Genealogical DNA -My Understanding of Identical By Descent and Identical By State

By this point, two months after receiving my results at FtDNA, I was still finding new ways to work with my results, and I still hadn’t looked at all the matches.  My list of Family Finder matches was now up to 30 pages and slowly growing by about about three distant cousins each fortnight.  With two confirmed relationships under my belt, I looked for new ways to identify my other matches.

My cousin from Prince Edward Island had mentioned triangulation.  This meant using the known details of two matching kits to identify the connection of a third definitely connected kit.  I have used a whole lot of ambiguously defined words there which I need to keep clear in my own mind.

If there is a common ancestor, lets say it is a man born in 1800, he will have passed some of his DNA on to his children.  We all understand this.  Working from ancestor to descendant is easy. Those children will pass their DNA on to their children and some of that DNA will be have come from their grandfather, intact and unchanged.  With each new generation, the chances are likely that less of that segment will be passed down.  The segment will be smaller. Similarities do exist between generations but it is rare for a child to be exactly like their parent.  They are a mix.

This is quite comprehensible and I don’t think anyone has trouble with this notion.  We might get our brown hair from Dad and our green eyes from Mum.  I have hazel eyes and my spouse has brown eyes, but two of our children have blue eyes.  This is because my father has blue eyes and so does my spouse’s father. Those genes have come down through us to them.

What is hard is going the other way.  We have inherited a whole lot from our ancestors and trying to work out which bit came from which ancestor is like doing a jigsaw puzzle with lots of shades in a few colours and no picture to refer to.  It can be done, with time, trial and error and a good system to keep track of what you have tried and what you have not.

I tend to think of it as words.  There are a whole lot of words made up of the same letters.  STOP has the same letters as POST which has the same letters as POTS.  The letters are the same but the order they come in matters.  They are very different words.

This is how I think of DNA segments, in the privacy of my own mind.  I’m sure a geneticist somewhere is having cold chills.  However, it works for me.  If my great grandmother’s DNA words are HORSE, TREE and RADISH, she might pass HORSE and TREE down to one child, and TREE and RADISH down to another.  If those children each pass TREE on to their children, the children will be a DNA match for the segment TREE.  This is Identical by Descent.

If a totally different, unrelated person has the words FOREST, BIRD and REEF in their DNA, they just might pass down FOREST and REEF.  Placed side by side, this looks like FORESTREEF .  Accidentally that DNA segment has the word TREE in the middle, but TREE isn’t one of their words.  This one will also show as a match.  It is identical by State.

In my own DNA test journey, with two confirmed matches, I knew I had to work out which words had come down from those two ancestor couples.  A word is a segment.  Segments can be viewed in the chromosome browser on either FtDNA or at gedmatch.  My first step was to work out exactly where we matched.

My confirmed 6th cousin, for instance, matches me on chromosome 4 for segments 1272091102 to 139828852 which is 10.32cM.  We also have a large number of smaller segments on other chromosomes which may be IBD, but the smaller they get the less sure one can be.  That segment on chromosome 4 is the only one big enough to call a word.

I started a new table in a new document with a two rows for each chromosome (one for paternal, one for maternal) and I entered that segment on my chromosome 4 maternal row.  I have a column for the name of my match, a column for the name of the MRCA and a column for the locality of the MRCA.

I then entered my chromosome matches with Sarah.  There were three of those, big enough to be sure of.  So now I had one maternal segment and three paternal segments identified.

One day, I hope, this nearly empty document will fill up. In the meantime, these successes had whetted my appetite and I went hunting for more.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s