Suppose I wish to find out the prevalence of a certain gene variant in the population, which database or resource can I use? Now I realize that this information isn’t probably present for most genes, but in many (hopefully large) studies, they will have recorded such data, and that will do, as well. There should be databases that have such information.
I would use OMIM (at least for human genes). That’s definitely your best shot.
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM
You may also try a genome browser. There is one also at NCBI. There is also one at UCSC:
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=Genome
http://genome.ucsc.edu/
A little word on allele prevalence. It is not as easy as you think, because it is often broken down among many different populations.
So for instance cystic fibrosis. The discussion of alleles are found not under the OMIM discussion of the disease that was just linked, they are under the discussion of the protein, CFTR. Under here we have a discussion of the alleles correlated with disease. Let’s take the most common disease allele, delta-F508. They have a bunch of different population breakdowns – different Quebecois populations, Ashkenazim, Turkish people, Pueblos and Navajos, etc. etc. The relevant literature will always be cited – you may have the most luck and the most specific information through the linked research. Many of these are available free online or free through PubMedCentral or another service; most are not. Your best bet is a large medical school or bioscience library to find the non-free ones – either they will have the journals or online access. Buying the individual articles can get very expensive very quickly.