Maria,

Your first thought is correct.  You and your brother share the same
mitochondrial DNA so there's no need for you to take the same test.  

If you were testing autosomal DNA then your results and your brother's
could be slightly different, but the usefulness and accuracy of such
tests is very limited for our genealogical purposes and it is not
recommended.

With your brother's Y-DNA and mt-DNA tests you will know your paternal
Y-chromosome lineage and your maternal mitochondrial lineage.  The
next logical step if you wanted to research your genetic origins any
further would to be to test either a maternal uncle's Y-DNA or a
paternal aunt's mt-DNA, or both.  

We have to remember that genetic lines (Y-DNA & mt-DNA) branch out and
multiply exactly like a genealogical tree.  We start with two parents
and then four grandparents and then eight great-grandparents.  

When we discover one of our genetic lines there is one ancestor at
each generational step that has the same genetic line just like
patrilineal surnames, with the difference that in old times people
used to change their surnames but here there's no way we can change
one's haplogroup.  If we had a way to genetically test all our lines,
as branches in our tree multiply going back in time it is very likely
that some genetic lineages would appear repeatedly as some families
tended to intermarry with a limited number of families, particularly
when population was scarce. Whenever this has happened in ancient
times these events are called a "bottle neck" event and are reflected
by a lesser genetic diversity.

Saludos,

Victor

--- In ranchos@yahoogroups.com, "readysetgo95814" <mcortez3@g...> wrote:
>
> 
> Victor,
> 
> Thanks for the wonderful explanation on DNA. It didn't come across
as patronizing at all.
> 
> I have a question about something you stated for you or anyone else
who may know the answer:  "Since we're not clones, half of those 46
chromosomes come from each of  our progenitors. 23 from the father and
23 from the mother to make 23 pairs..."
> 
> My brother is taking both the yDNA and the mtDNA tests.  I was
thinking that since we come from the same set of parents there would
be no benefit to having my DNA analyzed, but after reading the above I
am thinking otherwise.  Perhaps it would be more valuable to have a
great uncle or aunt from both sides of the family (since all my
grandparents are dead) have their DNA analyzed.  
> 
> Maria