The Combinatorial Nature of Odour Perception

The gene family, which encodes the receptor proteins is the largest in the genome and actually constitutes about 1% of the latter (Buck and Axel, 1991). Potentially, there are over a thousand different types of receptor proteins. However, not all are expressed in any one species and those that are expressed in one animal are not expressed uniformly across the epithelium. Thus, information about the nature of the odorant could also be gained from knowledge of the intensity of signals 

Most mammals use about 700-800 different types of olfactory receptor proteins in their noses. Humans, chimpanzees, gorillas, orangutans and rhesus macaques use only about half that number. Interestingly, these species are the only animals to possess colour vision and so it would seem that there has been an evolutionary trade-off between smell and colour vision. The rate of loss of olfactory receptor genes is higher in humans than in the other primates, indicating our increased dependence on vision rather than smell (Gilad et al., 2003). 

Each human uses only about 350-400 different types of olfactory receptors and there is variation between individuals as to which 350-400 of the 1000 are active genes and which are P-genes (Lancet, 2003). Thus, the subjectivity of olfaction starts at the most basic level in that statistics indicate that each of us probably detects odours with a unique combination of receptors. An example of individual variation is given by Lasker and Teubner (1999). They investigated the ability of subjects to discriminate between stereoisomers in 10 pairs of enantiomers and found that the degree of such ability was time stable for each individual but that there were big differences between individuals. 

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