What does it mean for genetic genealogy that sometimes people inherit genes asymmetrically from parents?

+6 votes
195 views
asked in The Tree House by Ian Mclean G2G6 Mach 1 (12k points)
This struck me as particularly interesting because I have found out recently that a number of my X Chromosome DNA matches do not in fact share DNA with my mother at all. There are also a number of tools for showing you what DNA you do not share with either parent.

I don't know I have ever seen a comprehensive treatment for how common or uncommon patterns of inheritance of types similar to this are in aggregate.
For lines of inheritance that include lots of male ancestors, X-DNA can come from ancestors too distant to show up in autosomal DNA. Look under DNA Ancestors in your WikiTree Family Tree and Tools to see where you X-DNA comes from, and about how much you might have inherited from a particular ancestor. You'll see that it's possible for a man to get one-eighth of his X-DNA (on average) from a 5G grandmother (percentage depends on the line).
I am aware of that. I am also aware of the fact that the matches can be the result of mutations. They can also be the result of a narrow band of recombinatorial events where my father's Y chromosome bonds with my mother's X chromosome, so they are not necessarily representative of a maternal match at all, but are sometimes actually Y chromosome matches by proxy.

My point is more to the fact that the practical situation is quite a bit more complicated, complex, and ambiguous than genetic genealogy how-tos seem to indicate. And from examination of my genetic matches and my mother's genetic matches, this kind of things is a lot more common than prevalent discussions would seem to indicate.
When you say "do not in fact share DNA with my mother" are you referring to autosomal or X-DNA?

Autosomal lack of match is understandable, but if you mean X then what does the full-resolution graphical  1:1 X-chromosome browser view look like between you and your mother? Are there one or two SNPs that don't match, or is there a big block?
X-DNA. My mom and I match on most of my X-Chromosome, but I have X-DNA that I do not share in common with my mom but I do share in common with relatives that I have now identified as being related to my father's side of the family.

I have two segments which are in the tens of centiMorgans in size that are no match to my mother's kit. Two big blocks, non-contiguous. One segment is at the centromere so perhaps error prone, and the other is off of it so less likely to be the result of an error.

I have an XY kerotype. Normatively, it is expected that I inherited pretty much my entire X chromosome from my mother exclusively. Practical reality is that there is a significant chunk of my X chromosome which is probably not attributable to my mother. Ergo either it is mutation (unlikely I think) or it is in fact Y chromsome DNA that has bled over onto my X chromosome due to recombination.

Interesting, no?

6 Answers

+6 votes
What it means is that you need multiple sources to develop a full picture of a genome. To really have your parents combined genome down really needs a minimum of 3 siblings taking autosomal tests. The case you referenced is pretty rare, but having an uneven amount isn't all that uncommon.
answered by Doug McCallum G2G6 Pilot (205k points)
Just asking, Doug. Are you saying it's NOT uncommon for an indvidual to have a chromosome (other than the "sex" chromosome) where one parent contributed BOTH strands?
It isn't common at all and I misstated. It is more common to have some partial cases (one chromosome) but even then it is pretty rare. Just not as rare as the nearly complete copies that this girl had.

I was thinking something else with the 3 siblings. That is to get the grandparents genome.
+5 votes

It means that whoever wrote the article needs to go back to school smiley

Your autosomal DNA is usually made up of two strands, one from your father and one from your mother, so that is 50% from each. 

answered by Lynda Crackett G2G6 Pilot (617k points)
Lynda, the article says it's a rare condition where her father contributed BOTH strands on DNA on most of her chromomes.

Further, the information for the article came from "the Journal of Human Genetics". The content is similar to what a number of other news organizations covered. Errors do happen in biology.

When she was 9 years old, a genetic test revealed that she had two identical copies of a rare gene. At first, the scientists thought it was extremely bad luck, inheriting the same super-rare gene twice, once from each parent.

But as it turned out, the rare gene was, “surprisingly, not in her mother,” wrote the scientific team, led by Irena Borgulová of the Centre for Medical Genetics and Reproductive Medicine Gennet in Prague. Which meant she had inherited two copies of her father's gene.

+5 votes
Just my thoughts:

That article says that only about 25 people in the world are known to have the condition. If that data is anywhere near accurate, the odds of running into the situation (abt 25 in 100 million? = 0.000025 per cent), are so slim that "Don't worry about!" is likely a good answer to your question. That doesn't account for unknown occurrences in past generations, of course, but with only 64 ancestors in 6 generations, the odds are something like 1 in 16 million of anyone having a problem.

Beyond 6 generations, things get pretty "iffy" in any case.

You would also need to take into account the probability that, in any particular family tree, other genealogical research (which has to accompany genetic genealogy at present) yields NO information on a particular individual.
answered by Bruce Veazie G2G6 Mach 5 (52.9k points)
This specific condition may be exceedingly rare. But it is also exceedingly obvious in its manifestation such that we actually know it exists from measurement.

As we have been decreasing the cost of imaging people's genetics, we have been steadily finding many exceptions to the grade-school classical genetics that we have been taught and which are used in communities like WikiTree to infer general patterns of human relations.

Suppose for a moment that we're considering this as one particular kind of genetic kerfluffle among a class of many other kinds of such kerfluffles. The question is less "how rare is this specific thing?" and more of "when we add all such occurrences together, how rare is it that people inherit both copies of genes or even entire chromosomes from a single parent?"

I could see this following a similar path as intersex conditions which were regarded predominantly as being so rare as to be negligible and are now known to be collectively as common as the occurrence of red hair.

For collective projects like WikiTree, this kind of thing isn't a "you never need to worry about it" kind of thing because when we ask about the probability that any one member of WikiTree or an adjacent genealogy project is going to run into the consequences of this kind of thing in their own family tree, I think the probability converges towards 1. This is absolutely a concern for genealogists as a whole and will effect entire families.
+4 votes

The article may be about gene expression or epigenetics.

Although she inherited half of her genes from each parent, I think the article means that her father's genes dominate her phenotype. However, the article says that her cells contain two full sets of her father's genes.

If you have a day to devote to learning some intricacies of genetics, look up the bolded terms on the Internet or Wikipedia.

answered by Ellen Smith G2G6 Pilot (841k points)

Article is not about epigenetic expression. It is about inheritance of genes. Specifically cited in the article.
 

When she was 9 years old, a genetic test revealed that she had two identical copies of a rare gene. At first, the scientists thought it was extremely bad luck, inheriting the same super-rare gene twice, once from each parent.

But as it turned out, the rare gene was, “surprisingly, not in her mother,” wrote the scientific team, led by Irena Borgulová of the Centre for Medical Genetics and Reproductive Medicine Gennet in Prague. Which meant she had inherited two copies of her father's gene.

 

+5 votes
Kind of makes you want to stick your fingers in you ears and loudly say something like "Blahblahblah!" until somebody makes it go away, doesn't it?

The thing is, while this level of the problem is rare, who says that you can't run into it on just ONE chromosome once in a while - enough to throw a monkey wrench into the works for somebody, here and there?

So what it means is that this stuff is not as simple as we like to think, and that more is found out about it all the time. We'll need to keep up with it, and maybe even find some ways to detect these issues and deal with them..
answered by Frank Stanley G2G6 Mach 1 (17.7k points)
+2 votes
It means we are human.  People want DNA like everything else in their lives to be predictable.  But nature is not predictable and it has mutations.  They are rare but they do occur. This is why doing genealogy requires a good paper trial and proof outside of DNA alone.  

Think of DNA as another type of proof for the paper trail not as a replacement for it.  

It also means that when dealing with DNA nothing is absolute.  All of what we do is based on probabilities not certainty and this just shows that in a very pronounced way.  

My own DNA is canted more to one side of my Dad's line because of how the DNA recombined.  With every generation the mixes can be more mixed than the averages predicted.
answered by Laura Bozzay G2G6 Pilot (443k points)
Good comment. The days of instant genealogy by way of DNA aren't here even if some unnamed TV ads sort of imply it can be done.

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