Monday, February 15, 2016

DNA: Not The Endogamy Of Cousins

We've been discussing endogamy in the ISOGG group. I took a poll at ISOGG a few weeks ago and learned that first cousin marriage was not uncommon in earlier times. This is true of certain groups. People living in isolated places with few prospective mates, for instance. I have not found that kind of cousin marriage in my family. Most of my lines go back 6-8 generations. My ancestors sometimes married outside of their ethnic group, and also married mates who came in different waves of immigration from the old world.

When I say not the endogamy of cousins I mean for most of us we aren't seeing any close cousin marriage out to 6 or 8 generations. We do see the affect of the smaller early American population. When my American ancestors came to America they initially settled in Pennsylvania and Maryland. They migrated from there to either the Midwest or South. Small populations in these areas, and common migration patterns,  could mean my matches' ancestors may have crossed paths with my ancestors more than once. I have found in a couple of instances that I could be related to a match through two different couples. This is a potential pitfall if you haven't carefully compared your tree with a match. This isn't the endogamy of cousin marriage, because it doesn't represent close cousins marrying, it's just that your ancestors crossed paths more than once when the population was smaller.

This is where mapping out chromosomes helps. Using the segments of matches, and your immediate family, going out to the 3rd cousin range you can begin naming your segments for family lines. I will never be able to do this for certain ethnic lines due to the lack of surviving records in the home country, so in that case I just name the segments according to ethnicity. Filling in the chromosome chart with named segments helps to identify matches who's segments overlap with confirmed family.

We are able to collect these segments at Family Tree DNA and 23andMe.  Family Tree DNA makes it easy by allowing us to see the segment information for all matches. At 23andMe you generally have to ask to share. 23andMe now has opt in sharing, which is working out for me better than expected.

At AncestryDNA there is no way to see the chromosome information. This creates a problem considering our ancestors may have crossed paths more than once. Without the possibility of mapping how do we know which or two, or more, couples we may have gotten our shared DNA from? AncestryDNA also has more of the segments we need to create such a map. I have closer matches there than at the other two companies. Their data could help me a great deal, and all of us.

Ancestry believes in DNA mapping because they recreated the genome of David Speegle using this technique. Some say we have segments going back to endogamy; if not more recently from America then going back to the old countries. These suppositions didn't seem to affect Ancestry's genome recreation?

Many will tell us compare at GEDmatch. Few of my matches have agreed to compare there. The process is confusing for those who aren't computer savvy. Others worry about the privacy of the site. The best solution, which would produce the most compliance, would be an opt in segment sharing system, like 23andMe.

Another problem with AncestryDNA is the problem plagued messaging system. If we don't hear from our matches it could be they aren't interested in sharing, or they didn't get the message at all?

Here is what Mapping can do for you:
  1. A well filled in map can help identify the ancestral route of a segment. This helps even when a match isn't cooperative.
  2. It can help to identify which of two or more couples a segment came from.
  3. It can help you eliminate IBS segments. You might find segments your parents don't match on.
  4. If you're using smaller segments as proof mapping can help confirm them.
  5. Ethnicity Chromosome Chart
  6. Chromosome matching segment maps can be compared to ethnicity chromosome maps to confirm ancestry. If you are 100% European the ethnicity chromosome chart won't help.
Without mapping we are hamstrung in certain situations.

This couple (below) now has 59 members in their circles. This could greatly help with chromosome mapping. Maybe Ancestry will sell us their genomes at some point?

1 comment:

Jason Lee said...

Chromosome mapping is very effective for predicting shared ancestry.