Could someone explain please ?

[deeahgwitch]

Puzzled
Ancestry result puts my mother's first cousin's son (714cM) a closer match to me than my half sister's son (620 cM).
I would have thought my "half nephew" would be a closer match.
On Ancestry it says that my mother's first cousin's son is my first cousin.
Could someone more knowledgeable than me explain please.

Any given number of cMs can have multiple possible relationships. Any given relationship (except parent-child) can have a wide range of cMs.

Gene inheritance is random. You get half your genes from each parent, but apart from sex chromosomes there is nothing specifying which half of theirs they pass on.

At a population level full siblings will share 25% of their DNA - half from each parent. But In ttheory at an individual level you could inherit exactly the same genes from each parent as your sibling, or none of the same genes.

Imagine your father is a bag of blue tokens, numbered 1 to 10. And your mother is a bag of red tokens, numbered 11 to 20. To make 'you', pull out 5 tokens from each bag and note the colours and numbers. Then put them back and do the same for a 'sibling'. You might get the same mix; you are most likely to get a different mix with some shared numbers; you might get no shared numbers. The probability of any individual combination of tokens is the same - so the chance of exactly 25% match is the same as of a full match or a zero match.

For a half sibling you'd use a different bag for 1 parent, so the maximum match you could have is 50% - if you get all the same tokens from the 'shared parent' bag. The minimum is still 0. The likelihood is somewhere inbetween.

Now do the same with millions of tokens, and repeat it for each step in the relationship. The cM number gives you a starting point for likelihood of a relationship, but it's not 'an answer'.

As the pp @NoBinturongsHereMate said (ps can you provide any explanation as to your handle? I did once see a Binturong in Paignton Zoo) inheritance can be a very tricky thing.

Actually, I think her example of blue and red tokens is quite a good metaphor.

The difference between 714 cM and 620 cM is really very small indeed.

Just to give some, quite random, examples. I only have one cousin on Ancestry (I know she is definitely my cousin, she is the daughter of my aunt) and we share 578 cM. She shares 1,651 cM with my dad (her uncle).

You might expect that we would share something like 825 cM (half of my dad's 1,651) but it doesn't work like that and what the pp said is much closer to reality.
.

"...my mother's first cousin's son (714cM) "

One of my father's first cousin's son shares 483 cM with him. He shares 357cM with me. And, yes, I am absolutely certain that he is the son of my father's cousin.

In contrast, the daughter of a different first cousin shares 679 cM with him and 349 cM with me.

Yet another example, the daughter of another first cousin of his shares 628 cM with him and 262 cM with me.
.

On my mum's side of the family there hasn't been anyone really close that has done a DNA test on Ancestry.

Just to show how things can be, her two half 1st cousins once removed share more DNA than the full first cousin once removed.

Just for context "first cousin once removed" refers to the child of the first cousin.

You might expect that a half cousin would share less DNA than a full cousin but it doesn't always work out like that.

The daughter of my mum's first cousin shares 352 cM with her (and 291 cM with me). Incidentally, her daughter (so the granddaughter of my mum's cousin) shares 175cM with her and 132 cM with me.

In contrast, my mum has two children of half first cousins (same grandfather but different grandmother) who have done DNA tests on Ancestry.

The children of the half-cousins share 428 cM and 393 cM with her (both more than the full cousin shares). They share 270 cM and 210 cM, respectively, with me - less than the full cousin does.

So, the children of the half-cousins share more DNA with my mum than the full cousin does. But, in contrast, the full cousin shares more DNA with me than the half-cousins do.

Small differences in the amount of shared DNA does not really matter all that much.

"The children of the half-cousins share 428 cM and 393 cM with her (both more than the full cousin shares)."

Actually, now I've got myself doubting everything. Could my mum's biological maternal grandmother have cheated on her husband? Could it be that the person I thought was my mum's full uncle (the father of the cousin mentioned above) was perhaps only her half uncle?

Perhaps this deserves more investigation? I really don't know.

For those real experts (I am certainly not one) out there. Is this any evidence of the cousin only being a half cousin or is it just within the range of possibilities for inheriting DNA?

can you provide any explanation as to your handle?

Er. Not really. My social media names tend to be binturong related because I like the furry oddities and think more people should know about them. And there was a thread on here (when the Aston data scraping kicked off, perhaps) where we were discussing the need to go incognito - so I switched from being a binturong to being an absence-of binturong.

I've not worked through your cousin/half-cousin numbers in detail but on a quick skim they look plausible. Although it would also be plausible for the 'full cousins' not to be, so ....

" Although it would also be plausible for the 'full cousins' not to be, so ...."

Yes, this is exactly what made me stop and reconsider things.

Your mother and her cousin are first cousins as they are the same generation and share the same grandparents. Your mother's child (you) and her cousin's child are second cousins as you are both the second generation and share the same great-grandparents.

When people talk about first / second cousins being 'removed', it is referring to you being different generations apart (ie: removed) from each other.

I think the OP understands that. But was confused as to why the Ancestry DNA result.is showing her first cousin once removed as being her cousin - putting them in the wrong place on the family tree.

Yes indeed @NoBinturongsHereMate

Thank you all who posted for your help.

I have matches that are the equivalent of yours. My parent's cousin's child and I share 370cM and my half aunt and I share 970 cM. As pp have said, it is random how much you inherit, although I do wonder whether there is a story around why you share so much dna with your second cousin.

“……although I do wonder whether there is a story around why you share so much dna with your second cousin.”

I wondered that too.