Fergus(s)on DNA Project

Interpreting Your Results

This is an example of comparing and analyzing the DNA of three persons known to be related. The table below compares their DNA haplotypes, i.e. the sets of numbers used to describe their DNA:

D
Y
S
3
9
3

D
Y
S
3
9
0

D
Y
S
1
9
/
3
9
4

D
Y
S
3
9
1

D
Y
S
3
8
5
a

D
Y
S
3
8
5
b

D
Y
S
4
2
6

D
Y
S
3
8
8

D
Y
S
4
3
9

D
Y
S
3
8
9
-
1

D
Y
S
3
9
2

D
Y
S
3
8
9
-
2

D
Y
S
4
5
8

D
Y
S
4
5
9
a

D
Y
S
4
5
9
b

D
Y
S
4
5
5

D
Y
S
4
5
4

D
Y
S
4
4
7

D
Y
S
4
3
7

D
Y
S
4
4
8

D
Y
S
4
4
9

D
Y
S
4
6
4
a

D
Y
S
4
6
4
b

D
Y
S
4
6
4
c

D
Y
S
4
6
4
d

D
Y
S
4
6
0

G
A
T
A
H
4

Y
C
A
I
I
a

Y
C
A
I
I
b

D
Y
S
4
5
6

D
Y
S
6
0
7

D
Y
S
5
7
6

D
Y
S
5
7
0

C
D
Y
a

C
D
Y
b

D
Y
S
4
4
2

D
Y
S
4
3
8

Colin_Ferguson

Ross_Ferguson

Bruce_MacMillan_Ferguson

Distance from reference:

Zero

One

Two

Three+

If you look carefully you will see that Bruce differs from Colin at DYS390. Since this is the only marker in which they differ, the genetic distance is said to be one; i.e. GD = 1. A genetic distance of 1 for 37 markers is a close match from which one would conclude that Bruce and Colin share a common ancestor. If you examine Ross you will see that he also differs from Colin but at GATAH4; in this case GD = 1.

Bruce, Colin and Ross are known via genealogy research to share a common ancestor within four generations. The table below shows how the line splits:

The vendor used for DNA analysis in the project is Family Tree DNA. Each participant is provided with a personal webpage for accessing and managing their results. One tool provided is known as the Family Tree DNA Time Predictor. When this tool is used to compare Colin and Ross one obtains the result shown below:

FTDNATiP Report
In comparing 37 markers, the probability that Colin Ferguson and Ross Ferguson shared a common ancestor within the last...
1 generation is 12.99%	2 generations is 29.62%	3 generations is 45.56%	4 generations is 59.15%	5 generations is 70.02%	6 generations is 78.36%

Colin and Ross actually share an ancestor within 4 generations and the theoretical comparison predicted the probability of that was 59.15% or 59%. Bruce and Colin share a common ancestor within 3 generations and in a separate calculation theory predicted the probability of that being the case is 45.41% or 45%.

TMRCA - Time to Most Recent Common Ancestor

In the descendancy chart above you can see that Bruce and Colin's most recent common ancestor was 3 generations ago whereas with Ross and Colin the TMRCA is 4 generations ago. These numbers are actual TMRCA based on the genealogy.

In computation of a theoretical TMRCA from the DNA data one method is to quote the time at which the actual TMRCA is equally likely to be greater than the estimate as it is less than the estimate. This is the median TMRCA and would be the time of 50% probability in the FTDNATiP Report above; i.e. between 3 and 4 generations or 3.3 generations. In this case it is in error by 0.3 generations when compared to the actual TMRCA.

Notice that the 5% point would be at 0.4 generations. Not shown is the 95% point which is at 10.2 generations. The confidence level for this same calculation is then said 0.4 to 10.2 generations. This means the probability that the actual TMRCA is less than 0.4 generation is 5% and less than 10.2 generations is 95%. You need to be prepared to accept that the actual TMRCA could fall anywhere within this range.

This is not unlike the roll of dice, your most likely result is a 7 and you are confident your result will be anywhere from 2 to 12. The probability of a 7 is 17%. If you add fuzz of plus or minus 2, the probability of a 7 ± 2 is 66%. Sooo... when you see a theoretical TMRCA don't take it too literally, instead think of some fuzz of ± 2 or 3 generations as your best estimate and about as accurate as your ability the roll of a pair of dice and get a 7 ± 2.

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