Essay on Polygenic Inheritance in Plants
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.
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.
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:
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
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
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.
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.
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