POLYGENIC INHERITANCE AND ASSOCIATED NUMERICALS

Polygenic inheritance: When a trait is controlled by more than one pair of contrasting gene ( generally a pair of allele) and follows non Mendelian inheritance pattern it is called polygenic trait and the pattern of Inheritance called Polygenic Inheritance.

It refers to Inheritance of quantitative traits which are influenced by multiple genes to different loci and environment may also be involved. A quantitative trait is a measurable phenotype that depends on the cumulative actions of many genes and the environment. These traits can vary among individuals, over a range, to produce a continuous distribution of phenotypes. Examples include height, weight and blood pressure.

 It is characterized by

Ø  Involvement of several loci in the expression of the trait.

Ø  There is no dominant and recessive relationship at each of the loci.

Ø  The loci at in an addition fashion, in adding and detracting a small amount from the phenotype.

Ø  The environment interacts with the genotype to produce the final phenotype.

 

Let's take the obvious example of height of the students in your class.  Unlike a simple Mendelian characteristic, human height displays continuous variation. Unlike Mendel's pea plants, there is no such two clear-cut “tall” and “short” varieties. In fact, we can't categorize the human height in four heights, or ten, or forty discrete entities. Instead, it’s possible to get humans of many different heights, and height can vary in increments of centi meters or fractions of inches. Height and other similar features are controlled not just by one gene, but rather, by multiple (often many) genes that each make a fraction contribution to the whole  outcome. This inheritance pattern is  called polygenic inheritance (poly- = many). For instance, a recent study documented over four hundred genes linked to variation in height.

 

Simple numerical

Calculation of no of gene pairs

No of gene pairs (n) that are involved characterizing particular polygenic trait can be determine if: The ratio of  F₂ progeny resembles either of two most extreme phenotypes

It can be calculating by using a simple formula ratio of F₂ progeny expressing either extreme phenotype =1/4ⁿ

Problem: A plant with a genotype aabb and height 40cm it crossed with a plant genotype AABB height of 60cm if each dominant allele contributes to height additive. What is expected height of F₁progeny?

SOLUTION:

The height of the plant containing AABB = 80 cm

The height of the other plant containing aabb= 40 cm

Thus, difference in height = (80-40) = 40 cm

Each capital allele contributes additively (rule of polygenic traits). So, for each capital allele, height will increase = 40/4= 10 cm

              P generation:  aabb              x              AABB

                  F1 genotype=                  AaBb

F₁ progeny has two capital allele A and B.

As each capital allele contributes an average of 10 cm (additive effect) to the height of the plant.

The height of the F1 plant=40+ (10+10)= 60 cm.

Suggested questions:

a) What do you mean by polygenic traits? Write down its important characteristics

b) Compare between polygenic traits and multiple allelism

c) What do you mean by quantitative traits?

 

Suggested readings:

1. Fletcher    H. and Hickey I. (2015). Genetics. IV Edition. GS, Taylor and Francis Group, New York and London

2. Gardner, E.J., Simmons, M.J., Snustad, D.P. (2008). Principles of Genetics. VIII Edition. Wiley India

3. Griffiths, A.J.F., Wessler, S.R., Lewontin, R.C. and Carroll, S.B. Introduction to Genetic Analysis. IX Edition. W. H. Freeman and Co.

4. Klug, W.S., Cummings, M.R., Spencer, C.A. (2012). Concepts of Genetics. X Edition. Benjamin Cummings

               5. Russell, P. J. (2009). Genetics- A Molecular Approach.III Edition     

                 Benjamin Cummings

6.   Snustad, D.P., Simmons, M.J. (2009). Principles of Genetics. V Edition. John Wiley and Sons Inc