My brother and I are thinking of taking an ancestry DNA test? Given that we have the same biological mother and farther, would the results be identical?
Your DNA is not identical, unless you are identical twins. As you did not mention that you have an identical twin, I will assume that you are not identical. In which case, you each get 50% of your DNA from each parent. But event this is not 100% accurate, and I will explain why. All human genomes are between 99% and 99.9% identical. So the 50% difference between sibling genomes is actually 50% of that 0.1-1% difference (https://www.ncbi.nlm.nih.gov/pubmed/20471132). More so, if two siblings would be of different sex, the male will have access to the patrilineal ancestry of the Y chromosome, which the female won’t have.
I recently came across an article which explains how DNA is passed down from generation to generation. You can read the full explanation, here: https://genetics.thetech.org/ask-a-geneticist/same-parents-different-ancestry
Unless you are identical twins, then it is very unlikely, almost impossible for the results to be identical. This is because each of you inherited 50% of each of your parents’ DNA. Based on probability, it’s impossible for two siblings to have inherited the exact same 50% from each parent. There is indeed a lot of overlap which will show up in your ancestry DNA results. In the same way the difference will be identified. Let’s say you have a parent with a high percentage of Eastern European ethnicity. It is likely that both you and your sibling inherited a considerable percentage of this. This percentage might not be the same size, so you might have 24% Eastern European, while your brother 32%.