Hi Sherry, we probably should have been clearer. What we mean in this case by the term 'maternal line' or 'paternal line' is those lines that are strictly through mothers only, or strictly through fathers only. They are sometimes called the motherline (mother, grandmother, great grandmother, great great grandmother, etc) and the fatherline (father, grandfather, great grandfather, great great grandfather, etc). mtDNA (mitochondrial DNA) is only passed down from mothers, so even though their sons receive it, they can't pass it on - only their sisters can. yDNA involves the Y chromosome, which only males have, so cannot be passed down through mothers.
What makes both these lines so interesting is that they aren't a mix of DNA from both parents, they are only a copy from one parent, and therefore they are unchanged for many generations (except for an occasional mutation). Because they change so slowly over time, we can dig up a body from thousands of years ago, and figure out if and how you're related to them (only by their yDNA and mtDNA though).
