Characteristics that exhibit continuous variation are generally controlled by

Step 1: Analyze the question The question asks what generally controls characteristics that show continuous variation. These are traits with a full range of values between two extremes and no distinct categories.

Step 2: Explain the concept

• Continuous variation: Traits such as height, body mass, skin colour, and yield of crops show a smooth range from one extreme to the other. Students in your class, for example, don't fall into just "tall" or "short". Heights vary across a whole spectrum.
• Discontinuous variation: Traits such as blood group (A, B, AB, O), sickle cell status, and the ability to roll the tongue fall into distinct categories with no in-between. These are usually controlled by one gene (or a few) with clear dominant/recessive alleles.

Continuous variation is polygenic. This means many genes each add a small effect to the trait. The environment (food, sunlight, climate) also influences it. Together, these produce the smooth range of values, which usually forms a bell-shaped curve when plotted.

Step 3: Evaluate the options

• A. A single gene: This gives discontinuous variation with distinct categories, so it is wrong.
• B. Recessive genes: Whether a gene is recessive says nothing about the continuity of a trait, so it is wrong.
• C. Epistatic interactions: This is one gene masking another, and it produces distinct ratios (like 9:3:4). It is not the general explanation for continuous variation, so it is wrong.
• D. Multiple genes: Many genes acting together (polygenic inheritance) give a continuous range, so this is correct.

Step 4: Answer The correct option is D, multiple genes.

Exam tip: Remember "Continuous = Combination of many genes + environment", and "Discontinuous = Distinct categories (one or few genes)". If you see height, weight, or skin colour, think polygenic.