Heterozygous A1298C has no impact unless there's another major missense mutation in the gene. Hence that's 27% right there to subtract from the total who have a genetic issue with MTHFR.
Additionally, you can't just add the two SNPs together, and compare it to the allele rate of a single SNP in the general population. I'd hoped to avoid doing the maths, but here goes.
C677T has a MAF of 32.5% in the general population. Approximately 10% of people will be homozygous for it and 44% will be heterozygous. 54% of the general population has a C677T mutation.
A1298C has a MAF of 22.8% in the general population. Approximately 5% of people will be homozygous for it and 35% will be heterozygous for it. 40% of the population has a A1298C mutation.
Because the two SNPs seem to be inherited entirely independent of each other, we can multiple the two values together to determine approximately how many people would have both mutations in each form:
C677T A1298C
Hom 10% 5%
Het 44% 35%
None 46% 60%
0.5% of people will be homozygous for both mutations.
3.5% will be homozygous for C677T and heterozygous for A1298C.
2.2% will be homozygous for A1298C and heterozygous for C677T.
15.4% will be heterozygous for C677T and A1298C.
6.0% will be homozygous for C677T with no A1298 mutation.
2.3% will be homozygous for A1298C with no C677T mutation.
26.4% will be heterozygous for C677T with no A1298C mutation.
16.1% will be heterozygous for A1298C with no C677T mutation.
27.6% will have no C677T or A1298C mutations.
Hence nearly 75% of the general population has at least one of these mutations. And if we subtract the heterozygous and homozygous A1298C combined with no C677C mutation (because it has no impact), that's about 54% of the general population with a meaningful MTHFR mutation.
If we look at the data I posted for 18 ME patients at
http://forums.phoenixrising.me/inde...thfr-mutations-do-you-have.31879/#post-491074 only 10 out of those 18 have a MTHFR mutations which are capable of causing an impact. That's 56% - yes, a whole 2% higher than the expected prevalence.
Unfortunately the poll in that thread was flawed at the outset - there was no option for having neither mutation, and the homozygous-heterozygous options are still not present. But even if taking the poll at face value, it's still just 60% of presumed ME/CFS responders who have a significant mutation.
15.4% of people will be heterozygous for both mutations.