GENETIC GENEALOGY, page 3
Risks and Privacy Concerns. The markers that are tested for family studies do not code for RNA or proteins and are thus unrelated to any physical or medical characteristics. This has two advantages for family history. First there is little danger if the privacy guarantees of the testing companies were somehow breached. Second, the markers are not subject to natural selection since they are unconnected to physical characteristics. Thus the markers have the values they do because your ancestor had those values--not because having a value of 12 on a certain marker gave your ancestor a reproductive advantage over the people who had a value of 13.
In addition, most sets of y-chromosome markers are not unique. Brothers, fathers, uncles, first cousins and even more distant relatives will all probably have identical y-chromosome markers. And even very distant relatives and non-related individuals (in a genealogical time-sense, since we are all related in the long run) may have identical markers. Thus the results of the y-chromosome test are of little use in making positive identifications in criminal or civil matters.
Many family studies do not include the names of participants in publications or public websites. Others do include names and email addresses, with the understanding and permission of the participants. Although there is little danger from the misuse of the actual DNA results, there are two issues, both of which may involve perception of misuse more than actual potential for misuse. First any public data that includes names of living individuals and their parents may cause concern among those worried about identity theft, since one's mother's maiden name is often used as a form of identity check, at least in the U.S.A. Second, individuals whose DNA does not match their known relatives may not want it known that there has been a biological break somewhere in their lineage. In our Bachmann study, one of my grandmother's distant cousins neither matches a close relative or more distant relatives known to come from the same line. Most likely there has been a "non-paternal event" somewhere in that cousin's lineage (an illegitimacy or more likely in this case, an unrecorded adoption). It is easy to imagine that someone might be embarrassed or unhappy to have this information prominently displaced with his name. To avoid possible embarrassing situations and as well as to reassure potential participants concerned about privacy issues, we have elected in our family studies to not display the names of living individuals or their parents, but to include only names from grandparents on back in displays of lineages.
The single largest potential risk of y-chromosome tests (unless one includes the costs) is the possibility that a participant will discover that he is not biologically related to someone else in the way expected. Illegitimacy rates vary by time, place, and economic and social status, but have always occurred. Adoptions have also always occurred, but are not always public knowledge, especially in times before birth certificates and vital record registration. Thus some unexpected non-matches can occur, although it is not always possible to know whether a non-match between two people believed to have a common ancestor indicates a "non-paternal" event or disproves the paper genealogy that led to the expectation of common descent.
Maternal or Matrilineal Studies.
Mitochondrial DNA (mtDNA) is found outside the nucleus of the cell and is thus independent of the chromosomes. Like the y-chromosome, mtDNA is not shuffled, and it is passed more or less unchanged from mothers to their children, both males and females. Males do not pass on their mtDNA, so it can only be used to study maternal lines. There are disadvantages to mtDNA in comparison to y-chromosomes as well as some advantages.
The mutations studied in mtDNA are base-pair substitutions, where one base in the chain of DNA is replaced by another. These mutations occur much less frequently than the changes in the number of repeats in y-chromosome short-tandem repeats (STR's)--although not as infrequently as base-substitution (SNP) mutations in y-chromosomes. In general, mtDNA is useful for long-term evolutionary studies. The research publicized in the book "The Seven Daughters of Eve" used mtDNA to classify all people of European descent into seven "clans" based on long-ago matrilineal ancestors. Some people have their mtDNA tested to learn about their long-term matrilineal background (which "clan" their maternal line descends from) rather than for genealogical purposes.
Because surnames are passed patrilineally in nearly all modern societies, mtDNA is not very useful in surname studies. It can be used to try to trace descendants or suspected descendants of a known female, but this requires that the paper genealogy be complete or nearly complete first. In general the most useful genealogical purpose for mtDNA is to try to solve specific puzzles or hypotheses about unproven relationships. In my own family, two women with the last name Hamman married two Ritters (brothers of my great-great grandfather) in 1839. A reasonable hypothesis would be that these two women were sisters and the daughters of Ritter neighbor Jacob Hamman (who had four daughters of approximately the correct ages in his household in 1830). An mtDNA test of matrilineal descendants of each of the two Hamman women could substantiate or disprove the possibility that they were sisters.
Males as well as females can be tested to determine their mtDNA. This is useful if a matriline has recently come to an end or is about to end for lack of daughters. For example my maternal grandmother (who had no sisters or maternal aunts who survived to adulthood) has no female great-grandchildren through females. But I and my sister's sons carry her mtDNA and could still be tested even if my mother and sister were deceased.
Unrecorded breaks in the matrilineal line of descent are less likely to occur than such breaks in a patrilineal line. Illegitimacy is rarely a problem, since there is rarely ambiguity about who was the birth mother in the same way that there may be questions about who was the biological father of a child. Adoptions of course may cause problems for both mtDNA and y-chromosome studies.
Finally, each cell contains many copies of mtDNA (usually thousands) but only one y-chromosome. DNA degrades rapidly, but the larger numbers of mtDNA make it more likely that it might be recovered in old or ancient samples. Thus mtDNA has been recovered from both Cro-Magnon and Neanderthal remains that are tens of thousands of years old, while it is currently very difficult if not impossible to recover complete y-chromosome sequences from such ancient samples.
Most of the major genealogical testing companies offer mtDNA tests. Rather than looking at the values of specific markers as with the y-chromosome, they generally sequence a portion of the mtDNA called the hypervariant region and then compare the results with a reference sequence and report the differences found. In addition, for a higher cost, at least one company will sequence the entire mtDNA chain, which is much shorter than the chromosomes found in the nucleas of cells.
Last Updated: Feb 2005; Copyright 2004-2005, Philip Ritter