Your example of someone who can smell but not produce the smell is familiar to me — up until very recently I thought this was my situation. But after dilly-dallying in the restroom after a recent asparagus-containing meal, I realized that in fact I am a producer. I just never stuck around long enough to find out! Maybe there are gradients of detection ability and production capacity — you and I might be on the low end of both.

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Researching in PubMed throws up two alternative points of view:

1. There are “producers” and “non-producers” of the smell.
2. There are “smellers” and “non-smellers” of the smell.

My own limited study (sample size one) is a subject who can smell others, but not their own – suggesting to me that they are a smeller but a non-producer.

I’d have thought it most likely that there is variation in both the ability to produce and to smell. But perhaps I am wrong. If we were to look at SNPs in olfactory and metabolic genes, would we see distinct subgroups of people?