict phenotype due to the complex pattern of polygenic inheritance.
  • Polygenic inheritance is statistically analyzed in order to estimate population parameters.
  • Polygenic inheritance is occurred by the continuous variation of the phenotype of a trait.
  • Environmental factors and the combined effect of many genes produced continuous variation.
  • Each gene has a certain amount of effect, and the more the number of dominant genes, more the pronounced its characteristics. so, quantitative inheritance is also called polygenic inheritance. 
  • The trait becomes much more pronounced due to the addition of several alleles.
  • Polygenic inheritance is also called cumulative inheritance because the phenotype depends upon the number of dominant alleles present.
  • Monogenic inheritance follows Mendelian inheritance, the expression may be predicted according to the phenotypic ratio, but polygenic inheritance doesn’t follow the Mendelian ratio.
  • Polygenic inheritance is different from multiple alleles. For example, human blood groups as multiple alleles. Here, three or more alleles are present on the same locus. The ABO blood grouping system is controlled by three alleles.
  • For F2 generation, the frequency of different types of phenotypes is always bell-shaped.

Examples of Polygenic Inheritance in human 

Skin color in humans, eye color, human intelligence, and height of humans are a type of examples of polygenic inheritance in humans.

The skin color of human

  • The inheritance of color of skin in humans study by Devenport in 1913.
  • It is controlled by around 60 loci.
  • Human skin color is regulated by three pairs of genes.
  • When a negro black (AA BB CC) phenotype is crossed with white (aa bb cc) phenotype, an intermediate phenotype (Aa Bb Cc) is produced.
  • In the F2 generation, (Aa Bb Cc× Aa Bb Cc) will produce different skin colors (i.e., very dark to light). Skin color depends upon melanin pigment. A negro black (with all dominant alleles (AA BB CC) would have the highest amount of melanin, and light color (aa bb cc) has a negligible amount of melanin. The ratio of skin color is following:

Negro black (1): vary dark (6): dark brown (15): intermediate (20): fair light (15): light (1).

Human height 

  • Human height is a polygenic trait determined by three genes that have six alleles (3 dominant alleles and three recessive alleles).
  • A tall person has all dominant alleles; however, a short person has all recessive alleles.
  • Human height follows a normal distribution curve (bell-shaped) wherein one end of the curve represents the tall population, and another end represents the small or short population, whereas the middle portion of the curve represents the average height population. 
  • A recent study concluded over 400 genes are linked to variation in height.
Polygenic Inheritance
Polygenic Inheritance

Examples of Polygenic Inheritance in plants

Kernel color in wheat, cob length of maize, and length of the corolla in tobacco are examples of polygenic inheritance in plants.

Kernel color of the wheat 

  • Nilsson-Ehle (1908) gave the first experimental evidence of the kernel color of wheat.
  • kernel color of wheat is determined by 3 independently assorted pairs of alleles.
  • The dark red wheat kernel has 3 dominant alleles AA BB CC and the white kernel have recessive alleles i.e. aa bb cc. These alleles crossed in the F1 generation produce intermediated red color have (Aa Bb Cc).
  • In the F2 generation, 1 white and 63 red with varying shades are produced. 

Environmental effect on polygenic inheritance

  • Polygenic inheritance (human phenotypes and other organism phenotypes) are highly affected by environmental factors.
  • Various environmental conditions or agents are responsible for regulating the function of genes resulting in varying gene functions.
  • The function of the gene may be to switch OFF or switch ON based on varying environmental conditions.
  • The range of phenotypes based on the different environmental conditions from the genotype is known as the ‘norm of reaction’.
  •  environment affects the gene expression which is addressed by the human characteristics like the height of human, skin color, intelligence, depression, and schizophrenia.
  • Phenotypic expression is dependent on both nurture and nature.
  • It can be best described by an example of the phenotypic hereditary disorder Phenylketonuria (PKU). In such disorder alleles of the PKU gene lacks an enzyme that helps in the breakage of amino acids Phenylalanine. Due to this, peoples retain amino acid phenylalanine in the body in high proportion resulting to build to a toxic level in their bodies. Keeping the patients on a specific diet from a very young age is a very simple method of treatment or reduced PKU.

Importance of polygenic inheritance

  • Polygenic inheritance determined quantitative traits rather than qualitative and does not lead to the sharp segregation shown by more familiar genetical differences.
  • Polygenic inheritance shows a continuous variation of the traits in a population, polygenic theory shows the relationship between continuous variation and discontinuous variation and also biometrical and genetic variation.
  • Polygenic variation or change is called polygenesis, which is responsible for the population variance or evolution of species. This is also referred to as the theory of polygeny for evolution. 
  • The variation is essentially responsible for adaptative changes, that may doffers according to specific requirements and needs of individuals or species.

Polygenic traits and Oligogenic traits

Polygenic traits

  • Many genes control the trait known as polygenic traits. 
  • The genes that control polygenic traits may be present near each other or in the separate chromosome.
  • It does not follow Mendel’s theory of inheritance.
  • The expression of an individual gene is difficult to detect due to the minor expression of genes.
  • Polygenic traits represented a range of continuous variations.
  • Polygenic traits are influenced by environmental factors.
  • mean, variance, and covariance are used as statistical parameters that can be used for the evaluation of the population.
  • Gene exhibits an additive effect.
  • Segregation of individuals into classes is not possible.
  • Skin color, eye color, height, and hair color are an example of polygenic traits.

Oligogenic traits

  • Oligogenic traits are traits whose expression is controlled by a few genes with large effects.
  • In the breeding programs or various crops, they are very important.
  • The expression of individual genes is detectable.
  • Plant resistance to disease is present in some of these traits. It resists diseases like rust in eucayptus, rice sheath of blight, and pea.
  • Genes exhibit a non-additive effect.
  • Oligogenic traits represented discontinuous variation.
  • Segregation of individuals into classes is possible.
  • Environmental factors not-influenced oligogenic traits.
  • Ratios and frequencies can be used as statistical parameters that are used for the evaluation of the population.

References

  1.  Pierce, B.A. (2016). Genetics, A conceptual Approach (fifth edition).
  2. Mather, K. (1943). Polygenic inheritance and natural selection. Biological Reviews18(1), 32-64.
  3. https://byjus.com/neet/polygenicinheritance/
  4. https://www.biologyonline.com/dictionary/polygenic-inheritance.
  5. https://www.khanacademy.org/science/ap-biology/heredity/environmental-effects-on-phenotype/a/polygenic-inheritance-and-environmental-effects.
  6. https://biologydictionary.net/polygenic-inheritance.
  7. https://www.thoughtco.com/polygenic-inheritance-373444.
  8. https://courses.lumenlearning.com/wm-biology1/chapter/reading-polygenic-inheritance-and-environmental-effects.
  9. https://bio.libretexts.org/Bookshelves/Genetics/Classical_Genetics_(Khan_Academy)/02%3A_NonMendelian_inheritance/2.04%3A_Polygenic_inheritance_and_environmental_effects.

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