This is a bit complicated but mutations in the MTHFR gene are not autosomal dominant. The full name of the MTHFR protein (coded for by the MTHFR gene) is methylene tetrahydrofolate reductase. It is an enzyme which, among other things, has a role in methylation of Vitamin B12 and folate to give their active forms.

The most common mutations in the MTHFR gene are C677T and A1298C. These are called Single Nucleotide Polymorphisms (SNPs) and their effect is to change the sequence of amino acids (one change for each mutation) in the MTHFR enzyme, leading to a decrease in its activity.

Everybody inherits two copies of the MTHFR gene, one from each parent.

Scenario 1: If both copies of the MTHFR gene have C at position 677 and A at position 1298 then the encoded MTHFR enzyme will have high activity. All other combinations can and do happen.

Scenario 2: One copy of the MTHFR gene might have T at position 677 and A at position 1298 and the other copy might have C at position 677 and A at position 1298. This will lead to slightly reduced levels of fully active MTHFR enzyme.

Scenario 3: Both inherited copies might have T at position 677 and C at position 1298. In this case the activity of the encoded MTHFR enzyme will be very low.

All of the other scenarios are somewhere between Scenario 1 and Scenario 3.