According to wikipedia humans have around 400 functional olfactory receptor genes and 600 pseudo genes. Most mammals have higher numbers of functional OR genes.

My question is whether it has been studied how the number or subset of these functional ORs varies between humans.

  • $\begingroup$ Can you elaborate on what you mean by "vary between humans"? Which characteristics amongst humans are you trying to associate with differences in the number of OR genes? $\endgroup$ – user22020 Aug 7 '17 at 13:10
  • $\begingroup$ My question is whether two randomly chosen humans have different numbers and different subsets of functional OR genes or whether they are very likely to have exactly the same subset of functional OR genes. I'm not trying to associate any characteristic to this subset, though of course one could assume that it might result in a specific range or acuity of olfactory sensing, but that's not very important to my question. $\endgroup$ – BlindKungFuMaster Aug 7 '17 at 13:18

Perhaps the most dramatic example of variation in olfactory abilities in humans is anosmia, or the inability to smell. However, this isn't always due to broken olfactory receptors -- some total anosmias are due to cilia (small hairs) that don't function properly. (This paper mentions that.)

Specific anosmias, or insensitivities to particular smells, are probably closer to what you're asking about. The textbook chapter I linked to below talks about how proportions of the population lack the ability to smell certain odorants -- for example, about 1/10 people cannot detect ethyl mercaptan, which is the scent given to natural gas leaks. These specific anosmias are generally restricted to just one odorant (smell), which suggests that it is likely a receptor deficit. (EDIT: The comments below have even more examples of scents that vary from person to person.)

Interestingly, many conditions are associated with a decline in smelling ability and range, like Alzheimers, diabetes, and taking certain medications. These declines can be because of fewer olfactory receptors or because of deficits further along in sensory processing.

I couldn't find any papers assessing smell variation across all humans -- doctors rarely think to assess smell, so there isn't much data on "normal" populations. Also, it doesn't seem like the genes controlling olfactory receptors in humans have been characterized well, so it would be extremely difficult to do a comparison on the genotypic level.

In sum, there are definitely differences in the ability to smell a wide range of scents. To my knowledge, this hasn't been characterized in an especially systematic way. (If it has, I hope somebody on here knows otherwise!)

This chapter of Purves' Neuroscience gives a basic run-down of olfaction and some possible variations in it. https://www.ncbi.nlm.nih.gov/books/NBK11032/

  • $\begingroup$ Hydrogen cyanide would NOT be used as a natural gas odorant, since it is highly toxic, quite aside from the fact that many people can't smell it. Mercaptans are generally used instead. Another instance of something smelt differently by people is the flower alyssum: to me it has only a faint grassy odor, while my ex thought it had a strong and very pleasant fragrance. $\endgroup$ – jamesqf Aug 7 '17 at 19:48
  • $\begingroup$ Coriander (fresh leaves, not the dried seeds), which is a common spice in asia, is percieved as 'soapy' by some people. There is an article, that also liks this to genetic variants. $\endgroup$ – Nicolai Aug 7 '17 at 19:52
  • $\begingroup$ @jamesqf : oops, I misread the a paper. Ethyl mercaptan is added to natural gas and some can't smell it. Some people can't smell hydrogen cyanide and that's dangerous because it could kill them. Editing now. $\endgroup$ – gabiwab Aug 8 '17 at 17:29

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