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Difficulty: MediumContinuous and Discontinuous Variation

A survey of students in a secondary school revealed varying physical traits among individuals. Height showed a wide range of values forming a normal distribution, whereas the ABO blood group fell strictly into four distinct categories. Which of the following statements correctly accounts for the genetic basis of these observed variations?

  1. Height exhibits continuous variation controlled polygenically with environmental influence, whereas ABO blood group exhibits discontinuous variation controlled by a single gene.Answer
  2. B
    Height exhibits discontinuous variation controlled by a single gene, whereas ABO blood group exhibits continuous variation influenced by environmental factors.
  3. C
    Both height and ABO blood group exhibit continuous variation because both traits are influenced by multiple environmental factors.
  4. D
    Both height and ABO blood group exhibit discontinuous variation controlled entirely by single-gene alleles without environmental effect.

Answer

Height exhibits continuous variation controlled polygenically with environmental influence, whereas ABO blood group exhibits discontinuous variation controlled by a single gene.
Continuous variation traits like human height are polygenic and influenced by environmental factors, producing a gradual continuum of phenotypes. Discontinuous variation traits like blood groups are governed by single genes with distinct alleles, producing clear-cut, non-overlapping categories regardless of environmental conditions.

Step-by-Step Solution

1
Analyze the distribution pattern of height.
Height displays a smooth range of intermediate values forming a normal bell-shaped curve.
Continuous variations show a quantitative spectrum of phenotypes controlled by many additive genes (polygenes) interacting with environmental factors such as diet.
2
Analyze the distribution pattern of ABO blood group.
Blood group phenotypes are distinct and non-overlapping (types A, B, AB, and O).
Discontinuous variations show qualitative differences determined by single gene loci (monogenic inheritance) unaffected by environment.
3
Compare the genetic mechanisms and match the correct statement.
Height is continuous and polygenic; ABO blood group is discontinuous and monogenic.
This accurately reflects phenotypic variation concepts in population genetics.

Key Concept

Continuous vs Discontinuous Variation
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Topics

Continuous and Discontinuous Variation

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