Genes-R-Us — You and Dad are cousins? How can that be?
Posted: Wednesday, November 23, 2011 5:01 pm
By NANCY LATIMER
Special to The Messenger
Each time a person is tested at a Direct-to-Consumer (DTC) genetic testing company, their DNA is compared to all the other people who have previously been tested. If there is a match of sufficient size along chromosomal segments then the testing company estimates the relationship between individuals based on how many and how long those shared segments are.
My birth parents, my oldest son and I have all had this testing done. Everyone’s relationship was correctly identified except that my mother was listed as my son’s aunt. How did they get that wrong? As it turns out a son would share about 25 percent of his genes with his aunt but also about 25 percent with his grandmother. But how is the aunt-versus-grandma relationship actually disentangled?
There is something called sexual recombination where little segments of DNA can swap between mom’s chromosomes and dad’s chromosomes even before taking up residence in either egg or a sperm. From grandma to son has two sexual recombination events; from grandma to aunt has one recombination event. So from an aunt to son there are three sexual recombination events.
More sexual recombination events provide more chances for separate segments to form. If there are more than 40 total shared segments of DNA then the relationship is predicted as aunt rather than grandmother. My son had 41 segments (and 25.7 percent shared DNA) with his maternal grandmother so he was definitely on the edge. For comparison, my son had 34 segments (and 25.3 percent shared DNA) in common with his maternal grandfather. The 34 segments are more in line with two sexual recombination events.
Geneticists measure the length of a shared segment of DNA along a chromosome using a very complex unit of measurement called a centiMorgan (cM). My son shares a total of 3747 cM (over 25 segments) with me. I share 3747 cM (over 24 segments) with my mother and 3743 cM (over 27 segments) with my father. These measurements are all very close, because each parent to child relationship involves only one sexual recombination event.
So why am I boring you with all this? I need you to get comfortable with this segment lingo so you can share my big “Aha!” moment.
My father, son and I all found out that we related to Pierre by DTC genetic testing. If we are all related through my father’s line, then all the segments I have in common with Pierre are fully contained in my father’s shared segments with Pierre, and my son’s common segments with Pierre are fully contained in mine.
I compared the shared segments, chromosome by chromosome, between each of us and Pierre. I used a look-up chart to estimate our relationship to Pierre based on the total cM of shared DNA segments. Pierre and my father share a total of 44 cM which estimates Pierre to be his fourth cousin. Pierre and I share 27.5 cM, which estimates Pierre to be my fifth cousin. So I fully expect Pierre to be my son’s sixth cousin, with around 12 cM in common between them.
When I compared my son’s shared segments with Pierre, he gets 12.5 cM from the chromosomes that my father and I had to pass along, making Pierre my son’s sixth cousin. But there is something unanticipated on chromosome 8.
Here is a single 13 cM segment that neither my Dad nor I had to pass onto my son. That means it had to come through my son’s father. This would imply that my son is also a sixth cousin independently through his father’s side, which makes his father a fifth cousin to Pierre.
Thus my son’s father and I are both fifth cousins to Pierre, which mean that we are fifth cousins to each other and have a common set of grandparents — those being the ones we share with Pierre. I know that my son’s paternal grandfather was French Canadian, so this is not a complete surprise, and I now know of my father’s Acadian French heritage.
My daughter was unnecessarily worried about the integrity of her DNA when I broke the news that her father and I were fifth cousins based on my DNA analysis.
She responded, “Umm … isn’t that kind of weird? Shouldn’t we just stop now and chalk this up as a mistake?” Somehow she only saw the cousin part and panicked. I reassured her that having a father and mother who were fifth cousins was way too many generations back to compromise her DNA. Ah, genetics is so … personal!
Editor’s note: Nancy Miller Latimer has worked in scientific research and development for 27 years. She blogs at neuronalbeauty.blogspot.com. Published in The Messenger 11.23.11