Scott FamilyTreeDNA Project
A Brief Analysis of Scott DNA Test Results
The following discussions make some necessary simplifications so that things don’t get too technical. For anyone who might want to read a more comprehensive explanation of how all this all works, go to http://nitro.biosci.arizona.edu/ftdna/TMRCA.html
First, let’s review what the individual tests show, why there are differences in some of the sets of numbers displayed for each person, and what these differences mean.
The individual test results displayed in the table show the values of each person’s DNA for a set of Y chromosome genetic “markers” (DYS ID#) that are used by FamilyTreeDNA Inc. for establishing probable ancestral connections between individuals. Basically, the better the “match” between individuals, the higher the probability that they have a common ancestor, and the more recent that common ancestor might be, although it’s not quite as cut and dried as that. As we shall see in a moment, both the number of markers included in the test, and the differences between them for any two individuals have a great bearing on the practical usefulness of the results in establishing a common ancestor.
It is important to understand up front that these tests cannot identify a common ancestor. But they can serve to suggest which individuals probably do (or do not) have a “recent” common ancestor, and help prove or disprove other information that may point to him. And we say “him” because these tests are for genetic values of the Y chromosome, which is passed down only through the male line of a family.
The values of these genetic markers can and do change (mutate) slowly over generations, which is why individuals even with a relatively recent common ancestor can have slightly different values for one or more of them. Since the genetic mutations of any given marker are random (that is, they don’t happen every x generations), we cannot predict exactly how often they occur. However, geneticists have been able to calculate the mean probability that a mutation will occur in any one generation, and this provides a way to estimate the time back to a “most recent common ancestor” (MRCA) based on the number of observed mutations (that is, the number and size of differences in the values of the markers tested) by which the Y chromosomes from two individuals differ. As one uses more and more markers, the probability distribution becomes tighter and tighter about its mean value, and estimates of the number of generations back to a MRCA have higher precision.
But keep in mind that since the estimates are based on probabilities, there are no absolute certainties. This is especially important when comparing individuals using only 12 marker test results. In this case, even a “perfect” 12 out of 12 match cannot, by itself, be interpreted to mean the two definitely have a recent common ancestor. Other evidence is required to validate this determination. In a study of 1386 DNA test participants representing 66 surnames, 971 individuals were found to share a unique 12-marker combination, but half of these shared it with someone with a different surname! And over 10% of the participants who had an exact match with another individual in the 12 marker test had a match of less than a 23 out of 25 in the 25 marker test, strongly suggesting that any common ancestor was many, many generations in the dim and distant past.
It is obvious from our observation of 1000's of samples that some markers change or mutate at a faster rate than others. While that actual 'faster rate' has not yet been definitively calculated, not all markers should be treated the same for evaluation purposes.
The markers in red have shown a faster mutation rate then the average, and therefore these markers are very helpful at splitting lineages into sub sets, or branches, within your family tree.
Explained another way, if you match exactly on all of the markers except for one or a few of the markers we have determined mutate more quickly, then despite the mutation this mismatch only slightly decreases the probability of two people in your surname group who match 11/12 or even 23/25 of not sharing a recent common ancestor.
When you compare a 12 Marker result to another 12 marker result of someone with the SAME surname, and the results match 12/12, there is a 99% probability that you two are related within the time frame included in the MRCA tables. If the match is 11/12, there's still a high probability that you are related if the 11/12 match is within the same surname. If you compare a 25 Marker result to another 25 marker result for the SAME surname, and the results match 23/25, 24/25 or 25/25, then there is also a 99% confidence that the two individuals are related…and at a much closer time interval then with the 12 marker test.
If two 12 marker results match for two participants with the same surname, and the genealogy research shows a common ancestor, the DNA test has validated the research and proven that the two descendents are related. In this example, you have two items of evidence to support that the individuals tested are related…a documented paper trail and the DNA results. In addition, the research provided a precise time frame for the common ancestor.
Without the genealogy research, and where 2 participants with the same surname match on the 12 marker test, then the scientific answer to the degree of relatedness is that 50% of the time the common ancestor would have occurred within 14.5 generations, or within approximately 360 years. The range of generations for the common ancestor extends to 76.9 generations, or almost 2000 years for those cases where there is not a surname in common. Therefore the importance of a surname link is paramount to provide a comfortable conclusion of relatedness. Most of the time random matches with people with different surnames do not stand the test for extended DNA testing.
II. INTERPRETATION OF SCOTT TEST RESULTS
Results from the Scott DNA project have been divided by Haplogroups. Haplogroups are a classification tied to deep ancestry (think 10,000 or 10's of 1000's of years).
J & J2 Haplogroup
J is found at highest frequencies in Middle Eastern and north African populations where it most likely evolved. This marker has been carried by Middle Eastern traders into Europe, central Asia, India, and Pakistan.
J2 originated in the northern portion of the Fertile Crescent where it later spread throughout central Asia, the Mediterranean, and south into India. As with other populations with Mediterranean ancestry this lineage is found within Jewish populations. The Cohen modal lineage is found in Haplogroup J2.
R1b Haplogroup R1b is the most common haplogroup in European populations. It is believed to have expanded throughout Europe as humans re-colonized after the last glacial maximum 10-12 thousand years ago. This lineage is also the haplogroup containing the Atlantic modal haplotype.