In contrast to monogenic inheritance, which occurs from the expression of a single gene, polygenic inheritance refers to the phenotype generated by the cumulative effects of several genes in this type of heredity (or one gene pair). In monogenic inheritance, phenotypic ratios that follow Mendelian inheritance can be used to predict expression. Polygenic inheritance is a non-Mendelian form of inheritance because several genes at different loci on different chromosomes express the same feature simultaneously. 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. If two pairs of genes govern colour and the dominant allele at both loci must be expressed to generate red flowers, crossing two heterozygotes (Aa Bb) will result in a 9:7 ratio of red to white flowers. This is a variation of the 9:3:3:1 dihybrid Mendelian ratio, in which all three progeny groups share the same phenotype.

Polygenic Inheritance Definition

In simple words, polygenic inheritance refers to a character or phenotypic characteristic that is controlled by multiple genes. It’s a sort of quantitative inheritance in biology in which two or more genes interact additively to affect a single phenotypic characteristic. In some aspects, polygenic inheritance resembles multiple factors, multiple genes, or multifactorial inheritance. As a result, phenotypic traits that are the product of the cumulative expression of multiple genes rather than a single gene feature are referred to as polygenic inheritance. Polygenic inheritance differs from Mendelian inheritance in terms of pattern and expression.

Characteristics of Polygenic Inheritance

• Polygenes are genes that have a small effect on phenotypes in combination with other genes.
• The impact of a single gene is typically insignificant and goes unnoticed.
• A large number of genes have the same impact.
• Individually, each allele makes a cumulative or additive contribution to the outcome.
• Epistasis is not engaged since the expression of one gene is not concealed by the presence of the other genes.
• There are no genes that are dominant or veiled in polygenic inheritance; they are either contributing (active allele) or non-contributing (null allele).
• In polygenic inheritance, the phenotype of a character is always changing.
• It’s impossible to forecast the polygenic inheritance pattern because it’s so complicated.

Analysis of Polygenic Traits

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

1. 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.
2. The population analysis for polygenic inheritance is done using mean values from many individuals. The variance or co-variance of the differences is calculated.
3. Quantitative genetics, often known as biometrical genetics, is a branch of genetics concerned with the genetic interpretation of quantitative traits.

Segmentation of Polygenic Variability

Variance is a metric for separating polygenic variation or variability in a genetic population. There are three types of polygenic inheritance variation. 

• Phenotypic: It’s the observable variability, and it’s usually the total variability, which can include both environmental and genotypic differences. It’s measured in phenotypic variance.
• Genotypic: Environmental variables have little effect on this diversity, which is inherited genetically. This is expressed as genotypic variance, which can be dominant, additive, or epistatic. This is the most widely exploited variability by plant breeders in the hybridization process.
• Environmental: These are non-heritable differences that have arisen as a result of environmental causes and fluctuate depending on the environment. The error mean-variance is used to assess this form of variation because it is fully unregulated.

Polygenic Inheritance Examples

1. Polygenic inheritance in humans

• Skin colour and pigmentation: Skin colour is inherited in a polygenic manner. It is governed by approximately 60 loci. Consider a pair of three distinct alleles present at unlinked loci, A and a, B and b, and C and c, to better understand the inheritance pattern of skin colour. 
• Human height: Human height is a polygenic characteristic governed by three genes, each of which has six alleles. 

2. Polygenic inheritance of the human eye colour: Eye colour is inherited in a polygenic pattern. Humans have nine distinct eye hues. Polygenic inheritance in plants

• Kernel colour of the wheat: The expression of three independently selected pairs of alleles determines the colour of the wheat kernel. The dominant allele, AABBCC, is expressed in the dark red wheat kernel, whereas the recessive allele, aabbcc, is expressed in the white kernel. 
• Length of the corolla in tobacco: The length of the tobacco plant’s corolla is determined by five genes. Polygenic inheritance is responsible for the variation in corolla length in tobacco.

Effect of Environment on Polygenic Inheritance

Environmental influences have a big impact on polygenes. Essentially, an individual’s genotype determines the quantitative trait’s range, but the phenotype of the trait is ultimately determined by environmental circumstances. Gene function is regulated by several environmental factors, resulting in variations in gene function. As a result, the gene function may turn on or off depending on the environmental variables. The ‘norm of reaction’ refers to the diversity in phenotypic manifestation of the same genotype under different environmental situations. The norm of reaction can be classified as follows, depending on the genotype involved:

• Human height, for example, is a narrow reaction standard.
• a broad range of reactions

Conclusion 

Any individual locus that is included in the system a polygenic locus is a group of genes responsible for the genetic component of variation in a quantitative (polygenic) trait. Allelic replacements play a role in the variance of a quantitative character. A polygenic locus might be a single gene or a complex genetic locus in the traditional sense, i.e., a single gene or a group of functionally connected genes.