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Hi, and thank you for coming here with your question.
The DNA from two brothers will likely have SOME of the same genes, but different in others.
Each parent has 2 copies of each gene. These two copies are often NOT alike.
Each parent gives 1 of the two copies to a child. Because they each have 2 unlike genes they can give either one.
so would I expect the dna to be positive and the one with the higher percentage is the father
No. It is all about statistics.
It might be more likely that they will have some of the same genes, but that is not for sure.
ie. When they test to see who a parent is they test BOTH parents (usually).
so when the dna is being done wil the lab send me the details of the result and one should be negative and one positive
No. They should both be mixed.
There are some specific markers. For example blood type.
how wil I know whose the father then
You can try a genetic test of the blood, but unless there are very different results you will still not know.
Here's an example
If a child is born to a mother with type A (also called AO) blood, and a father who is the same blood type, then if the child has any Type B blood in him it would show that one of these was not the correct parent
Because the B would have to come from somewhere.
Two brother would have a high chance of having the same blood type - and therefore it would not tell which brother the child belongs to.
Does this help?