No DNA test will tell you who your specific ancestors were. Now how genealogy DNA tests can be helpful is if you are into traditional genealogy work using documents/records and if the company you choose has cousins of your going back several generations in their database and they notify you of those cousin and you are able to make contact with them and they are also into traditional genealogy work, then you can collaborate information with them. They might have discovered ancestors you haven't and you might have discovered ancestors they haven't.

For he overall ancestral testing they use Autosomal along with the X. Autosomal is what most of your DNA is. 44 of your 46 chromosomes are Autosomes. You got it 50-50 from both parents but not normally 25% from each of your 4 grandparents. The reason why there usually will be some bias in what you inherited from grandmother and grandfather on both sides of the family and how you inherited any bias will not be how your siblings inherited it unless you have an identical twin. So if you and a full sibling were tested by the same company at the same time, the results will not be exact due to the differences in how you inherited Autosomal from your grandparents.

What they would tell you is that you are say, 25% British, 50% Scandinavian, maybe 10% Iberian Peninsula, 5% Slavic, 5% other and 5% MidEastern. That's just an example.

If you go to more than one company and have this type of testing the results will not be the same as the only thing companies can do is match you with population samples in their database and they each have their own database. So if one doesn't have certain population groups another has and vice versa, naturally the results can vary. Also it has been reported if you go back to the same company sometime later and have the test done again by that time they have received more population samples that can charge the picture for you.

The 3 most recommended are

FamilyTreeDNA. I understand they also do the test for National Geographic.

23andme and they will also advise you of any anomalies for health issues in your DNA

Ancestry.Com

She is your half grand aunt or great aunt. Most people drop the "half" if they get along with the person, unless they are in the genealogy clubhouse.

I prefer "grand aunt" because it is clearer, especially when you go back further.
Your great great great aunt is the sister of your great great grandparent. If you number the greats, you get 3rd great aunt is sister to your 2nd great grandparent. It is clearer if the numbers are the same; 2nd great grand aunt is sister to 2nd great grandparent.

Genealogy is about researching records your ancestors generated during their lifetime to find out where they came from, it is nothing to do with surname hunting, as surnames are words and all words come from language origins not countries and will never tell you which country the person spoke, heard or was influenced by the language to take a surname, which for many Europeans was 700-800 yrs ago, for black ancestors in America who were potentially slaves it could be less than 200 years ago

Deas is from the Olde English pre 7th Century lanaguge and means "dic", dike, ditch. Medieval ditches and dikes were formidable earthworks, often used for defence as much as drainage, and forming prominent landmarks. So a topographical surname denoting residence by a ditch or dike. If a slave was purchased by someone named Deas they may have just been given the surname, or they might have chosen it after slavery, it has zero to do with anyone who chooses a surname would have been born and it has zero to do with a slave breeding with someone named Deas and producing children.
So
"My last name is 'Deas' and presumably is of Scottish ancestry" NO, your surname is an Old English language word and English was spoken in three quarters of the Worlds countries, and tells you nothing at all about your ancestry as surnames were chosen/taken just like first name.

So you can start doing real genealogy and not making wild uneducated guesses based on what you think someone looks like or what word was chosen from what language and assuming you only got a surname because you were the product of a person from a certain country which is all completely wrong...

Start with YOU and your own records, they are at home, so ask your mum to get out so you can see your birth certificate as that is where you start, it will PROVE you and your parents and that is what genealogy is about finding real records that people generated during their lifetime and
proving ancestry, your mum will have her birth and marriage cert, those three real records prove 3 generations, giving you proof of the names they used and your grandmother will also have records, which will take you back at least another generation, all proved, not guessed, not assumption. http://familytimeline.webs.com/recordsin... then once you now how to research where the people are alive to ask questions of, you are then ready to start looking at other records and you will know who you are looking for