Essay on Polygenic Inheritance in Plants

Characters or traits are governed by one or few genes. Such traits are called as qualitative or oligogenic characters. Some characters are controlled by several genes.They are known as quantitative characters or polygenic characters.

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Features of Polygenic Traits:

i. Each polygenic character is controlled by several independent genes.

ii. Polygenic character exhibit continuous variation.

iii. Highly sensitive to environmental changes.

iv. Effect of individual gene is not easily detectable.

v. Expression is governed by additive gene action.

vi. Transmission is generally low because of high amount of environmental variation.

Analysis of Polygenic Traits:

i. Measurement based on degree of variation

ii. Observation recorded in population.

iii. Studies with the help of mean, variance and covariance.

Assumption of Polygenic Traits:

i. Each of the contributing genes involved in the expression of a character produces an equal effect.

ii. Each contributing alleles has either cumulative or additive effect in the expression of character.

iii. There is no epistasis among genes at different loci.

iv. The linkage is in equilibrium.

v. The environmental effects are absent.

Examples of Polygenic Traits:

1. Kernel colour in wheat

2. Corolla length in Tobacco

3. Skin colour inheritance in man.

Transgressive Segregation:

Appearance of transgressive segregants in F₂ is an important feature of polygenic inheritance.

The segregants which fall outside the limits of both the parents are known as transgressive segregants.

Example:

Polygenic Variation:

Three types —

1. Phenotypic Variability:

It includes both genotypic and environmental variation.

2. Genotypic Variability:

It is the inherent or genetic variability which remains unaltered by environmental conditions. It consists of additive, dominance and epistatic components.

3. Environmental Variability:

It is non heritable variation. This uncontrolled variation is measured in terms of error mean variance.

Table 7. Differences between Additive Variance and Dominance Variance:

Additive Variance:

i. Difference between homozygotes

ii. Genes show lack of dominance

iii. Associated with homozygosity

iv. It is fixable

v. Selection is very effective

vi. It is the cause of transgressive segregation

Dominance Variance:

i. Deviation of Aa from the mean of A A and aa

ii. Genes show incomplete, complete or over dominance.

iii. Associated with heterozygosity

iv. Non fixable

v. Selection is ineffective

vi. Cause of heterosis.

Table 8. Differences between Polygenic Traits and Oligogenic Traits

Polygenic Traits

i. Governed by several genes

ii. Effect of each gene is not detectable

iii. Variation is continuous

iv. Usually governed by additive genes

v. Highly influenced by environmental factors.

Oligogenic Traits:

i. Governed by few genes

ii. Effect of each gene is detectable

iii. Variation is discontinuous

iv. Governed by non additive genes

v. Not influenced by environmental factors.

Epistatic Variance:

It is due to the deviations as a consequence of inter-allelic interaction, is interaction between alleles of two or more different genes or loci.

It is of three types —

1. Additive x Additive

2. Additive x Dominance

3. Dominance x Dominance

Significance of Polygenes:

i. It has great evolutionary significance

ii. Increase the crop yield

iii. Variation for speciation