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.

         For many years I have been interested in the history of my family.  The advent of computers has sped up this process and recently a new tool has been added to the process–DNA analysis.  

           Recently when I was in Texas, I noticed that the genealogy web site, Ancestry.com, had a half price sale on their DNA tests.  I had been wanting to perform these tests for a long while but was unwilling to shell out the big money to do it.   I broke down and ordered the tests–the Y chromosome and mitochondrial DNA test for me and the mitochondrial DNA test for my father.

          There are several other companies that offer the most common tests being done at this time–the Y chromosome test and the mitochondrial DNA test.  The Y chromosome is of course the chromosome that determines maleness.  Males have the X and Y sex chromosomes, whereas females have two X chromosomes (XX).    During the formation of the eggs and sperm (meiosis),  all the so called homologous chromosomes which contain the same basic type of DNA, pair up and often exchange segments of their DNA (crossing over).  This results in the changing of the base sequence which is the genetic code. 

          The Y chromosome is a small chromosome and crossing over with the X chromosome normally doesn’t occur as often as it does with the other pairs of chromosomes (Check out my entry on June 21, 2007 for more information on the Y chromosome).  As a consequence the sequence of bases on its DNA doesn’t change around as much as other chromosomes.  Any changes in the base sequence usually comes about by mutations, or small changes in the base sequence.

          The rate of this mutational change can be estimated, and since the genetic code on the Y chromosome is not as subject to natural selection as is the code on the other chromosomes (autosomes), this mutation rate can be used to estimate how long ago a change occurred when comparing the Y chromosomes of two different men.  Also the Y chromosomes can be compared to see how closely related two men are.

          The mitochondrial DNA (mDNA) is a whole ‘nother creature.  Whereas the nuclear DNA is passed from one generation to the next via the eggs and sperm (gametes) and controls the various genetic traits through the synthesis of enzymes, the mitochondrial DNA is not found in the nuclear chromosomes but in the mitochondria which is a small structure (organelle) in the cell which is responsible for the aerobic reactions of cell respiration–combining food with oxygen to produce lots of energy.

          The mitochondria has a fascinating history, and is believed to be a once free living bacteria, partly based on it’s loop-shaped chromosomes, as opposed to the rod-shaped chromosomes found in us and other Eukaryotic creatues.  Check out the endosymbiotic theory when you have time.

         Once again, like the Y chromosome, the DNA in the mitochondria is not that subject to the influence of natural selection and it does not go through a shuffling process (independent assortment) or crossing over as the nuclear chromosomes do, thus it will not change unless a random mutation occurs.  Apparently the mutation rates in mDNA is higher than the nuclear DNA which makes it useful in comparing lineages.

       The Y chromosome is passed from father to son and can be used to determine information about a male’s paternal ancestry.  The mDNA is found only in the area outside of the nucleus, thus sperm can not pass on mitochondria from the father since only the head of the sperm (containing half the normal number of chromosomes) enters the egg at fertilization.

       The egg (oogonium), on the other hand being a large cell, contains mitochondria which are passed on to the cells of the resulting children after fertilization.  Thus the mDNA is only passed from mother to offspring of both sexes.  Therefore, the mDNA is useful for determining information on the maternal ancestry of a person.

          I am now receiving the information on these tests and will in the coming days talk about the results.

  I couldn’t resist including this model of DNA copied from Wikipedia