Half relationships in genetic genealogy

Posted on 19th Feb 2026 by English Ancestors

Getting your head around ‘Half’ relationships can be tricky. In regular life we probably wouldn’t make a distinction between a half cousin and a full cousin. Yet when we’re working with DNA, and indeed if we’re looking for a missing parent or grandparent, ‘Half’ relationships are important. The Shared centiMorgan Project chart shows that we are likely to share different amounts of DNA with full and half cousins. On average, we share 866 centiMorgans (cM) with a full cousin – although because of the random nature of DNA inheritance it could be as low as 396 and as high as 1397cM. By contrast, we share, on average, 449cM with a half cousin, although it could be as little as 156 and as much as 979cM. Similarly, with a second cousin twice removed the average is 71cM, but it could be as little as zero and as much as 244cM. For a half second cousin twice removed the average is 48, but the range is from zero to 144cM.

(If you’re having trouble working out what relationship a person is to you, I shared a Cousin Calculator a few years back which should help with that… but doesn’t include Half relationships.)

Half relationships, then, are important in genetic genealogy. The amount of DNA we share with someone is a clue as to how far back we connect, that is, where we should be looking for our Most Recent Common Ancestor. In fact, if you go back to the Shared centiMorgan chart, there’s a little box at the top where you can type in the amount of DNA you share with someone and it will provide you with a range of possibilities, and the likelihood of each.

What is it that makes someone a half cousin, a half aunt, or a half sibling?
It’s all about our direct line ancestry

We all have:
2 biological parents
4 biological grandparents
8 biological great grandparents
16 biological great great grandparents
and so on.

If, instead, we think of them as ‘pairs’, we all have:
1 pair of biological parents
2 pairs of biological grandparents
4 pairs of biological great grandparents
8 pairs of biological great great grandparents
and so on.

It doesn’t matter if these people were married, having a clandestine relationship or any variation on that. The fact is that any individual (let’s say person ‘A’) is born of two specific people. If one of these people – these biological parents – has another child (we’ll call that child person ‘B’) with a different partner, then biologically A and B are Half siblings, and anyone descended from each of them will also have the Half DNA relationship in relation to the other ‘branch’ of descendants..

The point to emphasise here is that whether someone is your Half sibling, your Half aunt, your Half cousin or your Half 3rd cousin twice removed depends on what happened in your direct line at the point where you and your DNA match’s lines intersect. If your Most Recent Common Ancestors are a pair, your relationship will be a full sibling/ aunt/ cousin or 3rd cousin twice removed, etc. If your Most Recent Common Ancestor is just one person, your relationship will be ‘Half’.

Cousins and Half Cousins
All of the above may be obvious, but it’s an essential foundation for what I think may be the bit most people have trouble with: first cousins.

If my uncle has children, my cousins, and then remarries and has more children, those younger children are half siblings to his older offspring. So are they my half cousins?
No, they are not. The reason for this is that for a Half relationship to exist between you and another person, there must have been a change of partner in YOUR direct line. The remarriage of an aunt, uncle, great aunt, great uncle, and so on, has no impact on YOUR direct line.

In the above scenario, the reason my uncle’s two sets of children are half siblings is because there is a difference in the biological partnering in their own direct line, that is: at the level of their own parents. However, the reason my cousins are my cousins is because their father is my parent’s brother. Our Most Recent Common Ancestor is our grandparent couple. The mother of my uncle’s children is linked to me only through that marriage/ relationship, and not through biology. Therefore his children may be half siblings to each other, but they are all full cousins to me.

How about if my uncle died and his wife – my ‘aunt’ only by marriage to him – remarried and had more children with her second husband. We may all get on like a house on fire. My aunt by marriage may be as much a part of our family as I am; and we may welcome her new husband and consider their children, alongside the children my ‘aunt’ had with my uncle, as our cousins. But in the true biological/ DNA sense, whereas her older children are my full first cousins, the second tier of her family has no connection to me whatsoever.

Whichever way you look at it, all of my uncle’s children are my full first cousins, regardless of how many partners he has had, because we are all descended from one Most Recent Common Ancestor couple: our grandparents.

Searching for an unknown father
Let’s suppose I’m searching for an unknown biological father. If he has other children, they and their descendants would share only one Most Recent Common Ancestor with me: that would be our father. We would have different mothers and would therefore be Half siblings. Children of the half siblings would be my half nieces and nephews, and they would be Half cousins to my children. Their descendants would always retain the ‘Half’ biological relationship.

But something changes when you start to work back from the biological father. Beyond him, right back into the past, all relationships are ‘Full’, not ‘Half’.

The biological father’s sister would be my full aunt, because our Most Recent Common Ancestor is a couple – her parents/ my biological grandparents; and her children would be my full cousins.

This applies at whatever position in your family tree you have an unknown ancestor: father, grandfather, a grandmother who ‘disappeared’ and turns out to have had a second family, and so on. Everyone descending from that SINGLE Most Recent Common Ancestor is a ‘Half’ relationship; all ancestors further back beyond that person is ‘Full’ – a full aunt, cousin, great uncle, and so on.

To conclude, here’s a family tree chart from one line of my own family tree. Right at the bottom in the centre you see me, my parent and my grandparent.

My great grandmother, Jane, was married to Edward and they had several children, including my grandparent and Maggie. When Edward died Jane married Thomas and they had one child, Alice.
Alice is half sibling to my grandparent and great aunt Maggie, although within the family she was simply their sister.
Maggie’s children are my parent’s full cousins. Biologically, because the Most Recent Common Ancestor is just one person – their grandmother Jane – Alice’s children are half cousins to Maggie’s children and my parent. Within the family no such distinction was ever made, but in DNA terms there is a distinction.

My great grandfather Edward’s mother, Harriet, also married twice. She was married to Marcus, and had several children, including Edward and Joe. Before marrying Harriet, Marcus had been married to Ann and they had a daughter, Amelia, who was brought up by Harriet after Ann died very young. After eight years of marriage to Harriet, Marcus also died. Harriet then married John and had more children, including Robert.
Joe and Edward are full siblings. Amelia is their half sister, because they share the one Most Recent Common Ancestor: their father, Marcus.
Robert is also half sibling to Joe and Edward, because they share the one Most Recent Common Ancestor: their mother, Harriet.
Amelia and Robert may well have considered each other as siblings, but biologically there is no connection whatsoever between them.
The children of Edward and Joe (including Maggie and my grandparent) are full cousins. Amelia’s children are their half cousins.
Alice, and Robert’s children, have no DNA connection to Amelia’s children, nor do they have a DNA connection to each other, although they may have thought of each other as cousins.

However, further back than Marcus and Harriet, all ‘Half’ relationships cease. If Marcus is the Most Recent Common Ancestor of Amelia, Joe and Edward, then Marcus’s parents are also common ancestors. Therefore any brothers or sisters of Marcus will be full uncles or aunts to all of Marcus’s children. Similarly, if Harriet is the Most Recent Common Ancestor of Joe, Edward and Robert, then her parents will also be their common ancestors, and as such any full brother or sisters of Harriet will be full uncles and aunts to all of Harriet’s children, regardless of who the father is.

The reason for including that chart and explanation was to illustrate some of the points raised above. It turns out also to illustrate how complicated this can be! So if you’ve been scratching your head trying to understand full and half relationships, and why the ‘Half’ has come about, I hope the first half of this post will help. If, after that, you can interpret the chart and work out who is ‘Half’, who is ‘Full’, who has no biological connection, how this impacts on previous generations, and how this affects DNA, then you have nothing to worry about. 😀

DNA: Chromosome mapping and chromosome browsers

Posted on 1st Apr 2021 by English Ancestors

This is the first part in the third and final ‘mini-series’ in my beginners’ guide to genetic genealogy. You’ll find links to all the previous posts [here].

In this mini-series we’re moving on to something called ‘chromosome mapping’ and as an essential part of that we’ll be looking at my favourite tool: the chromosome browser. We’ll focus on:

a definition of chromosome mapping
what a chromosome browser is
what it tells you about your match with another person
how we can use it to identify multiple matches who are descended from the same common ancestors
amount of shared DNA and length of segments as a guide to the closeness or remoteness of a match
‘triangulation’
a case study illustrating how this can all work together

What do we mean by chromosome mapping?
When you have a number of confirmed DNA matches and you’ve identified Most Recent Common Ancestors (MRCA) for each of them, you have already started to allocate matches to different branches of your tree. A second cousin, for example, becomes a ‘benchmark’ for more distant DNA matches along the same great grandparent line. We can do this far more accurately if we can actually see the specific segments we share with that second cousin. All these segments, we know, came to us (and to this second cousin) from that known set of great grandparents. If we now find a more distant match (say, a 4th cousin) on one or more of the same segments we can trace those specific segments back even further. Not only will this help to verify our documentary research going back to the next MRCA – likely to be around 3xG grandparents – but also we now know *which* of those previously identified great grandparents this specific segment of our DNA actually came from. I find that very exciting. However, before we can do any of this we need to be able to ‘see’ those segments, and for that we need a chromosome browser.

What is a chromosome browser?
Essentially, a chromosome browser is a visual representation of the parts of your DNA that you share with one or more other people. It looks like a series of ‘stripes’ – one stripe for each of your twenty-two autosomal chromosomes. If you tested with FTDNA or 23andMe there will be an additional stripe for the X chromosome.

The following is an example of one of my matches from MyHeritage. I have removed name and contact details, but we’ll call this person A.

Screen grab of DNA match information on My Heritage

The essential information provided here about my match with A on the general listing of all my matches is:

we share 155.4cM across eight segments
this equates to 2.2% of our autosomal DNA
the longest segment is 45.5cM
our estimated connection is ‘1st cousin twice removed – 2nd cousin once removed’

If I click on the pink ‘Review DNA Match’ button, I also see

a list of all the other people on the MyHeritage site who also match A and me
if A has a tree on MyHeritage I can look at that
lists of any surnames we have in common
a comparison of our ethnicities estimates
a chromosome browser representing our match

That’s a lot of information.

At its most basic level, the chromosome browser provides a graphical view of some of that information. It doesn’t provide any surname or ethnicity data, but it does show exactly where, on your chromosomes, those shared segments are.

Below is that match with A as viewed in the chromosome browser. The 22 grey lines represent me: my 22 autosomal chromosomes. The segments where A and I match are those pink lines (segments) on chromosomes 1, 7, 11, 13, 17, 19 and 21. Although you can’t see it on this screen grab image, if I hover my cursor over any of the pink segments while I’m on the MyHeritage website I can see exact locational information (start and end points). I can also download all that information.

These segments are where A and I have inherited exactly the same DNA from the same ancestors.

Example of a chromosome browser (one to one)

Using the chromosome browser ‘One to Many’ function
A and I now have a lot of information about our match but although we know we’re fairly closely related we still can’t say which of our ancestors we both share. For that we need to look at our documented trees. However, in this case, by a process of elimination of A’s close family members who have tested but don’t match me we have been able to conclude that we are connected on the one part of A’s tree about which nothing is known: a missing grandparent. The chromosome browser may be able to help.

Using the ‘One to Many’ tool, we can use the chromosome browser to compare overlaps between our own DNA and that of several other people – up to seven matches all in one go at MyHeritage. In the example below I’m looking at just two of my matches, comparing exactly where each of them matches me, and looking for any segments where all three of us match. Again, the grey lines represent my 22 autosomal chromosomes. The red segments show my match with A (you can see they are in exactly the same positions as in the above chart, although now they are coloured red). The mustard segments are another person with whom we both match at a close level. We’ll call that person B.

Example of a chromosome browser (one to many)

The chromosome browser here shows that I share more DNA with B (mustard) than I do with A (red), and the segments tend to be longer. It also shows that I share lots of DNA with each of them that I don’t share with the other. However, there is one more very important piece of information: A, B and I all share exactly the same DNA in two positions: on chromosomes 7 and 13. Taken as a whole this tells us:

All three of us are descended from the same fairly recent common ancestors.
Between the three of us, we have inherited different parts of the DNA of these common ancestors. What we inherited has come down the line from them to us via our own parents and grandparents – and just to recap a key point from my post earlier this year (Asking other family members to test) siblings (in this instance our respective grandparents) inherit a lot of the same DNA but not exactly the same – which explains why A, B and I don’t all have exactly the same autosomal DNA from those ancestors
I am more closely related to B than I am to A
Since I have previously placed B as my second cousin I already know which set of great grandparents are our Most Recent Common Ancestors (MRCA). Those two 3-way shared segments are proof that A is also descended from these same common ancestors. As a result A now has a name for that missing grandparent
Also as a result, I now know that all those segments coloured red and mustard on my maternal line have come to me from that particular set of great grandparents

Triangulation
What we have just seen at chromsomes 7 and 13 is an example of triangulation. To fully understand what this means, we need to understand a key point about chromosome browsers. The chromosome browser represents each chromosome as a single grey line, but that one grey line belies that fact that we get two copies of each chromosome: one from our mother and one from our father. The chromosome browser cannot tell these maternal and paternal lines apart. Therefore when you look at a match on a chromosome browser the segments on those grey lines could be maternal or paternal. They could even (more unusually) be a mix of both if your match is related to you via your maternal and your paternal lines. We have to find some way of working out which.

If you’re working on MyHeritage the Triangulation tool helps with this. It tells you if two or more people match with you at a given segment on the same copy of the same chromosome. In other words – either on your maternal side or on your paternal side. It still doesn’t tell you which side that is, but if you can work it out for at least one of the matches using your documented tree, then you know that this is also where the other ‘triangulated’ person matches you.

You can see how the triangulation tool works in the example above: there is a little box around the overlapping segments at chromosomes 7 and 13. This is MyHeritage’s way of saying that these two segments of DNA shared by A, B and me are indeed on the same copy, and we are therefore descended from the same line. A, B and I triangulate.

Segment length and amount of shared DNA as a guide to remoteness of the match
Obviously, these great grandparents from whom A, B and I inherited all these DNA segments, they too inherited their DNA from their own parents, and those parents from their parents, and back through time. Since we all inherit 50% of our autosomal DNA from our mother and the other 50% from our father, when my mother was born she recombined all of this DNA from the line we share with A and B with a whole new set of DNA from her other parent, and then I did the same. This means that DNA shared with closer matches (and inherited from closer ancestors) tends to have longer segments: there have been fewer recombinations. By contrast, shared segments with more distant matches have undergone more recombinations – the DNA is mixed up with that of far more other ancestors – and so segments tend to be shorter. Hence, alongside the total amount of shared DNA, we can also use segment length as a guide to closeness or remoteness of a match.

Once we have identified the MRCA (or MRCA couple) from whom we inherited a decent amount of autosomal DNA (as I did above with A and B) we can use that information to try to place other people who match us more remotely on any of the same segments.

Consequently, attributing a segment to one ancestral couple is not the end of the story because they got it from someone else – or rather one of them did. Therefore when we use new, more distant, DNA cousin matches to take that segment (or part of it) back further, we can work out which of the closer ancestral couple it came from, and which of their ancestors they (and we!) got it from.

A more remote example – a work in progress!
The final image, below, shows another One-to-Many chromosome browser example. This time the red lines are B (who was mustard in the last example) and the new person (mustard segments) is C. You can see here that I share far fewer segments with C, and that the segments are shorter. We actually share 31.9cM, and the longest segment is 17.4cM. Using all the information outlined above, we can say that my match with C is more remote than my match with B. Our estimated relationship is given as ‘3rd to 5th cousin’, suggesting MRCAs at 2xG grandparent to 4xG grandparent. Importantly though, MyHeritage’s triangulation tool (the box on chromosome 11) tells me that this match with C is further back along that same line that I share with B. If I can find where C and I connect I will know exactly which more distant ancestors these mustard-coloured segments of my DNA come from. But of course, beyond that set of great grandparents that I share with B there are four GG grandparents, eight 3xG grandparents, sixteen 4xG grandparents, and so on. Our MRCA could be any one of them.

So this one is still a work in progress. By making contact with C, I was able to use the same triangulation process (with one of his closer cousins) to determine which of C’s great grandparent lines we match on. So we know which of *his* great grandparents and also which of *my* great grandparents, but so far analysis and further building of his tree has drawn a blank.

Example of a chromosome browser (one to many)

I have reason to suspect that these short segments have come to me from an Irish 3xG grandmother along this line, precise origins unknown, but this is not my common ancestor with C. If I can find an Irish connection along the relevant part of C’s tree, or indeed if I can find a definite Irish lineage for one or more additional people who triangulate with B, C and me along this line, then I may even be able to place this Irish 3xG grandmother’s origins in a specific part of Ireland. I have some ideas for how to progress this, and if they work I’ll write about them in a future post.

In my next post I’ll be continuing this theme by looking at DNA Painter, a third-party tool that helps with the organisation of segment information.

*****

My DNA posts are intended as a beginners’ guide, building up the information in order, in bite-sized chunks. Click [here] to see them all in the order of publication.