What is a 2nd cousin--once removed?

 

Degree of cousin-hood:

I'm often asked questions like "what is a 2nd cousin--once removed" and "what is a half=cousin?" The answers all have to do with some simple facts about how you are related to the ancestor(s) you share with your cousin. 

1. The first question you must answer is, how many "g" s are there in the relationship that defines the shared ancestor(s) you have with this cousin?
2. Are the two of you the same number of generations removed from this shared ancestor?
   
3. Do you share a grandparent couple with this individual or do you share only one grandparent in some previous generation?
   

With the answers to these questions in mind, you are ready to define your relationship with your cousin. 

Question #1 is how many "g" s are there in the relationship that defines the shared ancestor(s) you have with this cousin? The most common shared ancestors would be a pair of grandparents. Note that there is one "g" in this the definition of relationship with the shared ancestor(s). Whether it is grandfather or grandmother or a pair of grandparents, there is a single "g" in the term that defines their relationship. Cousins who share grandparents with you are your 1st cousins.

By now you should have noticed all the "g's" sprinkled throughout my prose above. That is deliberate both to draw you attention to them but also to emphasize that they are present in "grandparents" and well as great-grandparents which I shall get to shortly.

If your shared ancestors with the cousin in question are a pair of great-grandparents, this cousin is your 2nd cousin. After all, your common ancestor(s) share 2 g's in their designation. Likewise, if your shared ancestors with the cousin in question is a set of great-great-grandparents, how may "g's" is that? The more astute of you have already concluded that this makes the cousin in question is your 3rd cousin. This pattern extends as far back in your family tree as your research may take you. Just count the number of "g's" and make sure to include the one in grandparents! 

 

Cousin's removed

So far, I've been describing cousins who share the same generation in the family that you do. But as you probably have noticed, many of your closest family members are either a generation older or a generation younger than you. The children of your parents' 1st cousins are your 1st cousins one generation removed. In like manner, the children of your own 1st cousins are also described as 1st cousins one generation removed. These relationships are generally expressed as your "1st cousin--once removed" whether the generation is one generation above you or below you in your family tree.

The same logic continues with additional generations. The grandchildren of your parents' 1st cousins can be characterized as your 1st cousin two generations removed. The mirror image of this relationship would be the grandchildren of your own 1st cousins. They would also be characterized as 1st cousins--twice removed. Again, it does not matter whether or no they are two generations above your or below you in your family tree.

In like manner, the children and grandchildren of your parents' 2nd and 3rd cousins can have the tag of "once removed" or "twice removed" added to their descriptors to help define their relationship to you. Again the mirror image also applies to the children and grandchildren of your own 2nd and 3rd cousins, etc. 

While these distinctions may not be necessary for some genealogical studies, often they become important in deciphering relationships based on the amount of shared atDNA (autosomal DNA) shared with a previously unknown individual.

 

Half cousins

Half-cousins are an extension of the concepts of half-siblings. It occurs when only one half of an ancestral couple is shared by you and your cousin. It can occur at any level of cousin-hood for any number of reasons. For example, if you had a great-grandfather who had 5 children by his first wife and four more by his second, you would be full 2nd cousins with the children with whom you shared a common biological great-grandmother. The children of the other wife would be your half-2nd cousins and would be expected to share about one half as much atDNA as a full 2nd cousin.             

Counting your "g s"

Remember to count the "g" in grandparents when counting the degree of cousin-hood. 

  

In the chart above, note that You are located in the box to the right in the row next to the bottom. All those represented in the same row to your left, are your same generation cousins. The one on your immediate left is your 1st cousin; the next one is a 2nd cousin; then 3rd cousin; 4th cousin and 5th cousin on the extreme left. In the bottom row (right to left) are possible 1st cousins -- once removed; 2nd cousins -- once removed etc.

So the answer to the question at the beginning of this blog, "what is a 2nd cousin--once removed", generally would be a person positioned 2 boxes to the left and one down from You in the chart above. If that was your answer, you may be giving a hint to your age. That indicates many of your cousins are younger than you. If you are taking this course for AP credit, you probably figured out that the person 2 boxes to the left of You and one row up would also qualify for this designation. If you caught both, congratulations. You well on your way to becoming a genetic genealogist. Humm. That has two "g s". Maybe 3.

Solving A Mystery With Women's DNA

Many still believe that only male DNA is useful for genealogical purposes. They need to wake up and smell DNA developments of the last 3 years. I recently helped a woman unravel an old mystery in her family using her autosomal and mitochondrial DNA test results. I will call her "Allyson" to protect the guilty dead and the living innocents. Here are some of the facts:

• The adopted daughter of Allyson’s great aunt was thought to be a family member;
• This adoptee was born in the 1920s;
• The adoptee’s daughter, who I will call "Bertha", is alive and has taken a mitochondrial DNA test and a Family Finder autosomal DNA test at FTDNA.

Speculation within the family included:

• This adoptee was the result of an illicit coupling of Allyson’s grandmother and her ex-husband long after their divorce;
• This adoptee was the result of a teenage fling of Allyson’s mother.
•  Allyson also has taken both a mitochondrial DNA test and a Family Finder autosomal DNA test at FTDNA.

The Mitochondrial DNA results:

Mitochondrial DNA is more definitive in ruling out potential relationships than in proving them. Their results could have ruled out Allyson’s mother and grandmother as potential mothers for Bertha’s mother. However, Allyson and Bertha are exact matches over all 16,569 locations on their mitochondrial DNA. They are the only exact matches for each other currently in the database. This means that the two share a direct maternal line (umbilical cord) ancestress probably within genealogical time. FTDNA says that such a match has a 50% probability of a common direct umbilical cord line ancestress within 5 generations and a 95% probability of a common direct umbilical cord ancestress within 22 generations. In this case the results do not prove how close the common ancestress actually lived, but they do add some credibility to the belief that this was an adoption within the family. Based on this result some hypotheses were formulated.

Research Hypotheses: 

#1. Bertha’s mom was the biological daughter of Allyson’s maternal grandmother and grandfather; 

#2. Bertha’s mom was the biological daughter of Allyson’s maternal grandmother and some unknown male – at least unknown to us; 

#3. Bertha’s mom was the biological daughter of Allyson’s mom and some unknown male.

No hypothesis was defined to include Allyson’s mom AND dad being the biological parents as existing letters clearly document that they did not meet until a few years after the birth of Bertha’s mother. On the other hand it can be documented that Allyson’s maternal grandmother and grandfather remained in contact long after their divorce.

Dick Eastman has famously blogged that there is no such thing as a ½ cousin. He quoted the venerable Black’s Law Dictionary to support his assertion. Well Dick, we would never be able to solve some of our genealogical mysteries unless we are able to split some cousins and other relatives in this manner.

If we want support for hypothesis # 1 above, we would need to find evidence that Allyson and Bertha could be 1^st cousins. To support hypothesis # 2 above, we would need to find evidence that Allyson and Bertha could be ½ first cousins. If hypothesis # 3 above is correct Allyson would have a ½ aunt relationship with Bertha. In these three cases Allyson and Bertha would be expected to share either about 12.5% (1^st cousins), about 6.25% (½ first cousins) or 12.5% (½ aunt). These percentages can be derived from the chart on the ISOGG Wiki: 

Cousin tree (with genetic kinship)

Relation with Adoptee’s Daughter “Bertha”	Maternal Grandmother is mother of Adoptee		Allyson’s Mother is mother
	Maternal Grandfather is Father Hypothesis #1	Maternal Grandfather is not Father Hypothesis #2	Allyson’s father is not Father of Adoptee Hypothesis #3
“Allyson”	1st cousin—12.5%	Half 1st cousin—6.25%	Half Aunt—12.5%
Male Cousin	1st cousin—12.5%	Half 1st cousin—6.25%	Half 1st cousin-once removed—3.125%
Female Cousin #1	1st cousin—12.5%	Half 1st cousin—6.25%	Half 1st cousin-once removed—3.125%
Female Cousin #2	1st cousin—12.5%	Half 1st cousin—6.25%	Half 1st cousin-once removed—3.125%

 Expected % of Shared Autosomal DNA 

for each Hypothesis

Oops! Using the autosomal data at hand we would not be able to distinguish between hypotheses 1 and 3. The expected amount of shared DNA would be the same in both cases. Fortunately there are three living 1^st cousins of Allyson who might be willing to test. Two are female and one is male. In this case their sex would not matter because we have already established the maternal haplogroup of the adoptee. All we would need is their autosomal matches with Allyson and Bertha. These cousins would not add to our ability to distinguish between options #1 and #2 because their amounts of DNA shared with Bertha should mirror those of Allyson in both cases. Both Allyson and each of the 3 cousins should be shown to be 1^st cousins with Bertha in #1 and ½ first cousins in # 2. So no increase in diagnostic ability is gained by collecting DNA from the 3 additional cousins.

But wait! We do not need them to help distinguish between #1 and #2. We might need their test results to differentiate between #1 and #3. Great! While Allyson would be a ½ aunt (sharing about 12.5%) in #3, the other 3 cousins would only be ½ 1^st cousins (sharing about 3.125%) a significant difference. So it looks like it may be possible to collect enough information to solve this mystery.

The Solution:

Actually, it turned out to be quite a bit simpler than all that. As we saw much earlier in this saga, FTDNA predicted that Allyson and Bertha were 1^st to 2^nd cousins. For our purposes that is quite an extensive range. If they share matching segments that would be expected of 1^st cousins, they would also share the amount Bertha would expect to share with a ½ aunt. But that is not the amount they share. They only share about ½ that amount. In this case it would have been easier to determine this if Allyson and Bertha had tested at 23andMe rather than FTDNA since the former reports matches in terms of % of match. FTDNA reports it in terms of centiMorgans (cMs) and SNPs matched. Although there may be handy conversion tables to translate these into % of match, Dr D had difficulty finding one. But he did find this one posted by Lindsay in her blog Confessions of a Cryokid:

According to Matt Dexter, on the FTDNA Forum, here is a breakdown of sum shared DNA ranges for various relationships:

• Parent/child: 3539-3748 centimorgans (cMs)
• 1st cousins: 548-1034 cMs
• 1st cousins once removed: 248-638 cMs
• 2nd cousins: 101-378 cMs

Although this table turned out to be very useful, it was compiled over 2 years ago. In autosomal DNA testing time that was almost light years ago. We now have tens of of thousands of more results that should have expanded our databases. Dr D would expect a currently compiled breakdown to show more overlap between the categories. 

Once the table in Lindsay's blog post was found, it showed that the 428.42 cMs shared between Allyson and Bertha were well within the expected range that would be expected for a 1^st cousin-once removed relationship.

But 1^st cousin-once removed is not one of the expected outcomes in the hypothesis table above. Or is it? Isn't it the equivalent of a ½ first cousin? I think we have a winner! The hypothesis that best meets the autosomal results is #2. The most likely biological parents of Bertha's mom are Allyson's maternal grandmother and an unknown male. Allyson's maternal grandfather seems to be ruled out as the biological father of Bertha's mother. Although testing additional cousins might lead to greater certainty, that does not appear to be necessary to identify the most likely candidate to be Bertha's grandmother.