LAB TESTS

1. Basics of DNA

One of the newest developments in genealogy is the use of DNA (deoxyribonucleic acid) as a source of genealogical information. DNA is the substance within every living cell that carries the code for passing on its exact makeup to new cells, and although DNA is uniquely different for each individual, it is similar in cells of related individuals. As applied to genealogical research, distinctive DNA patterns can be used to determine whether and how closely individuals are related to other individuals whose DNA patterns are known.

Genealogical DNA testing looks at the non-coding portions of the DNA strand (sometimes misleadingly called junk DNA) that have no known function. For the most part, these stretches of DNA remain unchanged from generation to generation. However, chance changes, called mutations or polymorphisms, do occur at infrequent intervals, and it is these changes that let us distinguish different lines of descent and determine how closely people may be related to each other from the closeness of their DNA matches. A DNA sequence that is passed on unchanged from one parent to a child is called a haplotype, and these are the distinctive patterns we use to establish genealogical links.

There are three main types of genetic ancestry test:

2. Y-chromosome DNA test

A Y-chromosome DNA (Y-DNA) test provides information about your male line ancestry only, which in most cultures corresponds with the inheritance of surnames. Only males carry a Y-chromosome, but a female can learn about her father line, for example, through her father or brother. Among the tests currently available there is much variety in the amount of information provided. The markers tested are of two types: STRs (short tandem repeats) and SNPs (single nucleotide polymorphisms). These markers have different mutation rates and so give information at different time depths. The information you receive depends on which and how many markers of each type are tested.

If two people have the same Y-DNA haplogroup, it means that they will usually share a common patrilineal ancestor more recently than two people from different haplogroups, but that common ancestor may still have been a long time ago. That time can be estimated, but such estimates are not precise with current standard tests, although comprehensive sequencing of the Y-chromosome is becoming available and will give improved precision.

3. Mitochondrial DNA test

A mitochondrial DNA test provides information about your female line ancestry only. Mitochondrial DNA is passed on by a mother to her male and female children but only females can pass their mtDNA on to the next generation (males are dead ends for mtDNA). This test, like the Y-DNA test, provides information about one specific lineage – your mother, your mother’s mother, your mother’s mother’s mother, and so on back in time. Again the amount of information provided varies among tests, but the mtDNA sequence is short (just 16,569 DNA “letters”) and so sequencing the whole mtDNA genome is already not very expensive.

As with the Y-chromosome, as you go further back in time your mtDNA represents a rapidly diminishing proportion of your total ancestry.

4. Autosomal DNA test

An autosomal DNA test provides information from the great majority of your DNA (the autosomes are the chromosomes other than the X, Y and mtDNA, and contain most of your DNA sequences, and genes). Although full genome sequencing is not far away, it remains unaffordable for most and autosomal DNA tests usually examine up to around 1 million genetic markers (SNPs) spread across the genome (1 million may sound a lot but there are over 3 billion DNA letters in the human genome, so it’s still a small fraction but the most informative sites are chosen).

Autosomal DNA tests can be used to identify individuals with whom you share one or more common ancestors up to a handful of generations in the past. This is done by looking for large chunks of DNA that you both share, indicating recent shared inheritance. Sometimes it happens that a large chunk of DNA is conserved in two individuals from a common ancestor more than 10 generations in the past, but this is rare: the great majority of common ancestors at that time depth will not be identified from the DNA of their descendants today. Although sharing one or more large chunks of DNA makes it almost certain that the two of you had at least one recent common ancestor, to ascertain the ancestor(s) is imprecise, particularly beyond about four generations ago. Also the tests have no ability to distinguish certain relationships: for example, using DNA alone the half-sibling relationship cannot be distinguished from the grandparent-grandchild relationship, and in the latter case we can’t tell from the DNA which is the grandparent and which is the grandchild. Algorithms that predict specific relationships are rarely precise beyond 1st degree, but they can identify more distant relationships approximately, with good accuracy out to about 2nd cousin, and the precise relationship may then be confirmed using additional information.

5. Genealogical Uses for DNA Tests

6. Additional Identity Item

For those ancestors at the head of an ancestral line, for whom we may know little more than a name and event date or place, a DNA sample from an appropriate descendant will provide the same pattern present in the ancestor, in the absence of any chance mutation along the way. For many family historians, a test of their own DNA is often their first step, providing a genetic signature for a distant paternal-line or maternal-line ancestor. Matching samples from two descendants through different lines provides assurance that the common ancestor’s DNA sequence descended unchanged, with no mutation in either line.

7. Verifying Probable or Suspected Relationships

Verifying relationships is perhaps the most frequent use being made of DNA, as tests can quickly determine whether any two men descend from a common ancestor through their all-male surname line or whether any two people of either sex are related through their all-female maternal lines to a common female ancestor. However, the number of generations to the common ancestor, if not known from other sources, can be only estimated. A widely publicized example of this application was the Jefferson-Hemings study. There were no sons from President Thomas Jefferson’s marriage, but DNA tests showed that a male-line descendant of his slave Sally Hemings shared the same DNA as descendants in two male lines from the president’s Jefferson grandfather, proving that a Jefferson fathered at least one of Hemmings’s children.

8. Sorting Family Lines

People with the same surname frequently come from very different ancestral origins. DNA can show which share a common heritage, can show which are unrelated, and, with enough samples associated with ancestral localities of origin, can point modern descendants to their family’s geographic origin. For example, there were four families named Smolenyak living near each other in the tiny Slovak village of Osturma, but DNA tests on male Smolenyak descendants from each of the four families showed they were unrelated through the surname line.

9. Family Reconstruction

Family and surname associations use DNA to confirm links in lines where records are ambiguous or less than convincing. Associations are also establishing previously unknown links of some members’ lines to known founder-ancestors. The Stidham Family Association sought proof that two lines, with problematic record links, truly descended from a seventeenth-century ancestor. DNA provided the assurance, but also revealed that another line, with clear documentary evidence of descent, was not biologically connected to the ancestor.

10. Future Promise

The DNA, called autosomal DNA, is widely used for forensic identification and for verifying paternity because individuals receive a full copy of DNA from each of their parents, which then pairs to form the individual’s DNA. During production of an individual’s egg or sperm, the paired DNA is randomly shuffled and recombined to create a combined version of a person’s parent’s DNA. This shuffled recombined copy is passed fully to the child. In each following generation, the genetic code is further randomly shuffled and recombined as DNA passes to a new generation. Small sequences of genes that pass unchanged over many generations are called haplotypes. A haplotype can occur when all the grandparents share an identical sequence of genes within a chromosome, so quite naturally no shuffling can occur on that particular segment for the resulting grandchild, because all the original combining segments were identical.

Most sections of one’s autosomal DNA represent a fully randomized mixture of unidentifiable DNA from your ancestors. The human genome consists of just over 3 billion DNA base pairs. But the shuffling process is very imperfect and oftentimes perfect, unshuffled, duplicate copies of DNA pass from a grandparent to parent to child. Over time homogenized groups of people who are relatively isolated also can come to share duplicated haplotype DNA sequences among the related population. In Tibet, for example, it was recently discovered that the Tibetan population shares a common, duplicated gene sequence that gives them resistance to high altitude cold weather. This haplotype sequence, unique to Tibetan’s, is believed to have occurred fairly recently.

Small haplotype sequences can be inherited from a relatively small number of unknown ancestors among the thousands we had tens of generations back. But newer haplotype sequences can also be inherited from more recent ancestors. Large, stable populations tend to result in a diversity of haplotype gene sequences. Autosomal DNA is likely to find more uses in genealogy as a result of research now underway to identify inheritance patterns for haplotype segments in the DNA of the recombining chromosomes.