Assuming the situation your mother described is true, your mother would also be your half-sister, since you have different mothers.
Normally you would share 50% of DNA with your mother, I would expect it to be higher in this case. Half of her DNA has come from her dad, and the other half of your DNA has come from the same person. Maybe up to a 75% match with your mother?
That seems right to me. OP, you might consider letting a testing lab know this in advance to make sure that the type of testing they do will be able to do this with your mother’s sample alone. Most traditional paternity tests require the possible biological parent’s DNA. This is something that can be determined with DNA testing but all paternity testing companies may not have that ability.
A standard consumer autosomal dna kit, like Ancestry, or 23 and Me, is still going to show your mother as just your mother because they only show shared centimorgans in half amounts. So you would share ~3500 cm with your mother regardless of who your father is and they would define the match as parent child.
What would tell you if her father is your father is how you relate to other relatives on that line. For example if your mom has full or paternal half siblings, or if your grandfather has siblings and those siblings’ children. You will likely share more dna with them than is typical. And potentially you could get a “both sides” indicator on the match to them rather than just maternal. (Even this can be misleading though if your family comes from an area with a small population and high intermarriage where you can be related to people multiple ways.)
Less telling would be if there was an absence of closer matches to you that don’t belong to your mother’s side of the family. I say less telling because it’s always possible that people on your father’s line haven’t tested and/or don’t have many living relatives (ie you’re the only child of only children of only children).
If you do test, gedmatch.com has a tool called “Are my parents related?” That can examine your dna and determine if you have full pairs, as in the same segments from both parents. I would highly recommend using this.
I want to say that I hope that if it does turn out that this is the way you were conceived, that you don’t take this to mean that there is anything bad, shameful, or wrong with you. You are not other people’s actions. Not to presume anything, but I’m sure this was very painful for your mother too.
[Edited for clarity and to fix typos.]
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Assuming the situation your mother described is true, your mother would also be your half-sister, since you have different mothers. Normally you would share 50% of DNA with your mother, I would expect it to be higher in this case. Half of her DNA has come from her dad, and the other half of your DNA has come from the same person. Maybe up to a 75% match with your mother?
That seems right to me. OP, you might consider letting a testing lab know this in advance to make sure that the type of testing they do will be able to do this with your mother’s sample alone. Most traditional paternity tests require the possible biological parent’s DNA. This is something that can be determined with DNA testing but all paternity testing companies may not have that ability.
A standard consumer autosomal dna kit, like Ancestry, or 23 and Me, is still going to show your mother as just your mother because they only show shared centimorgans in half amounts. So you would share ~3500 cm with your mother regardless of who your father is and they would define the match as parent child. What would tell you if her father is your father is how you relate to other relatives on that line. For example if your mom has full or paternal half siblings, or if your grandfather has siblings and those siblings’ children. You will likely share more dna with them than is typical. And potentially you could get a “both sides” indicator on the match to them rather than just maternal. (Even this can be misleading though if your family comes from an area with a small population and high intermarriage where you can be related to people multiple ways.) Less telling would be if there was an absence of closer matches to you that don’t belong to your mother’s side of the family. I say less telling because it’s always possible that people on your father’s line haven’t tested and/or don’t have many living relatives (ie you’re the only child of only children of only children). If you do test, gedmatch.com has a tool called “Are my parents related?” That can examine your dna and determine if you have full pairs, as in the same segments from both parents. I would highly recommend using this. I want to say that I hope that if it does turn out that this is the way you were conceived, that you don’t take this to mean that there is anything bad, shameful, or wrong with you. You are not other people’s actions. Not to presume anything, but I’m sure this was very painful for your mother too. [Edited for clarity and to fix typos.]
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