Discontinuous variation features distinct categories without intermediates

Steps:

• Recall discontinuous variation: traits controlled by single genes, showing clear categories (e.g., presence/absence of a condition).
• Examine options: identify if each shows gradual changes (continuous) or discrete states (discontinuous).
• Match sickle cell anaemia to discontinuous pattern due to its genetic basis.
• Eliminate continuous traits like measurements that vary smoothly.

Why D is correct:

• Sickle cell anaemia follows Mendelian inheritance, resulting in discrete genotypes/phenotypes: normal (AA), carrier (AS), or affected (SS), per the law of segregation.

Why the others are wrong:

• A. Body mass: continuous, influenced by multiple factors, varies gradually across a range.
• B. Heart rate: continuous, affected by environment and physiology, shows smooth variation.
• C. Height: continuous, polygenic trait with environmental influence, forms a bell curve distribution.

Final answer: D