Can half sibling category actually be full sibling?

Question

I was adopted and used AncestryDNA to help in my search.

Recently, I received an alert that a had a match to a "Close Family". People in that category include Aunts, 1/2 siblings.

Is there possibility that she is immediate family and in fact a full sibling?

Our Ethnicity is different but we have already established that that is quite possible given the mix from our parents.

Both potential parents are deceased so we are not sure how to determine?

My adoption records indicate that her father is my father but then she should have fallen in the Immediate Family category.

Answer 1

Let me try to give a simpler, more direct answer to your question. If you tested at Ancestry, you should be able to find the number of centimorgans you share with anyone in your DNA Match list. It will say something like "2,026 centimorgans shared across 69 DNA segments" (my sharing with one of my nieces).

The go to https://dnapainter.com/tools/sharedcmv3 and "plug in" the number of cM shared. You'll then see a number of relationships that are possible, given this number.

But remember, it's just the relationships that are possible. It can't tell you whether you are or are not full siblings -- just whether or not that relationship is one of the possibilities. Also, don't assume the ranges given are absolute -- they're based on submissions to date. But, you can tell whether you're even close or not.

Answer 2

Joao Ventura is not quite right when he says that full siblings usually share 37.5%.

As the International Society of Genetic Genealogy Wiki explains:

There are two simple mathematical methods of calculating the percentages of autosomal DNA shared by two individuals. Both methods give the same results except in the cases of parent/child comparisons, full siblings, double cousins, or any two individuals who are each related to the other through both parents.

Sites that use Method 1 (such as 23andMe) will show full siblings sharing on average 50% of their DNA. Other sites that use Method 2 such as FamilyTreeDNA and myHeritage will show full siblings sharing on average 37.5% of their DNA. However most sites (with the exception of 23andme) will also display a number of shared centimorgans.

If you have the number of shared centimorgans or a % derived by method 2, you can then use that information with this tool -- DNAPainter -- to see what the possible relationships may be.

There are also diagrams on the International Society of Genetic Genealogy Wiki that can help you turn shared centimorgans into a number of possible relationships.

Answer 3

To answer your question, we're talking about statistics here.. In theory you could be a 0% match with a full sibling (the halfs you get from each parent being mutually exclusive). Or even a 100% monozygotic twin born years apart (exact same mix of DNA from each parent). But the probabilities of these extremes are extremely low (low as in the probability of a 1 ton meteor of gold falling on your garden is higher). So yes, there is that possibility. Testing another sibling of hers might help to determine if you all share the same parents.