Is polygenic inheritance quantitative in nature?
In nature, a few characteristics have traits spread across gradients which is unlike the Mendelian inheritance pattern. In polygenic inheritance, two or more genes control one character. These are called polygenic traits. The word polygenic means literally “of many genes”. Polygenic inheritance is also influenced by the environment. Since the phenotypes of polygenic traits result from both gene action and environmental influences, they are often termed multifactorial, or complex traits.
In polygenic traits, each allele makes additive and cumulative effects. There is continuous variation across a range of phenotypes which can be measured and described in quantitative terms, so this genetic phenomenon is known as quantitative inheritance.
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Examples of Polygenic Inheritance
Following are some of the examples of polygenic inheritance:
- Human skin Colour
- Human eye Colour
- Human height
- Human intelligence
- Human body shape and weight
- Kernel colour of Wheat
- Wing morphology of Drosophila
- Cob length in Maize
- The yield of crop plants
- The number of seeds or fruits per plant
- Length of the spike of Barley rachis
- Length of ears in Rabit
- Bristle count in Drosophila
- Stem length in plants
- Ray size in flower heads in Bidens pilosa
- Human fingerprints
Determination of Human skin colour
The skin colour in humans is controlled by three genes – A, B and C. The dominant forms/alleles of the genes A, B and C are responsible for dark skin colour and the recessive forms/alleles a, b and c are responsible for light skin colour. The genotype with all the dominant alleles i.e. AABBCC will have the darkest skin colour (referred to as VERY DARK) and the genotype with all the recessive alleles i.e. aabbcc will have the lightest skin colour (referred to as VERY LIGHT). The genotype with three dominant alleles and three recessive alleles (AaBbCc) will have an intermediate skin colour (referred to as Mulatto). In this way, the number of each type of allele in the genotype determines the darkness or lightness of the skin in an individual.
Human height as a polygenic trait
Human height is strongly genetically controlled. There are many different genes that are responsible for it. Instead of tall or short people as two distinct alternatives, there is a whole range of possible heights. Recent height predictions focus on these large numbers of genomic loci behind human height. The graph in this case will be particularly a bell-shaped curve.
Image Source: www.autism.org
A bell-shaped curve in Polygenic Inheritance
Clinical significance of polygenic inheritance
To conclude, a lot of diseases can be polygenic that are particularly caused by the cumulative contribution of many genes that can act independently or interactively. Searching for polygenic combinations associated with a phenotypic trait (i.e., composite genetic markers) is the perfect method for studying polygenic diseases i.e. it can be used in population screening for common diseases {Reference- A Polygenic Approach to the Study of Polygenic Diseases (nih.gov)}
Hope the article could explain to you the quantitative nature of polygenic inheritance.
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