The terms “homozygous” and “heterozygous” refer to how many altered copies of any gene you have.  Homozygous starts with the greek root homo- which means “same” so this indicates two of the altered copy (both copies are the same). If you were homozygous for C677T, it would mean you have two bad copies of C677T.  Heterozygous starts with the Greek root hetero- which means “different.  Heterozygous mutations have two different copies of the gene – one altered and one typical.  So heterozygous A1298C means you have one altered copy and one normal copy of A1298C.  There is also something called “compound heterozygous,” which means you have one altered copy of two different MTHFR genes. This typically refers to one variant copy of both C677T and A1298C.