Y-DNA Basics

Like surnames (at least in the modern era) and status as a Levite or a Cohen, Y-DNA is passed down through the direct male line. Based upon their Y-DNA, men may be grouped into different Y-DNA haplogroups, which reflect major branches of the human family tree that date back tens of thousands of years ago.

Within Y-DNA haplogroups, men may be further sorted into subgroups comprised of men who are more closely related to each other, perhaps sharing a common ancestor who lived thousands of years ago. Those subgroups may be divided still further, into groups of men (“clusters”) who likely shared a common ancestor who lived centuries ago.

Two aspects of Y-DNA may be tested for genealogical purposes: (1) Short Tandem Repeats (“STRs”); and (2) Single Nucleotide Polymorphisms (“SNPs”).

STRs 

STRs are patterns of the DNA bases (adenine, cytosine, guanine, and thymine) that repeat a certain number of times at a particular locus on the Y chromosome. STRs are typically reported in terms of marker values (e.g., the term “DYS393=13” is used as shorthand to identify an STR repeating 13 times at the locus DYS393).

STR markers mutate randomly over time. Some markers mutate very slowly (e.g., at a rate of less than 0.01% per generation), while other markers mutate more rapidly (e.g., at a rate in excess of 3% percent per generation). As a result, some men who share a direct male ancestor who lived 500 years may have exactly the same STR markers over 67 or 111 markers, while other men who share the same direct male ancestor who also lived 500 years ago may differ on five or more marker values over 67 or 111 markers.

Certain patterns of STR marker values are characteristic of men who share a relatively recent direct male ancestor. Based upon a man’s pattern of STR markers, he may often be assigned not only to a haplogroup but also to a cluster and subcluster.

Since about 2000, STR testing has been commercially available for genealogical purposes. The price of STR testing has steadily declined over the past several years, as technology has advanced, and, as a result, Y-DNA testing has become increasingly popular.

Several companies offer testing of STRs. Family Tree DNA currently offers STR tests at 12 markers, 25 markers, 37 markers, 67 markers, and 111 markers. The more markers that are tested, the greater the ability to determine how closely men are related to each other.

Except in unusual circumstances, 12-marker test results will be sufficient to identify the haplogroup to which a man belongs. Upgrading those test results to 25 markers or 37 markers will generally be sufficient to identify subgroups of men who share a common ancestor within the past millennium. STR test results at 67 markers or 111 markers will allow one to determine, with some degree of certainty, how many centuries ago tested men share a direct male ancestor.

When men have chosen to make their STR test results publicly available, those results may be found on a variety of publicly available websites, including FTDNA project pages, the
YSearch website, and the Semargl search engine.

Computer programs are available that can calculate approximately how long ago the shared direct male ancestor of a group of men lived, based upon the variance in the men’s STR markers. 
 
SNPs 

Y-DNA SNPs are changes in a man’s DNA sequence at a specific locus on the Y chromosome. Y-DNA SNPs are typically (but not always) reported in terms of one or more letters followed by a number (e.g., Z2122 or CTS6), with a plus sign used to indicate that the man has the SNP and a minus sign used to indicate that the man does not have the SNP.

Y-DNA SNPs mutate considerably more slowly than STRs. As a result, Y-DNA SNPs are very useful for delineating Y-DNA haplogroups, subgroups, and clusters, and for identifying which subgroups and clusters are most closely related to each other. Put otherwise, Y-DNA SNPs may be used to generate a tree identifying which clusters of men are most closely related to each other, and when each cluster likely branched off from each other.

Y-DNA SNP testing has become commercially available for genealogical purposes more recently than has STR testing.  As discussed in more detail here,
Family Tree DNA, Full Genomes Corp., Scotlands DNA, and the National Genographic’s Geno 2.0 project all offer products allowing men to test a large number of Y-DNA SNPs.
2012 Israel Levite stamp

February 2012 Israeli stamp

1955 Levi stamp
1955 Israeli stamp from 12 Tribes
of Israel set