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Difficulty: HardContinuous and Discontinuous Variation

A study conducted on a human population recorded two phenotypic traits: resting systolic blood pressure (Trait I) and the presence or absence of the Rhesus D antigen on red blood cells (Trait II). Trait I produced a smooth, bell-shaped frequency distribution curve across a continuous gradient of values, whereas Trait II resulted in two distinct, non-overlapping categories with no intermediate forms. Which of the following correctly explains the genetic mechanism and environmental susceptibility responsible for these observed patterns of variation?

  1. Trait I is polygenic and modified by environmental factors, whereas Trait II is controlled by a single gene locus and unaffected by environmental conditions.Answer
  2. B
    Trait I is controlled by a single gene locus showing complete dominance, whereas Trait II is polygenic and strongly influenced by environmental conditions.
  3. C
    Both Trait I and Trait II are polygenic, but Trait II displays clear-cut categories due to strong environmental suppression of intermediate phenotypes.
  4. D
    Trait I is determined solely by environmental factors without a genetic basis, whereas Trait II is a multiallelic trait exhibiting quantitative inheritance.

Answer

Trait I represents continuous variation, which is polygenic (controlled by multiple genes) and influenced by environmental factors, whereas Trait II represents discontinuous variation, which is monogenic (controlled by one gene pair or single locus) and independent of environmental changes.
The option stating that Trait I is polygenic and modified by environmental factors, whereas Trait II is controlled by a single gene locus and unaffected by environment is correct. Continuous traits like blood pressure are polygenic (controlled by multiple genes whose effects combine additively) and show significant variation due to external environmental factors like diet and stress, resulting in a continuous bell-shaped curve. In contrast, discontinuous traits like the Rhesus factor are monogenic (determined by alleles at a single locus), resulting in distinct, clear-cut phenotypic classes that environment cannot alter.

Step-by-Step Solution

1
Analyze the phenotypic distribution of Trait I (systolic blood pressure).
The smooth, bell-shaped normal distribution curve indicates continuous variation with quantitative grading between extremes.
Continuous variation occurs when traits are polygenic (governed by multiple additive genes) and sensitive to environmental influences.
2
Analyze the phenotypic distribution of Trait II (Rhesus antigen presence/absence).
Two distinct, non-overlapping categories with no intermediate forms indicate discontinuous variation.
Discontinuous variation is driven by monogenic inheritance (a single gene pair or major locus) and is largely unaffected by environmental conditions.
3
Synthesize the genetic mechanisms and environmental impacts for both traits.
Trait I is polygenic and environmentally modified, while Trait II is monogenic and environmentally stable.
Matching phenotypic distribution patterns (bell-shaped vs discrete categories) directly to their genetic architecture resolves the question.

Key Concept

Polygenic inheritance causing continuous variation versus monogenic inheritance causing discontinuous variation
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Topics

Continuous and Discontinuous Variation

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