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Selection and Evolution: Question 1

Syllabus 17.1

Multiple choice A2 1 mark

A biologist studying a large population of adult volunteers records two features.

Trait 1. ABO blood group: each volunteer is classified as group A, B, AB or O, with no volunteers falling between these categories.

Trait 2, hand span: measured to the nearest millimetre, values form a continuous range from about 170 mm to 230 mm, with most volunteers close to the mean and progressively fewer volunteers towards each extreme.

Which row correctly identifies the type of variation shown by each trait and the genetic basis of that type of variation?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Identify the pattern each trait shows

Blood group has only four possible, clearly distinct categories (A, B, AB, O). No volunteer can be “between” groups. A trait that falls into a small number of distinct categories, with no intermediate forms, shows discontinuous variation.

Hand span, by contrast, is measured on a continuous numerical scale and forms an unbroken range of values with most volunteers clustered near the mean and progressively fewer towards each extreme (a typical bell-shaped distribution). This pattern, a continuous range of values rather than distinct categories, is continuous variation.

Step 2: Match each pattern to its genetic basis

Discontinuous variation is typically produced by one gene, or a small number of genes, with a limited number of allele combinations, and is usually not (or only slightly) affected by the environment. ABO blood group is controlled by multiple alleles at a single gene locus, which is exactly this pattern.

Continuous variation is typically produced by the combined, additive effect of many different genes acting together (polygenic inheritance), and is usually also influenced by environmental factors (for hand span, this could include nutrition during growth). This is why hand span forms a smooth range rather than distinct groups.

Step 3: Evaluate the options

  • A correctly pairs discontinuous variation with blood group (single gene, multiple alleles) and continuous variation with hand span (polygenic inheritance plus environment), this matches the reasoning above.
  • B swaps the two traits around and incorrectly claims hand span has no environmental influence.
  • C wrongly calls hand span discontinuous and wrongly claims neither trait is affected by the environment (blood group genuinely isn’t much, but ruling out environmental influence entirely for hand span is incorrect).
  • D wrongly calls blood group continuous, when it in fact falls into a small number of sharply distinct categories with no environmental overlap between them.

Final answer

  • Blood group is discontinuous (single gene, multiple alleles); hand span is continuous (polygenic inheritance plus environment), option A.