Posted By crates on October 7, 2008

See September 8 blog for an introduction to this topic. 

          As I talked about before,  the Y chromosome is only passed down from father to son, and can be used to find out information about a male’s paternal ancestry.  This chromosome contains about 58 million base pairs containing 86 genes or 27 depending on whom you read.  Except for small regions at the end of the chromosome, it does not cross over with the X chromosome.  It is this non-recombining area of the Y chromosome which is used to trace the paternal ancestry.

As shown by this really neat revolving model copied from Wikipedia, the DNA molecule is a double helix made up of building blocks (nucleotides) linked together.  This is shown in more detail below.  The “rungs” of the ladder consist of two bases with the following combinations: A-T, C-G.  It is the base sequence that actually form the genetic code.  When active the DNA molecule splits open and the genetic code is read from one side of the molecule only and consists of various combinations of the four bases: A, T, C & G.  These four bases make up the genetic code which controls the  construction of enzymes which controls pretty much everything going on in the cell.

              As mentioned above there is little crossing over between the female X chromosome and the Y chromosome as happens between the non-sex chromosomes (autosomes).  This results in the Y chromosome being passed down relatively unchanged from father to son.

     There are certain markers on the Y chromosome that have been identified.  These markers occur at certain known locations (or loci) on the Y chromosome, and their characteristics are known.  The markers generally consist of what is called Short Tandem Repeats (STR) which are made up of several bases that are repeated several times. 

        For example one marker may be made up of the following repetition of three bases (G, A & T): GATAGATAGATAGATAGATA.  This marker would consist of five repeats of GATA (guanine, adenine, thymine and adenine).

    Now these markers on the Y chromosome would be passed on from father to son unchanged if there wasn’t the fact that random changes, or mutations, occur at a rate that can be predicted.  These mutations often consist of an insertion or deletion of these repeats at these markers.  Thus a deletion of the above example would be: GATAGATAGATAGATA, consisting of four repeats.

    The test that I had done on my Y chromosome compared 46 markers.  The results came back looking something like this:

DSY19a     DYS385a <—Marker names 

 14                11        <—number of repeats

      DYS stands for: D=DNA; Y=Y chromosome; S=a standardized Segment of DNA.  The number with the DYS stands for a standardized location on the Y Chromosome.

          So by comparing these markers between different males (and using lots of statistics that are obscure to me), it can be determined how closely related the two people are.   For example if there is no difference in these markers between the two people then they almost certainly share a common ancestor.   What is usually calculated is Time to Most Recent Common Ancestor or TMRCA.

Different variations are grouped into Haplogroups.  My paternal haplogroup was R1b which is one of the most common groups in Western Europe.  It supposedly entered Europe 35,000 to 40,000 years ago.  About half of the men today of European descent belong to this group.

     This information is so generalized as to be useless in determing one’s ancestral history, but they do offer a way to compare your results with others in order to possibly find out relationships.

Categories: genealogy, Genetics, nature
Tags: DNA, DNA testing, haplogroup, nature, R1b haplogroup, Short Tandem Repeats, STR, y chromosome