|Genetics & Genealogy|
B. Y CHROMOSOME DNA AND THE Y LINE
We return again to discuss the sex chromosomes. The sex chromosomes, the X and the Y are unique because they are so different as a pair; the X chromosome is very large, and the Y chromosome is one of the smallest. The Y chromosome, has unique features that make it useful to genealogists: (1) The presence of a Y chromosome causes maleness. This little chromosome, about 2% of a father's genetic contribution to his sons, programs the early embryo to develop as a male. (2) It is transmitted from fathers only to their sons. A sperm will contain either an X, in which case the conceptus will be a female, or a Y chromosome in which case the conceptus will be a male. (3) Most of the Y chromosome is inherited as an integral unit passed without alteration from father to sons, and to their sons, and so on, unaffected by exchange or any other influence of the X chromosome that came from the mother. It is the only nuclear chromosome that escapes the continual reshuffling of parental genes during the process of sex cell production.
These three unique features mean that a male has a Y chromosome essentially identical to that of his father, and his father's father, and his father's father's father, and so on back indefinitely through all ancestral generations on the Y chromosome line, more succinctly called the Y-line. (For a recent more complete discussion of the Y line see this author's article: The Y chromosome in genealogical research. Nat. Geneal. Soc. Quart. 2000, Vol. 88, No.2, pp.122-143) But rare changes or mutations in the Y chromosome can occur over time. Therefore, over many generations, we develop altered molecular sites in our Y chromosomes. Some of these changes survive, and once they happen can persist over many generations or indefinitely. Thus, for genealogical research, we are fortunate that molecular variants in the Y chromosome occur, but that they are relatively rare.
Without the inherited variants (similar to being without variations in surnames) we could not derive or comprehend separate lineages. With the inherited variants we can provide evidence for genealogical relationships over a few or many generations. Very common molecular change or mutation would permit us only to identify connections between closely related families. Rare and unique changes, on the other hand, make it possible to identify connections into great antiquity well beyond historical records. Fortunately we have both kinds.
The Y line is also equivalent to the surname line in most cultures. But the term 'Y line' is more explicit in that it is defined as the true biological male to male line, uninterrupted by adoption, illegitimacy, or other non-paternal events. True, it is only a single ancestral line, and one that diminishes in importance to us by 50% in each generation as we continue back in our ancestral search. But it is a line for which we have a these molecular biological differences that we can identify and track among us living today.
It can be easily understood that just by chance alone a Y line could die out in succeeding generations. As a simple illustration, consider all permutations where a couple can have two children. They can have (A) two boys, (B) a boy and a girl, (C) a girl and a boy, or (D) two girls. The four permutations are nearly equally frequent. Already in the next generation, the Y line of Family D has become extinct. The Y line of Family A with two sons has a better chance initially of surviving than the Y lines of Families C and D. Over thousands of generations and thousands of years, all living males by chance will derive their Y line from a common ancestral male, Y Adam. This does not mean that we descend only from one male many generations ago, only that our Y line has one origin in one male many generations ago. In fact we as human beings probably descend from all or nearly all breeding males and females in that very small early human population. Y Adam was a real person, but probably was no different in appearance from any of the other males in those ancient populations. The Y chromosome implications do not mean that he was anyone special other than that today all human males through inheritance derive their little Y chromosome from him.
From anthropological and genetic evidence, it is believed that Y Adam lived in Africa about 144,000 years ago, give or take about 10,000 years. This was a time when the human population in Africa numbered about 2,000 breeding individuals and probably fewer than 10,000 total.
Let's take a specific question that might confront say the hypothetical Humpke Family. There is good evidence that those with this name came from only two different immigrating founders in the 17th century. Their European ancestry is expected to be German but is not known, and there is no evidence that they were in contact with each other after migrating to North America. But the Humpke Family Association, which admits members descending from both immigrants would dearly like to know if they are related, and if the Humpke name in these two families derives from a common male Y line ancestor. If all Humpkes come from one male who took the name as a surname, then we say the surname is "monophyletic," another useful term for genealogists.
So we contact one of the few commercial companies analyzing differences among Y chromosomes and obtain kits that provide a cotton tipped stick for one to take a swab of tissue from the inside of the cheek. A good scraping, which does not hurt, is sufficient for the company to make the analysis over several polymorphic sites. The Humpke Family Association gets three or four males of the name from one Humpke immigrant and three or four males of the name from the other immigrant, to get an inner cheek (buccal) sample, pays the money, and waits for the results. The results come back saying all eight men have identical or nearly identical molecular Y patterns (called Y haplotypes), thus confirming that all eight and thus including the two early immigrant Humpkes derive from the same Y line in relatively recent time. The conclusion is that the name is indeed monophyletic. The Y haplotypes are identical among these men or nearly so, and depending on closely they match, an estimate of the genealogical distance between the two Humpke immigrants could be made.
Going further, suppose one of the Humpke family visits Grupfenburg in Germany and is delighted to find a man living there named Humbke. Humpke thinks that possibly this Humbke could also be from this original family, and that the considerable number of Humbke individuals in the area of Grupfenburg suggests that this place is the early geographic origin of the family, if indeed it is one big family. Mr. Humpke then searches the phone books of Germany and finds nothing like the name Humpke or Humbke anywhere else than in the valley around Grupfenburg. He persuades three or four men there named Humbke to get their Y haplotype analyzed. The answer comes back that indeed they match the Humpke family haplotypes, not as closely as those among the descendants of the two Humpke American immigrants but nearly as closely. The conclusion then is that the two Humpke immigrants were closely related back in Germany, probably also coming from Grupfenburg or nearby and they in turn were both related a bit farther back with those of the Humbke Family. Another way of saying this is that the Humpke-Humbke name is monophyletic. The tourist Humpke also found a man named Von Humbke in Germany, got his Y haplotype analyzed and found he was nothing more than a Humbke. Resources permitting, one could run the study further and test other families with totally different surnames in and around Grupfenburg to see if there are other families with a similar Y haplotype. This haplotype from studies of other families might be shown to be so rare that it no doubt derives from this area of Grupfenburg, but in a few cases there are families with other surnames that have a very similar haplotype. Now we are showing relationships among families prior to the time of adoption of surnames.
With the Y pattern or haplotype analysis we have proved the common origin of the two Humpke brothers, and verified the genealogical work that connected the families. One must be aware that one or more Humpke members of the Humpke Family Association may have totally different Y haplotypes. One can only surmise that somewhere along their line from the immigrant to them, a non-paternity event occurred which did not break the surname line, but did break the genetic relationship. Non paternity events do not have to be scandalous. Adoptions are one such kind. But with DNA analysis, genealogists have to be prepared for these surprises.
A real study of this nature involving the Sykes family has been published in B. Sykes, and C. Irven. Surnames and the Y chromosome. Am. J. Hum. Genet. 2000;66:1417-1419. Another on the Mumma family is covered extensively on the Internet (http://www.mumma.org/DNA.htm) and the Cuni Family (http://www.paulcuni.com/family/dna/dna.htm) . Many further independent Y haplotype studies are underway including the Rice and Glennon Families of which you will soon read more.