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Why there are so many genes of olfactory receptors and meanwhile only a few of photo receptors?

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Except in the broadest sense (i.e. evolution and mechanistic reasons given by Endy) there is no simple true and detailed answer for this. To complicate matters, for many of the identified genes belonging to same family, many may not be functional. That is, they may not be expressed at all, carry inserts and so on.

If someone asks a question like this, one does not expect a correct answer as in multiple choice questions,but it is rather used to gauge how much you understand basic topics (the answers could be different if asked from a evolutionary, developmental, physiological or e.g. genetic point of view).

Instead, one would expect to give some kind of speculation that is well-founded in the material provided in class (bonus points if further sources are used).

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To add to what was said above, one must also take into account what species is being talked about. If no species is mentioned you should probably think about the relative importance of visual systems vs. chemical receptor systems in early evolutionary development. Specifically even some of the most primitive organisms have chemoreceptors that can be used to accurately detect specific compounds, while vision is an 'overview' of what something contains without specificity. Also, chemical receptors are used both internally and externally (though very specialized in olfactory systems) it was probably a very early and relatively easy adaptation to go from internal chemical receptors to external chemical receptors.

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