Discontinuous variation features distinct categories without intermediates

Steps:

• Recall that discontinuous variation shows clear-cut traits controlled by single genes, like blood types or genetic disorders.
• Identify traits in options: agility and obesity are influenced by environment and multiple genes; height varies gradually; sickle cell anaemia has discrete states.
• Classify each: continuous traits form a spectrum (e.g., height ranges); discontinuous traits do not (e.g., disease presence/absence).
• Select the option with no intermediates, matching sickle cell's normal vs. affected phenotypes.

Why D is correct:

• Sickle cell anaemia follows Mendelian inheritance, producing distinct genotypes/phenotypes (normal, carrier, sickle-cell) without blending, per single-gene control definition.

Why the others are wrong:

• A: Agility shows continuous variation, influenced by training and polygenes for a smooth range.
• B: Body height exhibits continuous variation, polygenic with environmental factors creating a bell-curve distribution.
• C: Obesity displays continuous variation, measured on a scale like BMI without discrete categories.

Final answer: D