Himalayan rabbits are homozygous for a mutation in the tyrosinase gene that causes temperature-sensitive pigment production. They develop black fur on their extremities (ears, nose, paws) where body temperature is lower, and white fur on their torso. What does this phenomenon best illustrate?

Core Concept

PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM

Step-by-Step Analysis

The Himalayan rabbit phenotype arises from a specific molecular lesion in the TYR gene encoding tyrosinase, the rate-limiting enzyme catalyzing the hydroxylation of L-tyrosine to L-DOPA and its subsequent oxidation to dopaquinone within melanosome compartments of melanocytes. The Himalayan allele produces a thermolabile variant of this copper-containing metalloenzyme. At the rabbit's core body temperature (approximately 38–39°C), the mutant polypeptide chain undergoes partial denaturation: hydrogen bonds stabilizing α-helices within the catalytic domain buckle under increased kinetic energy, and hydrophobic residues that should be buried inward become solvent-exposed. The active site geometry distorts, and the copper cofactor coordination collapses, rendering the enzyme catalytically dead. Melanin biosynthesis halts at the tyrosine substrate, and fur appears unpigmented (white). Conversely, in peripheral tissues—ears, nose, paws—ambient cooling drops local tissue temperature several degrees below the thoracic core. Under these reduced thermal conditions, weak intramolecular attractions (hydrogen bonds, van der Waals contacts) persist long enough to sustain a properly folded active-site conformation. The mutant tyrosinase retains enough catalytic throughput to produce dopaquinone, which then feeds the downstream polymerization cascade yielding eumelanin and visibly black fur. Thus, the identical homozygous genotype simultaneously generates two sharply different phenotypic outputs depending on the thermal microenvironment each hair follicle experiences.

Why Other Options Are Wrong

This mechanism underscores a central regulatory principle: enzymatic activity is conditional upon three-dimensional protein structure, which is itself sensitive to ambient temperature because the energetic difference between a folded and misfolded state can be mere kilojoules per mole. The Himalayan mutation pushes the enzyme's melting temperature (Tm) below core body temperature but above peripheral skin temperature, straddling a narrow functional threshold.

PILLAR 2 — STEP-BAY-STEP LOGIC

The question stem explicitly provides three critical data points: (1) the rabbits are homozygous for a tyrosinase mutation, (2) pigment production is temperature-sensitive, and (3) phenotype varies across body regions purely because of localized temperature differences. Since every somatic cell in the rabbit carries the identical TYR allele combination, no genetic segregation, allelic dominance hierarchy, or chromosomal linkage pattern can explain the spatial pigment pattern. The sole variable is the thermal environment surrounding each melanocyte population. Mapping these observations onto Unit 5 learning objectives, the phenomenon matches the standard describing how external conditions modulate phenotypic expression of a fixed genotype. The torso melanocytes possess the same enzymatic machinery as extremity melanocytes, yet only those operating in cooler tissue produce functional melanin. Therefore, the correct choice identifies environmental influence on phenotype, distinguishing this from inheritance-based phenotypic variation.

PILLAR 3 — DISTRACTOR ANALYSIS

Option A (incomplete dominance) ensnares students who observe two visibly distinct phenotypes (white and black) within one organism and leap to the heterozygous intermediate-expression model. The fatal flaw: the rabbits are homozygous, so no allelic dosage effect can exist; the phenotypic split stems from temperature, not allele dosage.

Option C (codominance) traps learners who conflate simultaneous expression of two traits with simultaneous expression of two alleles. Codominance requires heterozygosity at a single locus with both alleles producing detectable products—neither condition applies here because the organism is homozygous and no alternative allele product exists.

Option D (epistasis) appeals to students who recognize that gene interactions affect pigment pathways. While tyrosinase certainly sits within a multi-enzyme melanin pathway, epistasis demands a second locus masking or modifying the expression of a first locus. The Himalayan pattern involves only one mutated locus and an environmental variable—not a second regulatory gene.

Option E (polygenic inheritance) captures those who notice a smooth phenotypic gradient and assume multiple additive loci. However, the question pinpoints a single identified gene (TYR), and the phenotype changes categorically (black versus white) across body zones rather than varying continuously across a population. Polygenic inheritance cannot explain within-organism regional variation driven by temperature.