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                                      Theory Behind Interpretation of yDNA Profiles in Genealogy

DNA is found in every cell of the body.  Two copies of the entire DNA molecule are found in each cell nucleus, more specifically the nucleolus, and called nuclear DNA.  This DNA contains 22 pairs of autosomal chromosomes and one pair of sex chromosomes.  A child’s autosomal chromosomes are a 50/50 random mixture of the autosomal DNA received from each parent.  However, each child receives a completely intact sex chromosome from each parent, an X chromosome from the mother and either an X(female, xDNA) or Y (male, yDNA) chromosome from the father.  The chromosomes are made up of genes (coding region) and noncoding regions between the genes.  The latter includes activator/deactivator sites that turn genes on and off and many other areas of largely unknown use.  All of the genes within the chromosomes control the genetic characteristics that will be manifested by that person. 

Another form of DNA is found in mitochondria and is called mitochondrial DNA (mtDNA).  The mtDNA contains only that small part of the DNA that controls oxidative metabolism in the cell.  There are many copies/strands of mtDNA in each cell related to the oxidative capacity of the cell.  Endurance training will increase the oxidative capacity in the active cells and thus the amount of mitochondrial material and mtDNA in the respective cells.  The mtDNA comes only from the mother to all of her children, but only daughters pass it on to grandchildren.

The earliest humans had DNA profiles that they received from their ancestors. Generally the sex related DNA is resistant to change and is passed on to children pretty much intact.  However, with such complex molecules that are split and recombined during conception, there are bound to be some mistakes which are called mutations.  The mutations are cumulative so a person will tend to have all of the mutations from all of his ancestors.  There are two types of mutations that are useful in genealogy: 1. Short Tandem Repeat (STR) and 2. Single Nucleotide Polymorphism (SNP pronounced snip).  

A STR is a short segment (allele) of the DNA backbone that is repeated a variable number of times in a row.  On average, the number of alleles for a particular segment randomly change (mutate) about once in 500 births although some mutate faster and some mutate slower.  That is what I meant above when I stated that the sex related DNA is resistant to change.  There are many STRs and some of them have been identified as very useful in genealogy because they tend to mutate faster.  They are called markers and testing identifies the number of alleles for each marker.  Since each marker mutates about once in 500 births, two markers would be expected to mutate a total of about two times in 500 births or on average about once in 250 births (500/2).  With 5 markers you would expect a mutation every 100 births (500/5).  The standard tests look at 12, 25, 37, 67, or 111 markers with an expected mutation collectively every 42, 20, 14, 7, or 5 births, respectively on these tests.  On the Big Y-500 where up to 500 markers are tested, you would expect a mutation collectively up to one with each birth (500/500), although it more likely would vary from 0-2 for each birth. However, cost is a factor, so a compromise is to measure at least 37 markers in the Landry DNA Project.  Some Surname Projects and special purpose projects recommend 67, 111 markers or in some cases the Big Y-500 because there are many lineages with the same surname, location, or ethnic group.  So with a 37 marker yDNA test you would expect only a few mutations in 20 generations.  My second cousin and several other members of the Rene le jeune Landry group appear to have the same profile as Rene le jeune Landry probably had and several others including me only differ by one mutation.  So if you want to determine your paternal male ancestor 10-20 generations back, this kind of test is the gold standard.  FTDNA is the only commercial company that does these tests.  If you are a Landry male, the line in which you fall should be obvious from the yDNA test results.  See the group profiles in “DNA Results.”  There are two major lines: 1. Guillaume Landry and 2. The Acadian Landrys: Rene l’aine Landry and Rene lejeune Landry (2rd  cousins or farther removed.)  There are many differences between the Guillaume Landry profile and the Acadian Landry profiles.  Descendants of the two Renes differ only on two markers: DYS389i and DYS389ii which actually are coupled and represent a single mutation.  If you deviate by more than two or three steps on the 37 marker test, you may not be in these lines.  At  this point in time you will be in the ungrouped list until at least one other tester has a similar profile.  Your yDNA STR profile likely will only help you to identify your ancient ancestor - not your entire lineage unless you are extremely lucky, i. e. find a perfect match with a recent unique STR mutation and a known pedigree.  After you learn your ancient ancestor, then do an atDNA test (e. g. Family Finder) and look for matches.  Female to female lineage theoretically could be studied in a similar fashion (xDNA), but practical issues such as name changes and poor records make it extremely difficult.  Females are left with finding a brother or other direct surname male for a yDNA test or to test themselves with an atDNA test (Family Finder or Full mtDNA [for deep ancestry].)

To understand SNPs requires a little knowledge of the DNA backbone.  DNA is a very long double stranded helix, i. e. two long strands arranged in a barber pole fashion containing four types of units called nucleotides: Adenine (A), Cystosine (C), Guanine (G), and Thymine (T).  The nucleotides typically are coupled as A-T or C-G.  The two strands are held together by an A on one strand with a T on the other strand or a C on one strand and a G on the other strand.  A mutation occurs when an A or T is replaced by a C or G or vice versa.  SNPs develop much less often than STRs, e. g. generally 1,000 years to 10,000 years for one mutation.  Therefore SNP testing generally is used for studies beyond 20 generations, i. e. deeper ancestry, and haplogroup determination, especially the latter. As you will see below, SNP testing also is used for more recent matching.  The SNPs are sequential in occurrence.  In other words, A occurs before B which occurs before C.  If B is present, A will be there also.  If C is there, then A and B likely will be there as well.  In this example A, B, and C represent SNPs. 

There are multiple levels of SNP testing.  Recall that there are two sources of DNA: nuclear and mitochondrial.  If you want to determine your paternal line, then the test will use yDNA found only in nuclear DNA.  If you want to test maternal lineage, then mtDNA will be necessary.  A general DNA test uses the atDNA found in nuclei.  All major DNA testing companies use atDNA and some combine it with the sex DNAs as well.   The reports may or may not include paternal and/or maternal haplogroups (but not deep ancestry) as well.   Most companies have only one DNA test. The 23andMe Test includes health related SNPs (at an extra cost) which are not included by the other companies.  FTDNA has a wide range of SNP tests including single SNPs, many targeted groups of SNPs, Family Finder, BIG Y-500 (now 700), and Full mtDNA tests.  For most people the Family Finder Test (FTDNA’s version of the atDNA test) is the appropriate SNP test assuming you know in which general line you belong (yDNA results).  Your results will be compared with others for close matches.  Hopefully your matches will have posted their pedigrees and you will find common ancestors.  You may need to communicate with them to see their tree.  In this way you can add to your family tree.  This process assumes that your match has an accurate tree.  After getting your SNP profile, you look for matches who know their lineage.  You can look at matches from your account and FTDNA will email you with potential matches (incessantly and redundantly.)  When you find a match in a quality database with a common ancestor, you are on your way to identifying your lineage. You will need to work both backward and forward to complete your lineage.  Also you can look at my website for your lineage: .  It is quite comprehensive.  It requires a free subscription as a way to reduce hacker access.  Or you can ask me. (

The Big Y 700 includes 700 STRs and about 70,000 SNPs but is rather expensive.  If you want to study your deep paternal ancestry, this is the test to order.  If one or two people in your lineage have ordered this test, then you will not learn much additional information.  For example, four Acadian Landrys have Big Y-700 results showing that they are in Haplogroup BY53580 with an origin in France.  Probably all of the descendants of Rene le jeune Landry and perhaps the descendants of Rene l'aine Landry should be Haplogroup BY53580 as well.  Stay up to date by periodically checking the “DNA Results” page showing everybody’s yDNA profile including Haplogroup.  Nobody in the Guillaume Landry or Jean Jacques Landry groups has Big Y-500 results yet.

For goals of this website, click on "Goals" at the top of this page.

To see a discussion of the results,  table of family trees for males, stripped down pedigrees, and Acadian Landry Haplogroup Tree, click on "Results" at the top of this page.

To make a donation to the Landry Surname yDNA Project

   Donations will be used to help Landry individuals defray the cost of lab work - particularly those with a unique documented pedigree.


                                               Other Landry Websites  My Landry database with over 163,000 individuals and over 52,000 individuals with a Landry or Landry derivative surname.  Requires a free subscription to prevent hacker access.   An extensive Landry genealogy database maintained by Marcel Walter Landry (text is in French but labels can be set in English.)  Contains copies of numerous original records. (very much worth the effort) profiles of many Acadian women including Antoinette Landry and Perrine Bourg