Model Answer
0 min readIntroduction
Mendel’s laws of inheritance, while foundational, do not explain all patterns of inheritance observed in nature. Many traits exhibit more complex inheritance patterns, deviating from simple dominant-recessive relationships. Two such patterns are incomplete dominance and polygenic inheritance. Incomplete dominance occurs when the heterozygous genotype expresses an intermediate phenotype, while polygenic inheritance involves multiple genes contributing to a single trait, resulting in a continuous range of phenotypes. Understanding these concepts is crucial for comprehending the genetic basis of phenotypic variation in organisms, including plants.
Incomplete Dominance
Incomplete dominance is a genetic phenomenon where the heterozygous genotype exhibits a phenotype that is intermediate between the phenotypes of the two homozygous genotypes. Unlike complete dominance, where one allele completely masks the other, in incomplete dominance, both alleles contribute to the phenotype. This results in a blending effect.
- Example 1: Snapdragon Flower Color: In snapdragons (Antirrhinum majus), flower color is controlled by a single gene with two alleles: R for red flowers and W for white flowers. A homozygous red flower (RR) produces red pigment, while a homozygous white flower (WW) produces no pigment. A heterozygous plant (RW) produces only half the amount of red pigment, resulting in pink flowers. This demonstrates a clear intermediate phenotype.
- Example 2: Four O’Clock Flower Color: Similar to snapdragons, the flower color in Four O’Clock plants (Mirabilis jalapa) also exhibits incomplete dominance. Red (R) and white (W) alleles result in red, white, and pink flowers in the RR, WW, and RW genotypes respectively.
Polygenic Inheritance
Polygenic inheritance refers to the inheritance of traits that are controlled by multiple genes, each contributing a small, additive effect to the overall phenotype. Unlike traits governed by a single gene, polygenic traits typically exhibit a continuous range of variation, often following a normal distribution. Environmental factors can also influence the expression of polygenic traits.
- Example 1: Human Skin Color: Human skin color is a classic example of polygenic inheritance. At least three genes (MC1R, SLC24A5, and SLC45A2) contribute to melanin production, with each gene having multiple alleles. Individuals with more alleles promoting melanin production have darker skin, while those with fewer have lighter skin. This results in a wide spectrum of skin tones.
- Example 2: Grain Color in Wheat: The color of wheat kernels is determined by at least three genes. Each gene contributes to the intensity of the red pigment. The more dominant alleles present, the darker the kernel color. This leads to a continuous range of kernel colors, from white to dark red, rather than distinct categories.
Comparison of Incomplete Dominance and Polygenic Inheritance
While both deviate from Mendelian inheritance, they differ significantly. Incomplete dominance involves a single gene with two alleles and an intermediate phenotype in heterozygotes. Polygenic inheritance, however, involves multiple genes, each with a small additive effect, resulting in a continuous range of phenotypes. Incomplete dominance produces discrete phenotypes (though intermediate), while polygenic inheritance produces quantitative phenotypes.
| Feature | Incomplete Dominance | Polygenic Inheritance |
|---|---|---|
| Number of Genes Involved | Single gene | Multiple genes |
| Phenotype in Heterozygotes | Intermediate phenotype | Continuous range of phenotypes |
| Phenotypic Ratio | 1:2:1 | Typically a normal distribution |
| Environmental Influence | Minimal | Significant |
Conclusion
Incomplete dominance and polygenic inheritance represent crucial deviations from simple Mendelian genetics, demonstrating the complexity of trait inheritance in living organisms. Incomplete dominance showcases the blending of traits due to heterozygous expression, while polygenic inheritance highlights the cumulative effect of multiple genes on a single phenotype. Understanding these patterns is essential for predicting inheritance patterns, breeding programs, and comprehending the genetic basis of phenotypic diversity. Further research continues to unravel the intricate interplay between genes and the environment in shaping observable traits.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.