Elementary Idea of Polygenic Inheritance

Polygenic inheritance is a non-Mendelian form because numerous genes at different loci on separate chromosomes are expressed concurrently in the same characteristic. If one set of genes controls colour and red is dominant over white, then when two heterozygotes (Aa) are crossed, the red and white progeny will appear in a 3:1 ratio. Crossing two heterozygotes (Aa Bb) will result in a 9:7 ratio of red to white flowers if two pairs of genes determine the colour and dominant allele at both loci needs to be expressed to generate red flowers. This is a dihybrid Mendelian ratio variation in which all three progeny groups have the same phenotype. 

Characteristics of polygenic inheritance

• Polygenes are genes that have a small effect on phenotypes in combination with other genes. 
• Individually, each allele makes a cumulative or additive contribution to the outcome.
• The impact of a single gene is typically insignificant and goes unnoticed.
• Epistasis is not engaged since the expression of one gene is not concealed by the presence of the other genes.
• In polygenic inheritance, there are no dominant or veiled genes; they are either contributing (active allele) or non-contributing (null allele).
• In polygenic inheritance, the phenotype of a characteristic is always changing.
• Polygenic inheritance patterns can be statistically analysed to assist estimate population parameters. The majority of polygenic inheritance follows the standard distribution curve, with most people falling somewhere in the middle.
• Multiple alleles are not the same as polygenic inheritance. Three or more alleles are present on the same locus in numerous alleles, and an organism, such as the human blood type system, i.e., the ABO system, is controlled by three alleles.

Analysis of polygenic traits 

Polygenic inheritance is a quantitative inheritance, as we all know. As a result, it is critical to grasp the methodologies or foundations for distinguishing between quantitative and qualitative inheritance. Sir Ronald Aylmer (R.A.) Fisher did groundbreaking work in 1918 to comprehend quantitative features in terms of Mendelian genetics.

• Quantitative genetics, often known as biometrical genetics, is a branch of genetics concerned with the genetic interpretation of quantitative traits.
• Weight, length, width, height, duration, and other variables are used to measure polygenic features. Individuals, in this case, cannot be divided into groups based on colour or shape.
• The population analysis for polygenic inheritance is done using mean values from many individuals. The differences are expressed as variance or covariance.

Polygenic inheritance is also used in quantitative traits, where several gene loci contribute to the phenotype in a similar fashion, and the total number of contributing alleles determines the phenotype. Quantitative features in people include height, weight, and skin colour. The Mendelian ratios are substituted in quantitative traits by a normal distribution curve, with the two ends of the curve defining the phenotype’s two extremes. 

Examples of polygenic inheritance

• Eye colour

Polygenic inheritance can be seen in eye colour. Up to 16 distinct genes are considered to influence this feature. The inheritance of eye colour is complicated. The amount of brown pigment melanin found in the front region of the iris determines the eye colour. While melanin levels in black and dark brown eyes are higher than in hazel or green eyes, the iris of blue eyes is devoid of melanin. On chromosome 15, two genes that determine eye colour have been discovered (OCA2 and HERC2). Several other genes that affect eye colour also affect skin and hair colour.

Given that eye colour is determined by several genes, let us suppose that two genes determine it in this example. In this scenario, a cross between two people with light brown eyes (BbGg) would result in a number of diverse phenotypes. For gene 1, the dominant allele for black colour (B) is dominant over the recessive blue colour (b). The dark colour (G) is dominant in gene 2 and results in green colour. The recessive lighter hue (g) gives a light colour. There would be five basic phenotypes and nine genotypes resulting from this cross.

Black eyes: (BBGG)

Dark Brown eyes: (BBGg), (BbGG)

Light Brown eyes: (BbGg), (BBgg), (bbGG)

Green eyes: (Bbgg), (bbGg)

Blue eyes: (bbgg)

• Skin colour

Skin colour is inherited in a polygenic manner. Skin colour is thought to be influenced by at least three genes, with other genes perhaps playing a role as well. The quantity or proportion of dark colour pigment melanin in the skin determines skin colour. The genes that control skin colour each have two alleles and are located on different chromosomes. If we merely look at the three genes that have been linked to skin colour, we can see that each gene has one allele for dark skin and one for light skin. The dark skin colour allele  is dominant over the light skin colour allele . The quantity of dark alleles a person possesses determines his or her skin tone. Individuals who do not inherit any dark alleles have very pale skin, while those who receive solely dark alleles have a very dark complexion. People with varied mixes of light and dark alleles will have phenotypes with diverse skin tones. Medium skin colour is achieved by inheriting an equal amount of dark and light alleles. The darker you are, the more dark alleles you have inherited.

The inheritance of colour of skin in humans studied by davenport. Six types of phenotype of colour of skin are found in humans.

In a new discovery human skin colour and kernel colour in wheat is regulated by 3 pairs of alleles so phenotypic  ratio of F2 generation.

Ratio         1      :       6      :       15    :       20    :       6      :       1

             Full black                                   Intermediate                                             full white

 The type of heredity in which the phenotype is formed by the cumulative effects of numerous genes (or one gene pair) is referred to as Polygenic inheritance. In monogenic inheritance.

Conclusion:

Multiple genes, or polygenes, determine features in polygenic inheritance. Polygenic inheritance is a sort of incomplete dominant inheritance in which the expressed phenotypes are made up of a variety of inherited features. Polygenic features can manifest in a variety of phenotypes or visible characteristics. In a population, polygenic traits have a bell-shaped distribution. Most people acquire various combinations of alleles and lie somewhere in the middle of the curve for a given trait. Skin colour, eye colour, hair colour, body form, height, and weight are all polygenic features. Polygenes play a role in population variation and have evolutionary relevance.