Hi,
I am not positive of this, but my understanding is that the allele frequency of the mutant hemoglobin (HbS) is given in the table near the end of the page labeled "Population Diversity". I am assuming that the values in the table are for the frequency of the single allele which is much higher than the condition in which both chromosomes are mutated. If you do a search for allele frequency of sickle cell disease it says that in Africa the carrier frequency is 10%-40% (
https://en.wikipedia.org/wiki/Sickle-cell_disease). I would assume that by 'carrier' they mean one or both chromosomes affected but I don't know for sure.
I found a paper that reviewed the allele frequency for HbS and they show in figure 1 that Africa has a frequency that varies from 0% to 18% (
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3060623/). Since the term 'allele' refers to the DNA sequence on only ONE of the chromosome pairs, this would be the same as carrier frequency. A person could have one good Hb allele and one HbS allele and not have SCD.
Other experts PLEASE give your opinions on this. I'm not an expert on bioinformatics!
If you look at the NCBI Population Diversity table you will see that the frequency listings are broken down according to geographical location--East Asia, Europe, Africa, etc. For example, in Africa the normal allele is found about 90% of the time while the HbS allele occurs at a frequency of about 10%. In contrast, the frequency of HbS in America is only about 0.7%.
I hope this helps you to understand the NCBI SNP information site. If you have more questions, please post again.
Sybee
