Author Topic: DNA segments and proximity of relation  (Read 378 times)

Online Nick93

  • RootsChat Member
  • ***
  • Posts: 112
  • Census information Crown Copyright, from www.nationalarchives.gov.uk
    • View Profile
DNA segments and proximity of relation
« on: Sunday 21 June 20 18:36 BST (UK) »
Hello, everyone. I had a quick question about DNA segments, and how closely they imply a relationship to someone. As I understand, anything more than 12 centimorgans over a single segment implies consanguinity with a person, rather than simply a relation by shared ethnicity or region. But also, the closer related you are to someone; a cousin, second cousin, etc. the more segments you also share.

I was wondering that would mean for someone with, say, 12 or 13 centimorgans shared over two segments. Does that imply kinship by shared ancestry like 12 over 1, or does sharing multiple segments over a small number of centimorgans more like imply a relationship of shared regional background?

Thank you.

Offline davidft

  • RootsChat Marquessate
  • *******
  • Posts: 4,100
  • Census information Crown Copyright, from www.nationalarchives.gov.uk
    • View Profile
Re: DNA segments and proximity of relation
« Reply #1 on: Sunday 21 June 20 19:10 BST (UK) »
I am no expert but I disagree with pretty much all you have written there.

Rather than cloud the issue with my reasoning I shall wait and see what others say, and their reasoning
James Stott c1775-1850. James was born in Yorkshire but where? He was a stonemason and married Elizabeth Archer (nee Nicholson) in 1794 at Ripon. They lived thereafter in Masham. If anyone has any suggestions or leads as to his birthplace I would be interested to know. I have searched for it for years without success. Thank you.

Offline melba_schmelba

  • RootsChat Veteran
  • *****
  • Posts: 714
  • Census information Crown Copyright, from www.nationalarchives.gov.uk
    • View Profile
Re: DNA segments and proximity of relation
« Reply #2 on: Sunday 21 June 20 19:24 BST (UK) »
Hello, everyone. I had a quick question about DNA segments, and how closely they imply a relationship to someone. As I understand, anything more than 12 centimorgans over a single segment implies consanguinity with a person, rather than simply a relation by shared ethnicity or region. But also, the closer related you are to someone; a cousin, second cousin, etc. the more segments you also share.

I was wondering that would mean for someone with, say, 12 or 13 centimorgans shared over two segments. Does that imply kinship by shared ancestry like 12 over 1, or does sharing multiple segments over a small number of centimorgans more like imply a relationship of shared regional background?

Thank you.
I would say even 6cM on one segment indicates consanguinity, even a 5cM one, i.e. on GEDMATCH. I have several 6 cM matches on which I proved the connection via several matches and segment analysis on GEDMATCH, which date back to the early 1800s to the late 17th century. But I have also found 20-30 cM matches that have a common ancestor in the early 1700s. So you can't really treat a 6cM match much differently to anything under 35 cM.


Online Nick93

  • RootsChat Member
  • ***
  • Posts: 112
  • Census information Crown Copyright, from www.nationalarchives.gov.uk
    • View Profile
Re: DNA segments and proximity of relation
« Reply #3 on: Sunday 21 June 20 21:13 BST (UK) »
Hi, thank you both for your responses. The impetus for this question was a theory I had relating to a mid-1700s ancestor, born 1752-55 named Massey Christmas Medearis of Middlesex county Virginia. His mother was not a Christmas and as far as I can find there were no Christmas surnames in his family tree. There are theories that his grandmother's maiden name was CHISMAN, which is unproven, and a 1790 will in Orange county spells out his full name as Massey Chrismus Medearis and his will, his brother's will, and his revolutionary war pension application all call him by Massey C. Medearis. I'm not sure the name Chisman could be so jumbled to be written as Christmas

An hour away from where Massey was born, in Hanover Virginia was a family named Christmas, a Thomas Christmas (1690-odd to 1769) and his three sons. In 1790 Massey Christmas Medearis is living in Orange county North Carolina, the same county as John Christmas (son of Thomas) and his children. Looking at DNA results,  I found seven people; one in the 26 cM across one segment range, one in the 20 cM range over 2 segments, one in the 18 cM range across one segment, one in the 17 cM range, one in the 16 cM range, and in the 13 cM range, and one in the 12 cM range over one segment, all descended from a Thomas Christmas. But I'm not sure if those are too strong to be coming from an ancestor born in the late 1600s.

I wonder if Massey was  an illegitimate son of one of Thomas' sons and then adopted. One Medearis family tree blog offhand mentions that Massey was adopted but gives no proof regarding it. As for Medearis matches (other than descendants of Massey), my great uncle's only matches with that family above 12 cM was one with 13 cM over 2 segments, and one with 15 cM over 1, and 14 over cM over 1 as well.