Introduction
Inheritance is governed not only by single genes with distinct Mendelian ratios but also by multiple genes acting together. Many characters in plants, animals, and humans do not show sharp differences, but instead, exhibit a range of small, quantitative differences. These are controlled by polygenes and give rise to continuous variation. Such traits include height, weight, skin color, yield, intelligence, and seed size.
Polygenes
Definition
Polygenes (also called multiple factor genes) are a group of non-allelic genes that collectively influence a quantitative trait. Each gene has a small, additive effect, and together they determine the phenotype.
Characteristics of Polygenes
- Each polygene contributes a small and equal effect to the phenotype.
- Their effects are additive, meaning the total expression depends on the number of contributing alleles.
- Individual polygenes cannot be detected by simple Mendelian segregation.
- They do not show dominance; instead, their cumulative action produces the phenotype.
- Polygenes are responsible for traits that show quantitative inheritance.
Examples of Polygenic Traits
- In humans: Height, skin color, weight, eye color, intelligence.
- In plants: Yield of crops, fruit size, kernel color in wheat, cob length in maize.
- In animals: Body weight in cattle, egg production in poultry, milk yield.
Continuous Variation
Definition
Continuous variation refers to differences in traits that show a gradual change and do not fit into clear-cut categories. Such variations arise due to the action of polygenes and environmental influence.
Characteristics of Continuous Variation
- Traits are distributed over a range rather than distinct categories.
- Differences between individuals are slight and gradual.
- It is influenced by both genetic factors (polygenes) and environmental factors.
- Traits usually follow a normal distribution curve in a population.
- The exact genotype cannot be predicted from the phenotype.
Difference between Discontinuous and Continuous Variation
| Feature | Discontinuous Variation | Continuous Variation |
|---|---|---|
| Controlled by | Single gene (major effect) | Many genes (polygenes) |
| Categories | Distinct and separate | No sharp categories |
| Environment Influence | Minimal | Strong |
| Example | Blood groups, flower color | Height, skin color, yield |
Inheritance of Polygenic Traits
The inheritance of polygenic traits does not follow simple Mendelian ratios. Instead, it can be studied using Nilsson-Ehle’s experiment on kernel color in wheat.
Nilsson-Ehle’s Wheat Kernel Color Experiment
- He crossed a red kernel wheat with a white kernel wheat.
- In the F1 generation, all seeds were intermediate red.
- In the F2 generation, a wide range of kernel colors appeared (dark red, red, light red, pink, and white).
- This showed that kernel color is controlled by two pairs of polygenes (R1, R2).
Interpretation
Each dominant allele contributes a small amount of red pigment. The more the number of dominant alleles, the deeper the red color. Thus, polygenes explain continuous variation in kernel color.
Statistical Nature of Polygenic Inheritance
Since polygenes control quantitative traits, statistical tools like mean, variance, standard deviation, and correlation are used to study them. The distribution of such traits follows a bell-shaped normal distribution curve. This curve arises because the combined effects of many genes and environment produce a wide range of phenotypes.
Figure: Normal distribution (bell curve) showing continuous variation.
Significance of Polygenes and Continuous Variation
- Crop Improvement – Selection for yield, size, and resistance is based on polygenic inheritance.
- Animal Breeding – Helps in improving milk yield, meat quality, and egg production.
- Human Genetics – Explains the inheritance of height, weight, intelligence, and diseases like diabetes and hypertension.
- Evolution – Provides raw material for natural selection, since continuous variation ensures diversity in populations.
- Medical Research – Helps in studying complex diseases influenced by multiple genes.
