Help me understand DNA testing

[ForresterModern]

Doctor in the house. Doctor hat on.

The Y chromosome, as stated above, is only present in males (with rare exceptions) and is passed directly from one's father to the son, generation after generation. Since names in the western european model present in england and scotland also passed the paternal surname from father to son (at least for the last few centuries), the name would follow the Y chromosome.

The Y chromosome is actually this pitifully small thing, with very limited genetic material included in small packets called genes, each gene with its own individual purpose. The chromosomes are made up of deoxyribonucleic acids (or DNA), which itself is made up of 4 chemicals known as nucleotides(cytosine-C, thymine-T, Guanine-G, and adenine-A) which are chemically strung together in a long linear strand. DNA is made up of two complimentary strands, as certain nucleotides mate up with certain other nucleotides across from one strand to the other. Certain sections of each DNA strand/chromosome code for specific protein manufacture which ends up causing a specific end effect like blue eyes or blond hair, when combined with the protein effects of the other matching chromosome (we have pairs of 22 different chromosomes plus either an XY mismatched pair in males or XX matched pair in females). Each gene also may have minor variations in its chemical makeup---for example a certain sequence of ACTG nucleotides in a certain location in a certain gene on a certain chromosome may code for blue eyes while a different sequence of nucleotides in the same place in the same gene on another similar chromosome would code for maybe green eyes. Since most chromosomes have nearly identical mates (pairs of same numbered chromosomes) the genes do battle to see what the eventual expression of those different genes will be (dominant, recessive, or codominance--in my example green (dominant) wins out over blue(recessive) and the individual will have green eyes).

Since the Y chromosome has little genetic material to match up with the much bigger X chromosome many x-related genes express the mother's contribution in outward appearence simply because there is not mate to contend with on the smaller Y. However the Y chromosome does still carry a significant number of genes which can be traced by biochemical tests, and those individual variations in nucleotide group ordering in those genes can be identified (variations similar to the blue eye-green eye discussion above) by the DNA testing everyone is talking about. Y chromosome genes rarely mutate spontaneously so they are a good marker to follow from one generation to the next, and so forth, in the paternal line. Multiple diferent variations for each geen exist, and multiple different genes can be tested and typed as to which variation is present in each gene. Males of similar lineage should have similar Y DNA chromosomal genetic markers, regardless of how many generations apart they might be.

So, for instance, 20 different males with the last name Macdonald all get DNA tested in Scotland, and we find that 4 of them have identical Y genetic profiles. We can say that they all descend from a common ancestor while the others do not descend from THAT ancestor, despite the MacDonald name. You may be an american MacDonald and get your DNA tested and it matches those four other MacDonalds, at some time in the past one of your paternal lineage was a MacDonald, and you share a common lineage with those other four MacDonalds, i.e., you are paternally related. Same thing even if your name is not now MacDonald---maybe you were a Macdonald who segregated into a sept subsequently taking the sept name instead of the paternally inherited name, maybe you or someone in your paternal lineage were adopted yielding a name change, maybe there was some illegitimacy in your past and a paternal line was started using your mother's name. Your Y chromosome genetic marker makeup will tell your paternal heritage as long as you have something to compare it to (the huge Y chromosome DNA database). The more genetic markers tested the more acurrate the matings are, so if you are really going to do it to try to trace ancestry backward into the "old country", wherever that may be, try to do as many markers as you can afford or are available at the time you do the test. More markers are being defined every month and the number of testable genes is increasing regularly (more than a hundred now I believe).

Doctor hat off. I have not been tested yet, but will be in the next couple years, once I narrow down my american ancestry back to the first Foster/Forrester who came to the US. Also waiting for the number of testable markers to go up, and hopefully the price to go down. This will help me find direct line descendants back in the old country, and hopefully some distant relatives, maybe even royalty (there is alledgedly at least one
knighted member of my lineage).


jeff

[O'Callaghan]

Doctor in the house. Doctor hat on.

The Y chromosome, as stated above, is only present in males (with rare exceptions) ... <SNIP>

jeff

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.

[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".

[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.

[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]

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.

[Rob]

Phogfan86 requested a simple explanation of what DNA testing could tell him about his ancestry. However, most of what has been posted so far (although technically accurate) would most certainly go straight over the head of most 6 year-olds. I hope that the following is helpful.

There are two types of DNA that are used for ancestral testing. These are the non-recombining part of the Y-chromosome (NRY) and mitochondrial DNA (mtDNA).

NRY is passed down the direct male line unaltered apart from rare mutations. Therefore a man’s father will have the same NRY signature as the man himself, as also would his father’s father, his father and so on stretching back a thousand years or more. Testing for NRY will tell you nothing about your mother’s father, or his father, etc. Nor will it tell you anything about your father’s mother’s father, etc.

Similarly, mtDNA is passed down the direct female line unaltered apart from rare mutations. However, unlike NRY, it is also passed on by a mother to her sons, but the sons do not pass it on to their offspring. Therefore a man’s (or woman’s) mother (and her mother, etc.) will have the same mtDNA signature as the man or woman themselves. Testing for mtDNA will tell you nothing about your mother’s father’s mother, or her mother, etc.

This leaves an extremely large part of your ancestry about which neither NRY nor mtDNA can tell you anything at all. If we go back to the great great grandparents level (of which there would have been 16 in total), NRY and mtDNA tests would tell you about just 2 of them, i.e. your father’s father’s father’s father and your mother’s mother’s mother’s mother. This leaves 14 great great grandparents unaccounted for (genetically speaking).

Furthermore, DNA testing on its own cannot identify who the 2 direct line great great grandparents were. For this you will still need reliable family records, but once these two individuals have been identified, you can be sure that they will have the same NRY (direct male line) and mtDNA (direct female line) as you have. This only applies to males, as females do not have any NRY to test in the first place, so they can only trace one great great grandparent genetically, unless they can persuade a brother to be NRY tested.

Often a particular NRY signature is strongly linked to a particular surname if it is a patronymic surname, so it may be possible to tell if you are possibly linked to that branch of the family. I say ‘possibly’ as there are likely to be other branches with the same NRY signature.

DNA testing is probably most useful in determining likely ancient migrations of populations. For this reason, the field of study is referred to as population genetics and requires DNA samples to be taken from a significant sample of a current population. It can lead to some unexpected conclusions. For instance, in Victorian times it was assumed by historians that the ancient Britons were either exterminated, or driven out of what was to become England by the invading Anglo-Saxons. Population genetics studies have shown that this was far from being the case, as only a very small percentage of present day English people are descended from these 5th/6th century Germanic invaders, the vast majority being descendents of the ancient Britons.

I hope that this explanation of the capabilities and limitations of DNA testing has been useful.

[Alan H]

This leaves an extremely large part of your ancestry about which neither NRY nor mtDNA can tell you anything at all. If we go back to the great great grandparents level (of which there would have been 16 in total), NRY and mtDNA tests would tell you about just 2 of them, i.e. your father’s father’s father’s father and your mother’s mother’s mother’s mother. This leaves 14 great great grandparents unaccounted for (genetically speaking).

An excellent point and absolutely correct, thanks for mentioning it.

[wvpiper]

An excellent point and absolutely correct, thanks for mentioning it.

Which was alluded to in this story posted two pages back
http://www.cbsnews.com/stories/2007/...n3334427.shtml

"And that's the rub. This business of genetic genealogy is fraught with limitations. For one thing, it can only provide information about a tiny fraction of our ancestry. Because we get half our DNA from our mothers and half from our fathers, almost all of our DNA gets shuffled and remixed every generation, making it impossible to trace what comes from whom. There are just two bits of DNA that remain pure - the "Y" chromosome, which passes directly from father to son, and something called mitochondrial DNA, which passes unchanged from mother to child.

Hank Greely, a law professor at Stanford University, has studied this new field. He worries that people don't realize just how many ancestors they actually have.

"Eight generations ago both you and I had 256 great-great-great-great-great-great grandparents," Greely points out. "It doubles every generation. So you've got two parents. You have four grandparents. You have eight great grandparents. Sixteen great-great grandparents. And it adds up fast. It adds up so fast in fact that if you go back 20 generations you've got over a million grandparents."

1,048,576 to be exact. And in each generation, DNA testing can provide information about only two of them. "