Polygenic Inheritance: Polygenic inheritance refers to the kind of inheritance in which the trait is produced from the cumulative effects of many genes in contrast to monogenic inheritance in which the trait results from the expression of one gene (or one gene pair). In monogenic inheritance, the expression may be predicted according to a phenotypic ratio that follows Mendelian inheritance.

Polygenic inheritance is a non-Mendelian form since it is controlled by multiple genes at different loci on different chromosomes expressed together in the same trait. For example, if one pair of genes controls color, and red is dominant to white, then when you cross two heterozygotes (Aa), red and white progeny will appear in the ratio of 3:1.

However, if two pairs of genes control color and the dominant allele at both loci must be expressed to get red flowers, then crossing two heterozygotes (Aa Bb) will give you red and white flowers in a ratio of 9:7. This is a modification of the typical dihybrid Mendelian ratio of 9:3:3:1, in which three of the progeny groups all have the same phenotype.

Characteristics of Polygenic Inheritance

• A gene that employs a minor effect on a phenotype along with other genes is referred to as polygenes.
• The effect of an individual gene is too minor and often remains undetected
• Numerous genes exert an equal effect.
• Individually, each allele contributes to the result in a cumulative or additive manner
• The expression of one gene is not masked by the presence of the other genes, i.e., epistasis is not involved.
• The gene involved in polygenic inheritance is either contributing (active allele) or non-contributing (null allele); there are no genes as dominant or masked genes.
• There is a continuous variation of the phenotype of a trait in a polygenic inheritance
• The polygenic inheritance pattern is difficult to predict and it is highly complex.
• The statistical analysis of polygenic inheritance patterns can help to provide an estimate of population parameters. Most of the polygenic inheritance follow the normal distribution curve, wherein the majority of the people fall in the middle range of the curve.
• Polygenic inheritance is different from multiple alleles. In multiple alleles, on the same locus, three or more alleles are present an organism, e.g. human blood group system, i.e., the ABO system, is controlled by three alleles.

Examples of Polygenic Inheritance

• Polygenic Inheritance in Humans: There are many traits in humans, which show polygenic inheritance, e.g. skin and hair colour, height, eye colour, the risk for diseases and resistance, intelligence, blood pressure, bipolar disorder, autism, longevity, etc. Brief description of some of the traits are:
  • Skin color and pigmentation: Inheritance of skin pigmentation is polygenic inheritance. Around 60 loci contribute to the inheritance of a single trait. If we take an example of a pair of alleles of three different and unlinked loci as A and a, B and b, C and c. The capital letters represent the incompletely dominant allele for dark skin colour. The more capital letters show skin colour towards the darker range and small letters towards the lighter colour of the skin. Parents having genotype AABBCC and aabbcc will produce offspring of intermediate colour in the F₁ generation, i.e. AaBbCc genotype. In the F₂ generation of two triple heterozygotes (AaBbCc x AaBbCc) mate, they will give rise to varying phenotypes ranging from very dark to very light in the ratio 1:6:15:20:15:6:1.
  • 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.
  • 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.
    • 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: Polygenic inheritance patterns can be statistically analysed to aid in the estimation of population parameters. Brief description of some of the traits are:
  • The kernal colour of wheat: The colour of the wheat kernel is determined by the expression of three independently assorted pairs of alleles. The dominant allele, AABBCC, is found in dark red wheat kernels, while the recessive allele, aabbcc, is found in white kernels. As a result, when AABBCC wheat is crossed with aabbcc, the F1 generation has a red colour kernel intermediate to AaBbCc. Furthermore, one white kernel plant would be produced during crossbreeding in the F2 generation, while 63 red kernel plants of various shades would be produced.
  • Length of the corolla in tobacco: There are around 5 genes involved in the expression of phenotype for corolla length of tobacco. There is a wide variety in the length of the corolla in tobacco due to polygenic inheritance.

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By Team Learning Mantras