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Difficulty: HardContinuous and Discontinuous Variation

A botanical study recorded two phenotypic traits in a population of *Hibiscus sabdariffa*: total leaf surface area (measured in cm2\text{cm}^2) and petal pigmentation pattern (either solid crimson or white-striped). Leaf surface area exhibited a smooth, bell-shaped frequency distribution curve across the population, with mean values altering significantly when cloned specimens were grown in nutrient-deficient versus nutrient-rich soils. In contrast, petal pigmentation pattern showed strict discrete categories with no intermediate forms, remaining completely unchanged across different soil conditions. Which of the following statements correctly explains the biological mechanisms responsible for the variation observed in these two traits?

  1. Leaf surface area exhibits continuous variation controlled by polygenes and modified by environmental conditions, whereas petal pigmentation pattern exhibits discontinuous variation governed by monogenic inheritance and unaffected by the environment.Answer
  2. B
    Leaf surface area exhibits discontinuous variation driven entirely by environmental factors, whereas petal pigmentation pattern exhibits continuous variation under polygenic control.
  3. C
    Both traits represent continuous variation, but leaf surface area is polygenic while petal pigmentation pattern is determined by multiple codominant alleles.
  4. D
    Leaf surface area is governed by a single major gene subject to incomplete dominance, while petal pigmentation pattern is a polygenic trait suppressed by soil nutrient levels.

Answer

Leaf surface area exhibits continuous variation controlled by polygenes and modified by environmental conditions, whereas petal pigmentation pattern exhibits discontinuous variation governed by monogenic inheritance and unaffected by the environment.
The phenotypic trait showing a normal bell-shaped curve and response to soil nutrients (leaf surface area) is a classic example of continuous variation, which is polygenic and modified by environmental conditions. Conversely, the trait displaying clear-cut, non-overlapping phenotypic categories unaffected by environmental shifts (petal pigmentation pattern) represents discontinuous variation, which is governed by monogenic inheritance.

Step-by-Step Solution

1
Analyze the phenotypic distribution and environmental sensitivity of leaf surface area.
Leaf surface area forms a continuous bell-shaped curve and changes phenotypic expression when grown in different soil conditions.
Continuous traits are quantitative, controlled by multiple additive genes (polygenic inheritance), and significantly influenced by environmental factors such as nutrient availability.
2
Analyze the phenotypic distribution and environmental sensitivity of petal pigmentation pattern.
Petal pigmentation shows clear-cut, discrete categories (solid crimson vs. white-striped) without intermediate phenotypes, remaining stable across varied soil environments.
Discontinuous traits are qualitative, controlled by one or a few major genes (monogenic/oligogenic inheritance), and generally unaffected by environmental variation.
3
Synthesize the biological mechanisms for both traits to identify the correct option.
Leaf surface area = Continuous variation (polygenic + environmental impact); Petal pigmentation = Discontinuous variation (monogenic + uninfluenced by environment).
Matching phenotypic features to their corresponding genetic architecture and environmental susceptibility confirms the correct biological interpretation.

Key Concept

Continuous and Discontinuous Variation
Estimated Time:2m 0s
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Topics

Continuous and Discontinuous Variation

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