Explain the genetic basis of discontinuous variation and continuous variation
Explain the genetic basis of discontinuous variation and continuous variation
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The genetic basis of discontinuous variation and continuous variation differs in terms of the number of genes involved and how they interact to influence a trait.
Discontinuous Variation
Discontinuous variation is generally controlled by a single gene or a few genes with major effects. This type of genetic control is called monogenic or oligogenic inheritance, and the genes involved are often expressed in a dominant-recessive manner. Because only one or a few genes influence the trait, the resulting phenotypes are distinct and have no intermediates, resulting in separate categories.
Key Points:
- Few genes involved: Often, only one or two genes determine the trait.
- Distinct phenotypes: Individuals either have one form of the trait or another, with no blending.
- Dominant and recessive alleles: In many cases, the trait follows simple Mendelian inheritance, where dominant and recessive alleles lead to specific phenotypic categories.
Example: In pea plants, flower color (purple or white) is an example of discontinuous variation and is controlled by a single gene with two alleles. The presence of the dominant allele (P) results in purple flowers, while the recessive allele (p) leads to white flowers. Pea plants can either have purple flowers or white flowers, with no intermediate shades.
Continuous Variation
Continuous variation is usually controlled by multiple genes, each contributing a small, additive effect to the phenotype. This type of genetic control is known as polygenic inheritance. When many genes influence a trait, each gene contributes a small amount to the final phenotype, resulting in a continuous range of expression. Additionally, environmental factors often play a significant role in shaping the phenotype, further smoothing out the variation across a continuum.
Key Points:
- Multiple genes involved: Many genes, each with a small effect, contribute to the trait.
- Additive effects: Each gene adds to the overall expression of the trait, creating a spectrum of possible phenotypes.
- Environmental influence: Environment often interacts with genetics, adding further variability to the trait.
Example: Human height is influenced by many genes, each contributing a small effect on overall height. Some genes might contribute to bone length, others to muscle growth, and others to hormone production. Together, these genes produce a wide range of possible heights, resulting in a continuous distribution. Additionally, environmental factors like nutrition and health also impact height, increasing the range of phenotypic expression.
In summary:
- Discontinuous variation: Traits controlled by one or a few genes, with clear, distinct categories.
- Continuous variation: Traits controlled by many genes with small additive effects, creating a smooth range of phenotypic values.
Sourav
