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\b[Sensory genetics shows that evolution can change receptor molecules, their expression and neural organization, and that the relation between <genes> and <perception> is neither one-to-one nor confined to protein-coding mutations.] <Genetic architecture> concerns the number, effects, interactions and arrangement of contributing variants in a specified population and environment. Identifying a receptor is one achievement; explaining variation in sensory performance, its development and its evolutionary consequences requires several additional steps.
A foundational result is that a large sensory repertoire can arise by <gene duplication> followed by divergence. Cloning of human visual-pigment <genes> revealed related but distinct short-, medium- and long-wavelength-sensitive pigments, with particularly close sequence similarity between the latter two. <Molecular genetics of human color vision: the genes encoding blue, green, and red pigments by Jeremy Nathans and others>[Science 232, pp. 193–202; cloned visual-pigment genes and sequence comparisons]{ref} This links <colour vision> to a history of related <opsins>, rather than requiring a wholly new biochemical mechanism for every sensitivity. However, additional <gene> copies need not produce additional functional sensory channels: copies can be lost, expressed together, or differ too little in sensitivity to support useful discrimination.
The distinction follows from the physics of receptor stimulation. In a simplified model, input to receptor class $i$ is proportional to:
$$
R_i=\int I(\lambda)S_i(\lambda)\,d\lambda,
$$
where $I$ is the light spectrum reaching the eye and $S_i$ its effective <spectral sensitivity>. Different spectra can produce the same $R_i$, so a single receptor cannot independently determine wavelength and intensity. Comparing multiple receptor classes allows more information, but the animal also needs suitable <neural circuits>, a decision process and an appropriate task. Thus an <opsin> sequence is not a complete description of a perceived colour, and a <colour vision> test must vary brightness to prevent intensity from supplying the answer.
Regulatory differences can be as important as changes in the receptor sequence. Crosses between <cichlid> species expressing different <opsin> combinations identified relatively few regions contributing to expression differences, including both <cis-regulatory elements> and <trans-regulation>. <Genetic basis of differential opsin gene expression in cichlid fishes by Karen L. Carleton and others>[Journal of Evolutionary Biology 23, pp. 840–853; crosses and cis- and trans-acting regulatory regions]{ref} The same inherited receptor repertoire can therefore support different visual sensitivities through deployment of different members. A trans-acting regulator can coordinate several <genes>, whereas a local regulatory change can alter a more restricted expression pattern. These are mechanistic possibilities, not a rule that cis changes are always specific or trans changes always costly. Developmental stage and light exposure also matter, so comparisons need common rearing, matched ages and measurements beyond a single transcript sample.
Experimental addition of a receptor can test whether downstream processing can use the new input. Adding a human long-wavelength <opsin> to adult dichromatic <Saimiri sciureus> produced new colour-discrimination performance. <Gene therapy for red-green colour blindness in adult primates by Katherine Mancuso and others>[Nature 461, pp. 784–787; adult squirrel monkeys, added L opsin and colour discrimination]{ref} The result challenges the assumption that a new sensory channel always requires a matching circuit assembled only in early development. It does not imply that every sensory deficit can be corrected this way or that the monkeys' subjective experience became identical to a human's. Stable pre-intervention performance, tests across chromatic directions and controls against brightness cues make the inference stronger than expression of the introduced <gene> alone.
<Olfaction> reveals a related architecture at a larger receptor repertoire. Molecular cloning identified a very large family of candidate <olfactory receptor> <genes> with expression in olfactory tissue. <A novel multigene family may encode odorant receptors: a molecular basis for odor recognition by Linda B. Buck and others>[Cell 65, pp. 175–187; cloning a large receptor gene family from olfactory tissue]{ref} This supplies diversity of molecular recognition, but it does not imply one <gene> for every odour or one receptor for every behavioural response. Odour mixtures and concentrations can engage overlapping inputs, and the same sensory input can acquire different significance through context and <learning>. Establishing receptor function requires physiological responses to defined stimuli, appropriate expression controls and comparison with cells lacking the receptor.
<genes> also help organize the neural map receiving those inputs. Labelling mouse neurons expressing a particular receptor showed convergence at characteristic locations in the <olfactory bulb>. Receptor-swap experiments changed targeting, indicating a contribution of receptor identity to <axon guidance>. <Visualizing an Olfactory Sensory Map by Peter Mombaerts and others>[Cell 87, pp. 675–686; labelled receptor-expressing neurons and receptor-swap experiments]{ref} The receptor was not the sole determinant: the resulting projection was not simply an arbitrary map wholly specified by the substituted molecule. This connects molecular recognition with <neural development> and illustrates <pleiotropy>, while leaving room for other guidance mechanisms and activity-dependent refinement. A <gene> can influence the wiring through which its own sensory signal will be interpreted.
Sensory loss likewise need not mean simple destruction of a receptor <gene>. In cave populations of <Astyanax mexicanus>, increased embryonic Hedgehog signalling was associated with lens apoptosis and arrested eye development; experimental overexpression in surface fish reproduced important elements of the <phenotype>. <Hedgehog signalling controls eye degeneration in blind cavefish by Yoshiyuki Yamamoto and others>[Nature 431, pp. 844–847; midline expression, lens apoptosis and experimental overexpression]{ref} Manipulating a developmental pathway therefore provides causal evidence beyond observing an association between cave residence and blindness. It does not identify every naturally selected causal variant, prove that eye reduction was directly favoured rather than indirectly associated with other traits, or establish the same mechanism in all cave populations. Developmental competence and the adult <phenotype> can differ substantially.
These findings also connect sensory genetics with <speciation>. In Lake Victoria <cichlids>, differences in the light environment, visual pigments, mating signals and preference were associated with reproductive differentiation. <Speciation through sensory drive in cichlid fish by Ole Seehausen and others>[Nature 455, pp. 620–626; light gradients, visual pigments, colours and preferences]{ref} The connection is that selection on obtaining information can change the relative effectiveness of signals; it is not a claim that every <opsin> variant is an isolating barrier. One must separately establish expression, sensitivity, preference and reproductive consequences.
Several general methodological lessons follow. A <quantitative trait locus> is an interval associated with variation, not automatically the causal <gene>. A large detected effect does not exclude many small effects, and laboratory crosses may sample only a subset of natural variants. Fine mapping, <allele> replacement, reciprocal genetic backgrounds, a <genetic rescue experiment> and neural or behavioural measurements can connect successive causal steps. <Pleiotropy>, <genetic linkage> and environmental covariance require different explanations even when all produce correlated sensory traits. Conversely, absence of segregating variation in one laboratory stock says little about all evolutionary routes available to a lineage.
\b[Sensory systems can evolve through changes in molecular recognition, <gene> regulation, developmental organization and use of existing circuitry. Their <genetic architecture> both enables and constrains these routes, while functional experiments are needed to connect a genomic difference to an animal's sensory performance.]
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