Help me understand DNA testing

[Alan H]

mtDNA is different from X and Y chromosome DNA.

mtDNA is DNA from the mitochondria. It tracks the maternal line. Mitochondria are little organelle's within cells which have a lot to do with providing energy for the cells functions. The theory goes that way back in the mists of time, one bacteria ate another bacteria but didn't digest it. The "eaten" bacteria discovered that living inside another bacteria had advantages. The "eater" bacteria discovered that having this particular other bacterium inside itself also conferred some advantages. Thus, the advent of the mitochondrion.

Whatever the case, mitochondria have their own DNA, which has nucleotide codes for molecular processes that occur inside the mitochondria. Many of those processes have to do with dealing with an important biological molecule called ATP. ATP is the "powerhouse molecule" of the cell. It's the molecule that cells use to power the work that they do. Sperm SWIM, don't they? That's a lot of work, isn't it? Sperm have a LOT of mitochondria. However, the sperms mitochondria are not added to a fertilized oocyte. That oocyte only contains maternal mtDNA.

You see, during the process of fertilization, the sperm attaches to the oocyte (the egg) cell membrane and injects chromosomal DNA. However, it ONLY injects chromosomal DNA, no mitochondria are injected. Therefore, the mtDNA is passed on from generation through generation through the mother, essentially unchanged except for the random and exceedingly rare process of mutation.

Different human population groups contain mitochondria which are somewhat distinct from each other, in terms of the various forms of the genes inside them. These "various forms" are call polymorphisms and are represented by subtle variations in the order of four molecules that Forrester Modern talked about: Adenosine, Thymidine, Cytosine and Guainidine.. Thus, an/some ancestral group(s) of people might be identified, by working backwards from current arrays of mtDNA polymorphisms. There are computer programs and fancy algorithms which use the principle of maximizing parsimony (parsimony = the simplest answer is the most likely answer) which can make an educated guess at what these ancestral polymorphisms would be like, by choosing the simplest possible series of mutational events that might result in the modern array of polymorphisms.

In fact, since there are estimates of the average rate of mtDNA mutation, some researchers even try to pinpoint general times when different polymorphisms occurred, thus putting some sort of timeline on the human mtDNA family tree.

To simplify...If Panache and Alan H share 12 polymorphisms in their mtDNA....and Panache and McMurdo share 8 polymorphisms, then Panache and Alan H are more closely related than Panache and McMurdo.

[Alan H]

I defer to you as the doc, but AFAIK, those women who do have a Y chromosome are not able to pass it on, as they are not fertile. That is a complicated subject in it's own right.

There are women who are genetically XXX, and multiple X, generally. It's quite an abnormal situation but it does occur. You can read about these sorts of sex chromosome abnormalities on the 'web.

However if a person acquires a Y chromosome at all, they will be physiologically "male" if they survive to birth. XXY is rare, but it does occur, the syndrome is called "Klinefelters Syndrome".

[Alan H]

An interesting sidelight to all of this may be found in this tale...

The Borders have long been a place where races mixed. Anglo Saxons, Celts, Danes, Romans....and...

people from steppes of Central Asia????:

Yes. OK, I'm over-simplying here, for the sake of the story, but this will illustrate the principle....

There's a haplotype..meaning a distinct subset of mtDNA polymorphisms, which is associated with a people called the Sarmatians. The Empire of Sarmatia co-existed with the Romans, in fact the Romans and the Sarmatians fought for hundreds of years. Sometimes one side would win a few decisive battles, then the other side. Well, at some point, the Romans won an important campaign against a Sarmation Lord, and to buy the peace, they demanded that he hand over several thousand of his cavalrymen.

This practice was pretty common.

OK, so what is Rome going to do with a couple thousand trained Sarmatian warriors? Leaving them in Sarmatia would be pretty stupid, bringing them to Rome is asking for an insurrection, so....what? What they did was post the Sarmatians, as Roman Soliders...to the lonely, distant outpost of Gaul.

Which is now the border of England and Scotland.

OK, so take a couple thousand young lads and a few hundred of their women, move them away from their homes and dump them in a new country. What's going to happen?

Imagine how surprised researchers were when they discovered that a whole lot of people from the Scottish Borders share an mtDNA haplotype, AND a few other significant chromosomal DNA markers with peoples from the Steppes of Asia...Uzbekistan, places like that.

Interesting.

[ForresterModern]

I defer to you as the doc, but AFAIK, those women who do have a Y chromosome are not able to pass it on, as they are not fertile. That is a complicated subject in it's own right.

You are in general correct, that the presence of a Y chromosome, regardless of how many x chromosomes my also be present (but only to a degree) would make the person genetically a male, who should (theoretically) have the outward expression of a male. Developmentally we all start out as very tiny one celled females and those with the Y chromosome develop testosterone which typically causes them to develop male physical characteristics during the early and middle phases of fetal development.

But there are rare cases of exceptionally high numbers of x chromosomes (like 8 or 10) in cells with a single Y where the sheer preponderonce of x's outweighs the presence of the Y. You may also have situations where the Y may be present but not complete and therefore not carry ALL the genetic material necessary to express the full Male phenotype (appearence), which may look female. Lastly, there are spontaneous mutations of specific genes for metabolising certain sexual hormones which may make a genetic male (XY) not manufacture testosterone or not have appropriate cell membrane receptors and related mechanisms for getting cellular testosterone effects to occur, which would also leave you with the female appearence (although no normal internal female anatomy)---a phenomenon known as testicular feminization. The first and second groups COULD theoretically pass on Y chromosomal material if they were "female" enough anatomically and fertile, although the likelihood would make lottery odds look small. The last scenario is not compatible with fertility or reproduction so is not an issue.

Hence the reason I hedged a bit on my initial discussion.

[gilmore]

DNA testing has been commercially available only for seven years or so, but is proving to be a useful tool.

As was pointed out, men inherit Y DNA from our fathers, just as we usually do our surnames, hence that is tested for more often than the other kinds.

The larger the database of results, the more likely one is to match one's DNA with with some one else's. Family Tree DNA at www.familytreedna.com has the largest in the world, and is therefore the most useful. You can test for 12, 25, 37 or 63 markers. 12 is too few to tell you very much, and 63 is more than you need. You can upgrade and test more markers later, since FTDNA keeps the samples for 25 years.

Your DNA may match exactly with that of a man who has meticulous and reliable records going back over a thousand years. Or you may find no matches at all. Or you may find that your matches all hve surnames different from yours and come from the other side of the world than you thought your family did. It's a crapshoot.

Probably the most helpful thing that Y DNA tells you is who you are NOT related to. That is, by showing you who your close matches are, it gives hints as to where research is likely to be beneficial, and where it is likely to be useless.

Another thing to remember is that the results are in terms of probability, and rarely exactitude. That is, you will discover men with whom you have a common ancestor within a range of time, a range of generations.

DNA testing isn't really a substitute for old fashioned paper documentation, but can be a helpful adjunct.

[O'Callaghan]

There are women who are genetically XXX, and multiple X, generally. It's quite an abnormal situation but it does occur. You can read about these sorts of sex chromosome abnormalities on the 'web.

However if a person acquires a Y chromosome at all, they will be physiologically "male" if they survive to birth. XXY is rare, but it does occur, the syndrome is called "Klinefelters Syndrome".

Ah, but to what extent will they be male physiologically? Internally, yes, but not necessarily externally. Apparently there is something called AIS where someone can have ordinary male XY chromosomes but their body cannot respond to male hormones, so they are born female and develop as females, but lack any internal female plumbing of any kind, and instead have undescended you-know-what.

AIS came up in the news because someone suggested that it might apply to the 800m champion, Caster Semenya, but apparently if she had that syndrome she would look and sound like a woman, which unfortunately she doesn't. Some others have suggested she might have another problem called CAH, but that raises other inconsistencies. It all goes to show that, especially if you aren't qualified, you can't diagnose someone by looking at them, especially if they have clothes on.

AIS was only suggested, it seems, because some other athletes who failed a gender test had it (who, BTW, were eventually reinstated, because they can't gain muscle due to testosterone), but it doesn't fit her atall.

Reading the news on this reveals that there are many different syndromes, and a lot of them don't even involve abnormal combinations of chromosomes, although they may not be the right ones for the person's apparent gender.

My apologies for the thread hijack. This has almost nothing to do with using DNA tests to trace your heritage.

[KFCarter]

Thanks all, for this great discussion on DNA testing. I've thought about doing it, but did not really have a clear picture of what it could (and couldn't) provide. Thanks for the education!!

[ForresterModern]

Ah, but to what extent will they be male physiologically? Internally, yes, but not necessarily externally. Apparently there is something called AIS where someone can have ordinary male XY chromosomes but their body cannot respond to male hormones, so they are born female and develop as females, but lack any internal female plumbing of any kind, and instead have undescended you-know-what.

AIS came up in the news because someone suggested that it might apply to the 800m champion, Caster Semenya, but apparently if she had that syndrome she would look and sound like a woman, which unfortunately she doesn't. Some others have suggested she might have another problem called CAH, but that raises other inconsistencies. It all goes to show that, especially if you aren't qualified, you can't diagnose someone by looking at them, especially if they have clothes on.

AIS was only suggested, it seems, because some other athletes who failed a gender test had it (who, BTW, were eventually reinstated, because they can't gain muscle due to testosterone), but it doesn't fit her atall.

Reading the news on this reveals that there are many different syndromes, and a lot of them don't even involve abnormal combinations of chromosomes, although they may not be the right ones for the person's apparent gender.

My apologies for the thread hijack. This has almost nothing to do with using DNA tests to trace your heritage.

Doctor hat on.

The AIS you speak of (I am not personally aware of what the acronym stands for) is the same as the series of abnormailities I spoke of more commonly known as testicular feminization syndrome, a genetically XY person who for whatever reason does not manufacture or respond to testosterone as a normal person would. Going back to the discussion earlier, since we all start out as litle females and only develop into males if testosterone effects occur these children/people develop into perfectly nromal little girls who unfortunately find out the genetic truth usuall only in their teenage years when they fail to start their menses by the usual time, and are then chromosomally tested. You can only imagine the psychological stresses this causes on all family and friends involved, especially when the "girls" are told that they are genetically male, and effectively sterile as they lack ovaries and internal female organs but also have atrophic male organs. Interestingly there is a higher proportion of them who have more athletic dispositions than the background female population, which may have something to do with other CNS effects of the XY chromosomes other than testosterone.

CAH is another unfortunate group of genetic enzyme abnormalities where the body lacks a certain ability to metabolize an adrenal hormone from one form to another form, instead leaving excess of the first form circulating to cause excessive effect on the developing body. This usually is in genetic females who get inordinate testosterone effects from the excessive variant hormone (similar to but not identical to testosterone) and thereby become genetic females with some tendency toward inordinate virilization, often leaving newborn babies with "ambiguous genitalia" requiring extensive hormonal and enzymatic evaluation along with chromosome evalutation to determine short and long term therapies, both medical and reconstuructive surgical. Many of these children may also have severe metabolic problems due to coincident salt-wasting problems also related to the excess of the variant hormone.

Hopefully the last of the thread hijacks. Doctor hat off.

Back to DNA testing for heritage tracing, as Gilmore said above it is most useful when one has done as thorough a job as possible to trace one's own paternal lineage backwards as far as can be obtained, then supplement that with the information about potential close relatives that may be divulged by the DNA search, which may allow you to pick up the trail again and connect things by backtracking from there to make your needed lineal connection. Otherwise the DNA tests will give you wild hairs all over the world sometimes, when you are most concerned about a specific lineage in a specific place. The others in the world may share a common distant ancestor but may have no bearing on the tracing of your own paternal ancestral lineage.



Jeff :ootd:

[ForresterModern]

As Alan has said above it was common during the Roman Empire, as well as during the British Empire, for the powers that be to relocate troops obtained in one part of the world to a very distant and often remote part of their empire for their military postings. This has sense, as Alan alluded to, in that locals fighting against local might tend to be less likely to fight a "neighbor or potential relative" of a similar heritage than they would a total stranger in a foreign land with a totally alien heritage. Hence the use of mongol troops to man the frontiers of the britannic roman empire, and the use of Ghurkas as storm troops and military guards by the UK in places across the world but not their own homelands (in general). It makes sense---it is easier to justify fighting against someone who you do not look like, share a common language with, or have any cultural relationship to than it is to fight a neighbor or potential relative to whom you may have more sympathy/empathy. A tactic that has been in use for thousands of years.

This may explain a lot of the aberrancies in DNA testing, as obviously these relocated troups, if left there long enough, would be likely to intermarry with the locals over time, even possibly becoming settled there for the remainder of their lifetimes and thereby become "locals" themselves after a generation or two. That combine with general migration and repetitive warring invasions (e.g., the vikings over virtually all of coastal western Europe) alone would cause such a melange of genetic material that would likely make ones head spin were he to try to make sense of it all in a vacuum.

[vmac3205]

There are always going to be people with surnames and genetics that don't match - so even if it is possible to trace a family line on paper, the genetic results will sometimes be a surprise.

It has been known for samples to get switched at laboratories, babies to be switched in hospitals, and all sorts of other odd occurrences.

Anne the Pleater :ootd:

Not to mention babies getting switched at hospitals. I swear I can't be related to half my family. I must have been switched at the hospital.