Temperature-sensitive gene expression (a full environmental example)

A genuinely different environmental mechanism from a simple mutagen: some genes code for enzymes whose ACTIVITY itself depends on temperature, not on whether the DNA has mutated.

The Himalayan-rabbit mechanism, in full

THE GENE is unchanged (not mutated) across the rabbit's whole body — every cell carries the same allele for the melanin-producing enzyme.
THE ENZYME'S SHAPE (and therefore activity) is sensitive to TEMPERATURE: below about 25°C the enzyme keeps its functional shape and converts tyrosine to melanin; above about 30°C it denatures or loses activity and produces NO melanin.
BODY TEMPERATURE VARIES BY LOCATION: the rabbit's core body stays close to 37°C (too warm for the enzyme to work), but its EXTREMITIES (nose, ears, feet, tail) run cooler, since they lose more heat to the environment — cool enough for the enzyme to stay active there.
RESULT: the SAME gene and the SAME enzyme produce DIFFERENT visible phenotypes (dark fur at the cool extremities, no pigment on the warmer body) purely due to a temperature difference across the animal's own body — not due to any genetic difference between those cells.

Worked example Explaining why a Himalayan rabbit is dark at its extremities but pale on its body Discuss why Himalayan rabbits have dark melanin expression at their extremities (nose, ears, feet, tail) and not on their core body. THE GENE AND ENZYME ARE IDENTICAL EVERYWHERE: every skin cell across the whole rabbit carries the same melanin-pathway gene and produces the same temperature-sensitive enzyme. CORE BODY TEMPERATURE: sits near 37°C, well above the enzyme's ~30°C threshold — the enzyme is inactive there, so no melanin is produced, giving pale/white fur. EXTREMITY TEMPERATURE: nose, ears, feet and tail lose more heat to the surrounding air and run cooler, often below 25°C — cool enough for the enzyme to remain active and produce melanin, giving dark fur. CONCLUSION: the phenotype difference is caused ENTIRELY by a temperature difference across the body, not by any difference in genotype between the cells producing dark vs pale fur. ✓ The key move is stating explicitly that the GENE is the SAME everywhere — the visible difference comes from the ENZYME's temperature-dependent activity, not from any genetic difference between body regions.

What conditions produce an ALL-BLACK or ALL-WHITE Himalayan rabbit

ALL-BLACK: raised in a COLD environment for its whole life, so the enzyme stays active across the WHOLE body, not just the extremities.
ALL-WHITE: raised in a WARM environment for its whole life, so the enzyme never activates anywhere, including the normally-dark extremities.

Explaining this with a mutation instead of enzyme temperature-sensitivity

This is NOT a mutation-based explanation — the gene and enzyme are identical in every cell across the rabbit's body. The correct mechanism is that the SAME enzyme behaves differently at different temperatures (active when cool, inactive when warm) — introducing a mutation into this explanation is a common but incorrect substitution of mechanism.

Body location	Local temperature	Enzyme activity	Fur colour
Core body	~37°C (above threshold)	Inactive	Pale/white
Extremities (nose, ears, feet, tail)	Below ~25°C (within range)	Active	Dark

Practice for this standard covers the whole topic — flashcards and quiz.