Which of the following is an example of discontinuous variation?

Step 1: Analyze the question We are asked to identify an example of discontinuous variation.

Step 2: Explain the concept

• Continuous variation: A trait shows a full range of values between two extremes, with no clear-cut categories. It is usually controlled by many genes (polygenic) and influenced by the environment. Examples: height, weight, skin colour. A graph of these traits gives a bell-shaped curve.
• Discontinuous variation: A trait falls into distinct categories, with no intermediates. It is usually controlled by one or two genes and is little affected by the environment. Examples: sex (male or female), blood group (A, B, AB, O), tongue rolling, ability to taste PTC. A graph of these traits gives separate bars.

Step 3: Go through the options

• A. Height of a pea plant from 30 cm to 50 cm: Height can take any value in the range, so this is continuous variation.
• B. Offspring of two male cats and three female cats: Sex has only two distinct forms, with nothing in between. This is discontinuous variation.
• C. Colour of pear plants grown in the dark: Plants grown in the dark lose their green colour (etiolation). This is an environmental effect, not a clear-cut genetic category. It is not the standard example of discontinuous variation.
• D. Adult body weight from 50 kg to 95 kg: Weight changes gradually across a range, so this is continuous variation.

Step 4: Answer The correct answer is B. Sex is a clear-cut, either/or trait.

Tip: Ask yourself, "Can I measure it on a scale, or can I only count it in categories?"

• Measured on a scale (height, weight, length) means continuous.
• Counted in categories (sex, blood group, tongue rolling) means discontinuous.