Genetics

 

Y-DNA

 

A son gets his Y-DNA from his father, and with the exception of occasional random mutations, this will be an exact copy of his father's Y-DNA. All human males have similar Y-DNA, but there will be many small differences due to genetic mutations. Y-DNA tests focus on the parts of the Y-DNA with the greatest potential to differ, and uses these to construct a characteristic "fingerprint". Individuals with similar fingerprints are highly likely to share a common recent patrilineal ancestor.

 

Y-DNA testing is perfectly suited for one name studies, but the tests can be taken only by males. They are by far the most useful DNA test for pre 1700 genealogical research. This section describes the available tests in detail, together with the types of deductions which can be drawn from them.

 

For illustrative purposes, the specific tests offered by FamilyTreeDNA.com (FTDNA) will be discussed. Currently this is the dominant company in Y-DNA genealogy testing.

 

Potential complications

 

The use of Y-DNA for surname studies typically relies on descendants sharing the surname of their common patrilineal ancestor. This is expected on the Isle of Man (as in many other countries) since surnames are normally passed from father to son. There are occasions in which this doesn't happen though (including because of adoption, or illegitimacy) which are described as non-parental events (NPEs). FTDNA estimates the probability of an NPE as between 1% and 3% per generation. John Creer estimates the probability as being lower for the Isle of Man (at about 0.5%).

 

Y-STR tests

 

DNA contains sections in which certain short sequences are repeated multiple times. These repetitive sequences occur in fixed places within the human genome, but the number of repeats each contains can change through mutations. Such sequences are known as Short Tandem Repeats (STRs) and pre-determined sets of them (chosen have high mutation rates) are used for the tests. Each individual STR functions as a genetic marker, and the result of a Y-STR test is a set of numbers corresponding to the repeat lengths of the selected basket of markers.

 

An example of how FTDNA presents an Y-STR result is given below. In this case 12 STRs have been tested. The table shows the scientific name for each marker, and the corresponding repeat count which was identified during the test. Two of STRs are connected and are therefore grouped together under the same label, the corresponding values are commonly denoted DYS385a and DYS385b.

 

Marker	DYS393	DYS390	DYS19	DYS391	DYS385	DYS426	DYS388	DYS439	DYS389I	DYS392	DYS389II
Value	13	25	15	11	11-17	11	13	11	14	11	30

 

To make use of a set of Y-STR results they need to be compared with those of others. FTDNA has a large database of hundreds of thousands of Y-STR test results, and automatically looks for near or exact matches with each of these. The aim of these comparisons is to identify those people with a common patrilineal ancestor within the genealogical period (say 25 generations). The accuracy of the matches increases with the number of markers compared, but can still be somewhat hit or miss. One problem is there are large groups of men in some haplogroups (i.e. sharing a common patrilineal ancestor thousands of years ago) and the comparison struggles to differentiate all of these distant relatives from more recent ones. Perhaps because of this, FTDNA is rather conservative in setting its criteria and some "recent" relatives may be excluded.

 

In most cases two men sharing a recent patrilineal ancestor will also share the same surname. FTDNA hosts thousands of single surname Y-DNA projects in which the Y-STR results of corresponding individuals are grouped together. This makes identifying matches much easier, and also enables follow-on analysis to partition family groups into individual lines of descent. More detail about how to analyse and interpret the results is given in the appropriate section of this site.

 

The Y-STR specific tests currently offered by FTDNA are the Y37 (which tests 37 markers) and the Y111 (which tests 111 markers, including the pre-mentioned 37). The Y37 is generally sufficient to prove a link with others with the same surname, but other connections generally need more markers.

 

Y-SNP tests

 

The human genome is made up of billions of individual base pairs, each of which contains a small amount of information. Each generation, a tiny proportion of these of these will mutate and take on new values. This type of mutation is known as a single nucleotide polynomism (SNP). A Y-SNP test looks for differences (mutations) between the tester's Y-DNA and the reference human Y-DNA chromosone.

 

The FTNDA Y-SNP test is called BIG Y-700 and it is an enhanced version of a previous test called BIG Y-500. It works by explicitly testing for about a million possible individual Y-DNA SNPs. The results returned will show each SNP found, and there are likely to be several thousand of them. FTDNA will also show any matches with other testers (those with similar or identical Y-DNA SNPs).

 

For the purpose of matching, and proving ancestral links, the Y-SNP test is very powerful. Although it is possible for the same SNP mutation to occur independently in different people, clusters of SNPs will almost invariably be unique to a specific ancestral line. In this respect, Y-SNP markers are superior to those from Y-STR.

 

When trying to reconstruct the relationships between several people, it will typically help to combine Y-SNP and Y-STR results. On average, it is estimated that a new SNP (which is detectable by this test) will occur about every three generations on average. A Y-STR change could easily have occurred within those missing generations STR results will therefore provide greater resolution. The main FTDNA Y-SNP test (Big Y) is significantly more expensive than their Y-STR offering, but also incorporates Y-STR testing. Another consideration is that although there are currently far more individuals with Y-STR results than Y-SNP results, and that provides a much larger comparison pool.

 

There is a much cheaper test which will allow a Y-DNA sample to be checked for a specific SNP. This is only likely to be of use if testing whether an individual sharing patrilineal ancestry with a specific pre-determined group of individuals.

 

More detail about the Y-SNP results and their interpretation is provided in the relevant section.