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Difficulty: HardHuman Morphological and Physiological Variations

A biological study recorded two distinct human characteristics across a high school population: Trait X (skin pigmentation shade measured by spectrophotometry, exhibiting a continuous bell-shaped distribution curve) and Trait Y (phenylthiocarbamide tasting ability, where individuals strictly belong to either the taster or non-taster phenotype). Based on these phenotypic distribution patterns, which of the following statements correctly contrasts the underlying genetic mechanisms and environmental influences governing both traits?

  1. Trait X is a continuous morphological variation governed by polygenic inheritance and modified by environmental factors, whereas Trait Y is a discontinuous physiological variation controlled by a single gene pair with negligible environmental modification.Answer
  2. B
    Trait X is a discontinuous morphological trait determined solely by non-genetic environmental factors, whereas Trait Y is a continuous physiological trait governed by codominant alleles.
  3. C
    Trait X is controlled by a single gene pair showing monohybrid dominance, whereas Trait Y is a polygenic trait displaying cumulative environmental modification.
  4. D
    Both Trait X and Trait Y represent discontinuous physiological variations governed by multiple alleles demonstrating codominance.

Answer

Trait X (skin pigmentation) is a continuous morphological variation under polygenic control and environmental influence, while Trait Y (PTC tasting) is a discontinuous physiological variation under single-gene control.
The correct answer accurately identifies Trait X (skin color spectrum) as a continuous morphological trait governed by multiple additive genes (polygenes) and modified by environment (sunlight), while correctly identifying Trait Y (PTC tasting) as a discontinuous physiological trait governed by a single pair of Mendelian alleles unaffected by environmental conditions.

Step-by-Step Solution

1
Classify Trait X based on its phenotypic distribution curve
The continuous bell-shaped spectrum indicates quantitative (continuous) variation, which is characteristic of morphological features like skin color controlled by multiple gene pairs (polygenic) and influenced by solar exposure.
Continuous variation produces a gradient of phenotypes without clear-cut divisions.
2
Classify Trait Y based on its categorical grouping
Clear-cut categories (tasters vs non-tasters) without intermediate forms define qualitative (discontinuous) physiological variation, driven by a single gene pair (e.g., TAS2R38 gene locus).
Discontinuous variation results in distinct phenotypic classes unaffected by environmental factors.
3
Synthesize and contrast the genetic and environmental controls of both traits
Trait X is continuous, morphological, polygenic, and environmentally modified. Trait Y is discontinuous, physiological, monogenic, and environmentally stable.
Matching phenotypic frequency patterns with genetic basis is key to differentiating human variation types.

Key Concept

Classification of continuous versus discontinuous morphological and physiological human variations
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Topics

Human Morphological and Physiological Variations

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