An understanding of genealogical relationships is necessary before diving into genetically equivalent relationships in your family history. This article will provide an overview of both concepts.
Correctly evaluating shared DNA within the context of genetically equivalent relationships first requires mastery of genealogical relationships. Here we review important genealogical relationships based on some of the pertinent variables. For the sake of simplicity, we limit our analysis to biological relatives and exclude in-law and step relationships.
Immediate Family: These relationships are straightforward: father, mother, sister, brother, son and daughter.
Immediate Family of Ancestors: Your mother’s brother is your maternal uncle. Your father’s sister is your paternal aunt. Your sibling’s child is a niece or nephew. Considering the immediate family members of more distant generations gets more complicated: Your grandfather’s sister is a grand-aunt (sometimes referred to as a great-aunt). The brother of your second great grandmother or (great-great-grandmother) is your second great granduncle (or great-great-granduncle). The relationship to a sibling of an ancestor carries the same prefixes as the relationship to that ancestor. Therefore, a sibling of your third great grandfather will be your third great grand-uncle.
Cousins of the same generation: A first cousin is a child of your aunt or uncle – another grandchild of your common grandparents through a unique descent line. A second cousin is a grandchild of your grand uncle – another great grandchild of your common great grandparents through a unique descent line. If you and a cousin share the same third-great-grandparents, then you are fourth cousins to each other. The number associated with the cousin level is one more than the number of greats in the relationship level of your common ancestors. Therefore, fourth cousins share 3rd great grandparents, fifth cousins share 4th great grandparents and so on. Keep in mind that your relationship to others is dependent on your most recent shared ancestors and not their shared relationships to others. Therefore, you can have third cousins who are in turn, siblings, first cousins or second cousins to each other.
Removed relationships: When you and a relative are not of the same generation, then we describe the relationship by the number of generations removed. Your mother’s first cousin is a first cousin once removed, and you are the generation removed. Your first cousin’s son is also a first cousin once removed, but he is the generation removed. Your grandmother’s second cousin is a second cousin twice removed, and you are two generations removed. Your third cousin’s son is a third cousin once removed, and he is the generation removed. If your head is spinning, consider reviewing our article, “Third Cousins Twice Removed and Consanguinity: Figuring Out How You’re Related to Your Relatives”.
Half relationships: Most relatives share two ancestors in common – a man and a woman who are both members of an ancestral couple. When two individuals share only one common ancestor, we consider them to be half relatives. A son of your mother by a man other than your father is a maternal half-brother. A grandson of your grandfather descended from a woman other than your grandmother is a half first cousin. Note that half relationships only apply if you descend from different partners or spouses of your common ancestor at the generation of your most recent common ancestor. If two of your first cousins were born to your uncle by different spouses, they will still both be first cousins to you even if they are half siblings to each other.
Genetically Equivalent Relationships
Once potential relationships have been estimated, consider genetically equivalent relationships and the genealogical context of a match’s tree. The amount of DNA that two individuals are expected to share with each other depends on the number of generational steps between them and their common ancestor(s). Thus, second cousins and first cousins twice removed are both expected to share similar amounts of DNA since both levels of relationship include six generational steps. In most cases, two genetic cousins will be related through two common ancestors: an ancestral couple. When two individuals only share one ancestor, the amount of DNA they are expected to share in common is cut in half which is equivalent to adding another generational step. As a result, some levels of relationship are genetically equivalent. Half siblings (2 generational steps with one common ancestor) share about 25% of their DNA. An aunt and her nephew (3 generational steps with two common ancestors) also share approximately 25% of their DNA. A grandparent and a grandchild (2 generational steps with one common ancestor – the grandparent) also share approximately 25% of their DNA. A first cousin (four generational steps with two common ancestors) shares about the same amount of DNA as a half-uncle and his half-nephew (three generational steps with one common ancestor). A first cousin once removed (five generational steps with two common ancestors) shares about the same amount as a half first cousin (four generational steps with one common ancestor).
When evaluating the amount of DNA shared with an unknown genetic cousin, consider potential equivalent relationships. When evaluating the amount of DNA shared with known relatives, determine if their amount of shared DNA would be more typical of a half relationship.
Understanding how you relate to others in your family tree can be confusing at times. We’ve created an easy-to-read DNA & Relationship Quick Reference chart to help take the guesswork out of determining family relationships. Download the full-resolution version for free using the button below.
If you need help determing your relationship to a genetic match, or would like help extending your family lines, let Legacy Tree Genealogists provide the research and the extensions to your family tree. Contact us today for a free consultation.
Paul - Legacy Tree Genealogists Researcher
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How is a person my 4th cousin but not related to my sister.
At some levels of relationship, it is entirely possible to be genealogically related and yet share no DNA with each other. This begins around the level of third cousins. At the level of fourth cousins, around 29% of cousins will share no DNA with each other. The probability of no shared DNA increases the further the relationship is until at the 8th cousin level, there is only a 1% chance that relatives will share DNA. Because you and your sister inherit different portions of your parents’ DNA, and because the DNA you inherit is the basis for the genetic relationships that are determined, it is entirely possible for you to have fourth cousin matches who share DNA with you, but who do not share DNA with your sister.
If I understand what I have read which can be quite confusing. I believe it’s Because you and your cousin share more DNA than your sister shared with that cousin.
What do I call our daughter’s relationship to her father’s (my husband’s) great-grandfather’s daughter (for whom she’s named)? Is our daughter a great-great-granddaughter of her namesake? (or is our daughter the namesake?)
Hi Marianne. Your daughter would be the namesake of her great-grandaunt.
My grandfather married wife A and had a son (#1). He later married wife B and had son (#2) and son (#3). Son (#3) is my father. I know that son (#2) is my uncle. Is son (#1) my uncle or half-uncle?
Hi Nate! Since son #1 had a different mother than son #2 and #3, but all share the same father, son #1 would be a half-uncle.
I share 23.6 % DNA with a women who is supposed my Grandmothers Brothers Daughter (First cousin once removed?). Is it possible to share this much DNA with her?
My biological father says that his parents were “Cousins somewhere down the line”.
Hi Sabrina. This amount of shared DNA is more typical of a half sibling or aunt/uncle relationship. It might also be observed between double first cousins or some other combination of multiple close relationships.
I met a guy and my parents said we are related?
My great great grandfather and his great great great grandfather was siblings
What does this make him of me?
Hi Robin. You would be 4th cousins. You can also download our free DNA & Relationship Chart…this is a great quick-reference tool, and shows the approximate amount of shared DNA for each relationship.
Hi, sorry for jumping in on this question such a long time after it was asked and answered… But they would be 4th cousins without any generation removed? Aren’t they on a different generation from each other though, if it’s one person’s 2nd great grandfather and the other’s 3rd great grandfather? Sorry, this one just has me confused.
Robert, you are absolutely right. Although there are ways to compute this, the easiest way is to just draw the small tree:
……….MRCA (your GGGgpar’s, his GGGGgpar’s)
GGgpa….GGGgpa (Brothers)
Gg…………GGg (1st cousins)
g……………Gg (2nd cousins)
par…………g (3rd cousins)
YOU………par (you are 4th cousin of his ma/pa)
……………….HIM
This verifies that he is your 4th Cousin once removed (down)—–if you want to distinguish the two different relationships when there are removeds. In this case from his perspective you are his 4th cousin once removed (up).
I was adopted a few days after birth.
I know who my bio mom is.
I have determined thru Ancestry and 23andme,
that my bio dad is one of 2 or more brothers.
An older (1999) paternity test said the one she thought my bio dad, was not.
Recently I made contact with several descendants of the brothers.
I was pretty sure I had it narrowed down to one of the brothers.
If so I have a 1/2 brother, who has a son.
The son would be my what????
Additionally the son is on 23andme and is showing up as;
my 1st cousin once removed,
that they defined as;
the child of my 1st cousin,
Or a parent of your second cousin. (which isn’t possible in this case.)
So am I back to looking at a different brother?
(I believe my 15 year old mother was with at least 3 of the 5)
5.16% DNA shared, 17 segments
Under the scenario you have proposed, the son of your half brother would be your half nephew. However, the amount of DNA that this individual shares with you is too low for that proposed relationship. More likely this individual is your first cousin once removed and you are the son of one of the other brothers.
If I have a 3rd cousin what would he grandchild be to my grandchild?
Hi Tammi, the relationship would be fifth cousins. You may find this free download helpful: https://legacytree.com/dna-relationship-chart.
Basically not even kin! Thank you
What would I call my great great grandfather’s brother and my great great grandfather’s brother’s wife?
Great Great Grand Uncle & Aunt.
My great grandparents were 1st cousins once removed. I have a living cousin who is my 2nd cousin twice removed through the great grandmother and 3rd cousin once removed through her husband. What is this living cousins actual relationship to me? Is there a way to combine the two relationships to determine a single cousin relationship?
Hi Robert, our free relationships chart may be helpful to you. You can download it at https://www.legacytree.com/dna-relationship-chart.
There is no way to combine these two relationships into a single genetically equivalent relationship level. However, in analyzing the amount of DNA that she shares, it might be worth considering that we would expect roughly 2/3 of any DNA she shares to be from the second cousin twice removed relationship and 1.3 of the DNA she shares to be from the third cousin once removed relationship level.
My mother doesn’t know who her father is we were always told that my grandmother was raped by her father but we’ve taken a DNA test and it shows that my grandmother‘s dad niece is my great aunt would that actually make my mom‘s dad her great grandfather and not her grandfather
My mother’s mom he’s right when she was a kid we were always thought it was her father but now we are assuming that it might’ve been her grandfather we did the 23 and me test and it shows that my grandmother’s cousin is my great aunt
Sorry there was a problem with my phone my mom’s mother was raped when she was 16 we were always told that her her father raped her but me and my siblings had taken the 23 and me DNA test and it shows that my grandma’s cousin is my great aunt wouldn’t that make my great great grandfather and my mom’s dad
If your mother’s parents were closely related, then the double relationship could result in a higher than expected amount of shared DNA with individuals from the same family. Without being able to view the results directly and consider the amounts of shared DNA, it would be difficult to pinpoint the identity of your mother’s biological father. We might recommend ordering a short consultation with a genetic genealogist who can review the results and offer some guidance and interpretation.
is it possible that a nephew could have more of a DNA match than my half sibling
Yes, because uncle/nephew and half-sibling relationships are genetically equivalent, and because there is a range of possible shared DNA, it is entirely possible for a half sibling to share less DNA than a nephew.
Hi Jessica,
Follow up. Is that the same case for a cousin and half-sibling? sorry, I was wrong about it being a nephew.
While not genetically equivalent, there is some overlap in the observed ranges of shared DNA between half siblings and first cousins. While rarer, it is possible for a first cousin to share more DNA than a half sibling.
I was researching endogamy and how it affects DNA results, and came across this article. I have a match on Ancestry DNA that I am puzzled about and I was thinking that the results could be affected by endogamy, as I am Acadian. My match and I share 912cM across 35 segments with the longest segment being 93cM. Ancestry lists the relationship as 99% 1st cousin or half niece. However, I know this match’s mother as my 1st cousin, so I am puzzled why she only has a 1% chance of being my 1C1R. Should my 1st cousin be tested to see if she is my half sister? Or can these results be due to endogamy? Can you provide some clarity for me? Thanks!!
Hi Liz,
I believe one of our researchers who specialize in DNA could certainly help you figure this out. They would pull our current data together and combine that with other DNA resources to fill in some of the blanks for you. Please contact us on our Get in Touch page. We can point you in the right direction and give you a free estimate if research is needed.
Is there a way to determine if my relative is my “1st cousin” or “half niece” based on the CMs alone since the CMs are about the same for either relationship. Famiy history says cousin, suspicion says half niece.
One of our researchers who specialize in DNA can not only answer that question but can also provide the next steps to help you learn more about your family. Please contact us by filling out the form on our home page https://www.legacytree.com. We can point you in the right direction and give you a free estimate if research is needed.
I have a match ancestry that is 1590 cm across 33 segments longest segment is 190. It says close family-1st cousin? Just wonder if this is correct
Donna, that sounds correct, but one of our researchers who specialize in DNA can not only answer that question but can also provide the next steps to help you learn more about your family. Please contact us by filling out the form on our home page https://www.legacytree.com.
Determining if two relationships are genetically equivalent is straightforward without using a table (although a table is useful for viewing groups of relationships that are genetically equivalent). This method also includes all of the half relationships not in the table and is not limited (i.e. any of the parameters IG or “Cousin N” or “Removed M” may be arbitrarily large):
Background:
IG (i.e., Implied “G”s in PREFIX) == count the number of “G”s in the PREFIX in its unabbreviated form:
e.g. implied “G”s in PREFIX that is missing: 0
implied “G”s in PREFIX of Great-Great grandpa = 3
implied “G”s in PREFIX of 4 times Great grand uncle = 5
[Half] is optional and if left out means also to leave out [0.5 +] in the definition
Definition of Genetically Equivalent Cousin (GEC):
Self: GEC = -0.5
PREFIX(parent/child): GEC = IG/2
[Half] sibling: GEC = [0.5 +] 0
[Half] PREFIX(aunt/uncle/niece/nephew): GEC = [0.5 +] IG/2 + 0.5
[Half] Nth cousin M times removed: GEC = [0.5 +] N + M/2
Two relationships are “Genetically Equivalent” if they have the same GEC value.
The average amount of shared DNA is simply 1/(2^(2 x GEC + 1))
e.g. 2nd cousin has GEC=2.0 so the average amount of shared DNA is 1/(2^(2 x 2.0) + 1) = 1/32 = 3.125%
Half 2nd cousin has GEC=2.5 so the average amount of shared DNA is 1/(2^(2 x 2.5) + 1) = 1/64 = 1.563%
e.g. For all of the following the computed GEC = 3.0: 3rd Cousin, 2nd Cousin twice removed, 1st Cousin 4 times removed, Half thrice great grand aunt, great times 5 grandmother
I leave it to the readers to figure out why GEC for Self is -0.5
What would be my relation to someone who is the grandaughter of the brother of my grandfather?
Hi Lori,
If you would like research assistance, we are here to help. Our team conducts detailed, personalized research in millions of family history records, spanning hundreds of years to tell you who your ancestors were, where they lived and much more. To get started, please complete this form to request a free quote, and a member of our Client Solutions Team will reach out to you.
My Grandfather’s Sister is the Great Grandmother of a friend, how are we related?
Hi David,
We would love to help you but would need some additional information. Please contact us by filling out the form on our Get in Touch page. We can point you in the right direction and give you a free estimate if research is needed.
Hello, I got a question I need to ask.
I’ve been taking quite lot of DNA tests and it is very complicated story but this cousin partly she is claiming to be my 1st half cousin but the DNA companies like Ancestry and My Heritage is saying something completely opposite and I don’t go off what people say, I go off what DNA is telling me.
Anyway according to the DNA results both of them partly saying she share same DNA as me at 637.8 cm and it is at 9% Shared DNA – partly she is from my great aunt – 1st cousin once removed that what my Heritage is saying and another was Ancestry partly they are saying she is my second cousin like great grandparent generation, her DNA combine to my DNA is: 579 cm and it is at 8% Shared DNA.
Can you please help me? Because I’d thought I was going over my head and I was trying figure it out. I’d thought my calculation was right and thought she was indeed from my great grandparent generation, not my 1st half cousin.
Hi Lauren,
I believe one of our researchers who specialize in DNA could certainly help you figure this out. They would pull our current data together and combine that with other DNA resources to fill in some of the blanks for you. Please contact us for an estimate on research.
Hello, I am wondering if it is possible to have a 1st cousin once removed who shares 8% dna and nearly 600cM? This seems to me to be more than usual but not enough to be full first cousin. I can’t find anything that gives a range of measure dna for this relationship. I’m also questioning what the other possibilities of how we are related could be.
Diana, as much as I would like to help you, DNA questions are better answered by our DNA researchers. Please fill out the form below for a free estimate on a consultation package or research. https://www.legacytree.com/contact-us?utm_source=blog-reply
If my aunt is Portuguese does that mean I am to? Cuz that’s my dad’s sister
Hi Genevieve,
You can download our free DNA & Relationship Chart, a great quick-reference tool. We would also love to help you but would need some additional information. Please contact us by filling out the form on our Get in Touch page. We can point you in the right direction and give you a free estimate if research is needed.
I have identified three brothers.
I share 6% DNA (second cousin maybe) with a Great Grandson of one of the brothers.
I know which one.
How likely is the known brother my Grand Father and not one of the other two brothers?
Appreciate any comment.
Hi David, that’s a good question, although family connections can be complex. Please contact us for a free consultation by filling out the form on our Get in Touch page https://www.legacytree.com/contact-us. Our Client Solutions Specialists can help point you in the right direction or set up a research project if needed.
My 3x great grandma on my mother’s side is my friends great aunt, how are we related?
Hi Jan, we would love to help you but we would need some additional information. Sometimes family histories can seem confusing, especially without detailed research. Please contact us by filling out the form on our Get in Touch page. Our Client Solutions Specialists can point you in the right direction and give you a free estimate if research is needed.
Hi, I have a dna match on 23andme we share 264 cm. His sibling is also on 23and me but we share only 85 cm. Comparing them on the chromosome browser on 23andme, they are full siblings. Is the difference in cm between me and the sibling that I share 85 cm based only on recombination ?
Hi Ed, sometimes family histories can seem confusing, especially without detailed research. Our team conducts personalized research in millions of family history records, spanning hundreds of years to tell you who your ancestors were, where they lived, and much more. Please consider scheduling a consultation with one of our Client Solutions Specialists on our Get in Touch page. They can point you in the right direction or set up a research project if needed.
I match with someone 1,145 cm and share about 16% DNA with this person. Is this person my great aunt or as you would say grand aunt? I recently found out my father is not my father and when doing a DNA test found this information she is my highest match on the website.
Hi Haley,
We would love to help you but we would need some additional detailed information. Please contact us by filling out the form on our Get in Touch page. Our Client Solutions Specialists can point you in the right direction and give you a free estimate if research is needed.
Would a 1st cousin of my 4×great-grandfather have any relationship to me?
Hello Delores, it would be a distant relationship through your common ancestors, the great-grandparents. If the cousin had a descendant in the same generation as you, you would be seventh cousins. According to the shared cM chart (beta), seventh cousins share an average of 14 cM of DNA (very little), with a documented range of 0 to 57 cM.
My friends aunt could be her sister is there a test that can be took to know if it’s true as the dad is passed
Hi Emily,
A DNA test can help your friend fill in information about the family that may not otherwise be available. For example, children share 50% DNA, but would only share 25% with an aunt). We have researchers who specialize in DNA. Please contact our Client Solutions Specialists through the form on our Get in Touch page. They can point you in the right direction and provide a free estimate if research is needed.
I live in the smallest rural town ever. Where literally everyone is kin to everyone. I just need to know if we are close kin haha. Seriously though. Like if we were to have a child would it be affected. Here’s what I know about how we are kin. My maternal grandmother’s maternal grandfather is Brothers to my boyfriends maternal grandmothers maternal mother’s father.
That can be a complicated situation to understand your relationship to others in your town when it is so small and so many are related to you. To best determine this, it would be best to schedule a consultation to get a clearer picture on your relationship to your boyfriend. You can schedule those at https://www.legacytree.com/. Thank you!