O Level Biology 9 Views 1 Answers
Describe that discontinuous variation is usually caused by genes only and continuous variation is caused by genes and the environment
Describe that discontinuous variation is usually caused by genes only and continuous variation is caused by genes and the environment
Answer
Discontinuous vs. Continuous Variation
Variation among individuals in a species can be broadly categorized into two types: discontinuous variation and continuous variation. Understanding the differences between these two forms of variation is crucial for studying genetics, evolution, and ecology.
Discontinuous Variation
Definition
Discontinuous variation refers to traits that exhibit distinct categories or phenotypes with no intermediates. Individuals fall into separate groups based on their observable characteristics.
Causes
- Genetic Factors Only: Discontinuous variation is primarily driven by genetic factors. Traits are typically controlled by one or a few genes, where specific alleles dictate the phenotype.
- Examples:
- ABO Blood Groups: The presence of specific alleles (A, B, and O) results in four distinct blood types (A, B, AB, and O) with no intermediate types.
- Pea Seed Shape: In pea plants, seed shape is determined by a single gene with two alleles: round (dominant) and wrinkled (recessive). Individuals will display either one phenotype or the other, with no intermediate shapes.
Continuous Variation
Definition
Continuous variation refers to traits that exhibit a range of phenotypes without distinct categories. Individuals show gradual differences in characteristics.
Causes
- Genetic and Environmental Factors: Continuous variation is influenced by multiple genes (polygenic inheritance) as well as environmental factors. This interplay leads to a spectrum of phenotypes.
- Examples:
- Height in Humans: Human height is influenced by multiple genes (at least 700 identified) and environmental factors such as nutrition and health during development. This results in a continuous range of heights rather than discrete categories.
- Skin Color: Skin color is another example of continuous variation, where multiple genes contribute to pigmentation levels, further influenced by exposure to sunlight and other environmental factors.
Key Differences
| Feature | Discontinuous Variation | Continuous Variation |
|---|---|---|
| Phenotype Categories | Limited number of distinct categories | Range of phenotypes without distinct categories |
| Causes | Primarily genetic factors only | Genetic factors and environmental influences |
| Examples | ABO blood groups, pea seed shape | Human height, skin color |
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