In 1588, the Spanish Armada (also called the Invincible Armada) was launched by
Philip II of Spain in an attempt to overthrow the Protestant Queen Elizabeth I and
gain the throne of England for himself. After several skirmishes in the English
Channel, the Armada was finally shattered when the English set fire to several of
their own ships and sent them into the Armada’s anchorage off Calais, causing it to
break formation and scatter. The Armada was subsequently defeated by a close
range attack at Gravelines. Thanks to a change in wind the Duke of Medina was
able to lead the Armada north around Scotland and along the west coast of Ireland
in his escape. But the terrible weather he encountered was too much for the fleet;
many of the Spanish ships were destroyed and many sailors drowned.

But is it possible that of the few survivors, a sailor made it to shore in County
Clare? Is it possible that this sailor changed his surname to mask his identity and
settled for the rest of his life in this area of western Ireland? Is it possible that he
married and had a son whose male descendant's genetic profile would confound a
DNA study of this "Irish" surname over 400 years later?
* * * * *
As any fan of the popular television shows CSI, Forensic Files, and Medical
knows, DNA can be a powerful tool in solving a mystery. Just as a trail
of blood that is left at the scene of a crime can be analyzed to narrow down the
possible suspects, the trail of Y-chromosome DNA left by an ancestor along the
line of his male descendants can be analyzed to narrow down who he might have

The chapter The DNA Detective has something for researchers of all levels of
interest. For those who don’t sweat the details, there is an introduction to the
basics of DNA-based genealogy. For those with intermediate or advanced interests,
there is more detailed information on DNA markers, surname studies, haplogroups,
and clades later in the chapter. There are also step-by-step instructions for
generating cladograms from your DNA study results, and an explanation of
pairwise mismatches.

Most importantly, The DNA Detective shows that DNA is not just a tool for
identifying which participants in a surname study are closely related or for
determining which clan mother you descend from along your exclusively female
line. This chapter shows you how DNA can be used to tie your family to world
history, how it can give you the geographical location your ancestors came from in
recent centuries, and how it can reveal unsuspected liaisons between seemingly
unrelated families through nonpaternity events. There is as much information as
you probably will want to know about all of these topics and more in The DNA
Cladogram of the Fitzpatrick DNA study.

Yellow circles represent the DNA profiles or haplotypes of one or more participants.
Red circles represent individuals whose DNA profile is not yet included in the study.
Red labels represent markers where a mismatch exists between haplotypes.

Click on thumbnail to see a larger image.
For those of you already involved in a DNA study of your surname, here are two
Excel spreadsheets you can use to calculate your Most Recent Common Ancestor
(MRCA). One is based on the binomial expansion and the other uses the Poisson
distribution for calculating the probability that two people are related based on the
number of markers tested, the number of mismatches observed, and the mutation
rate. These spreadsheets allow you to experiment with the values of each of these
three variables, so that you can gain insight into how they affect the results of the
calculations. A discussion of the statistical basis for the calculations is included in
the chapter The DNA Detective.

For those of you who are not interested in the details of the calculations, The DNA
Detective chapter includes lookup tables you can use to determine the probability of
your MRCA based on the number of markers tested by the most popular DNA
testing laboratories.
MRCA Spreadsheet Based on The Binomial Expansion

MRCA Spreadsheet Based on the Poisson Probability Distribution
The DNA Detective