Find Common Ancestors by Comparing Genomes in deCODEme
We have just made some updates to the deCODEme ancestry service. Now you have more power and flexibility when you compare your genome with that of friends or individuals from different populations around the world.
Your genome can be viewed as a mosaic or tapestry made up of fragments of chromosomes from your ancestors. Fragments of chromosomes inherited from very recent ancestors, say grandparents, are expected to be large – typically tens of millions of nucleotides in size. As ancestors become more ancient, then the size of the chromosome fragments inherited from them become smaller – down to a few thousand or hundred nucleotides for ancestors born thousands of years ago.
Our new and improved genome comparison tool enables to you compare your genome with another individual in order to determine which chromosome fragments you share and to see how much of your genome is shared. The fascinating thing about this analysis is that each shared fragment represents a common ancestor. The number of shared fragments and their size reflects the number of common ancestors and how far back in time they are found. In other words, you can see how closely you are related.
When genomes are compared, your chromosomes are broken down into fragments of a particular size and sharing is evaluated for each fragment. Before the fragment size was fixed at 1 million nucleotides. Now you can change the size of fragments that are compared, from a minimum of 250 thousand nucleotides (250Kb) to a maximum of 20 million nucleotides (20Mb). The minimum fragment size will reveal shared chromosome fragments from common ancestors going back thousands of years. The maximum fragment size will reveal only shared chromosome fragments from very recent common ancestors – i.e. going back only a few generations. Setting the fragment size thus lets you select how far back in time you want to hunt for common ancestors.
This image shows results of a comparison between an Icelander and an Orkney Islander using a fragment size of 3Mb. The brown lines are shared fragments, inherited from common ancestors from more than 1000 years ago!
In fact, even shared segments at 5MB can indicate common ancestors going back many thousands of years, depending on where they’re located. That’s because some areas are highly conserved. For example, if you show medium sharing in the middle (centromere) of chromosome 11 with a Yakut, Icelander, Bedouin, etc. this probably indicates you shared a common Paleolithic Eurasian ancestor who now has descendants living all the way from Europe to East Asia.
Davidski
14 Jan 10 at 12:11 am
The fragment size of the image is 500Kb, your article says 5Mb. So you are saying that a fragment of 500Kb like the image shows, indicates shared common ancestors from more than 1000 years ago?
Ponto.
Ponto
14 Jan 10 at 7:03 am
Thanks for pointing this out Ponto. We have uploaded a new picture and changed the reference to 3Mb.
admin
14 Jan 10 at 2:10 pm
Thanks deCODEme for this new feature. It has certainly made comparisons between friends more informative and interesting. Perhaps one day it might be possible to develop an algorithm to correlate these fragments to time to most recent common ancestor (TMRCA)?
rogers
14 Jan 10 at 10:46 pm
You mention instances of medium sharing. How would you interpret the one instance of high sharing on chromosome X?
jim berry
20 Jan 10 at 2:45 pm
How do you determine what is significant though? For example, if I share 0.8% medium at 5Mb with someone, is this saying I share an ancestor with them? If so, how far back roughly?
Reiver
23 Jan 10 at 5:46 pm