Human Olfactory Phenotype Variability

Human beings are considered microsmatic organisms (organisms with a more feeble sense of smell). Although we are less dependent on olfactory cues for survival, we utilize them extensively in enjoying perfumes, food, and beverage, avoiding poisons and stale food, as well as in subtle social interactions. It stands to reason that the relative small number of OR genes in the human genome, as opposed to other macrosmatic mammals (animals with enhanced sense of smell, e.g., mouse and dog) (8-10,20), is the reason for our somewhat poor olfactory sensitivity. Nevertheless, a recent study demonstrated that in human and other primates, typical detection thresholds toward various odorants are comparable to those of rodents (21). It appears that the massive OR gene loss in human could underlie more subtle olfactory deficits.

It has indeed been known for decades that human beings are highly variable in their olfactory sensitivities. This inter-individual variability is in the form of cases of significant threshold deficiencies toward particular odorants, termed specific anosmia or "smell blindness" (1,22-24). Such human deficiencies have been studied for dozens of odorous chemicals (24). An ambitious study in this field was carried out under the auspices of the National Geographic magazine, examining six distinct odorants in nearly 1.5 million of its readers (25). Pronounced variability was observed, with the most notable cases (approx 30% prevalence) of specific anosmia being toward Androstenone and Galaxolide (musk). This survey as well as other olfactory studies also demonstrated that olfactory sensitivity toward particular odorants may vary significantly according to age, gender, geographical location, and various environmental factors suggesting a complex trait (26-29).

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