I'm using Promethease to examine genome data originally downloaded from 23andMe. Some genes I notice do not show allele information (eg. (G;G), (C;G)) and instead seem to be blank (-;-) like in the screenshot I provided. Can someone please tell me what this means? Thanks in advance.
I suggest you to take the tutorial you can found from here
Before the $(-;-)$ there is the specific location of the gene in the DNA. Between the brackets you have your specific genotypes at the location marked before. So, whatever is inside the brackets is referred to a single nucleotide polymorhism (SNP) found in your gene. If there is nothing, as the case of the screenshot that you provided, is possible that they are not able to determine the SNP (so the SNP is not found and this information is missing).
Hope this answer helps.