Irish Roots

July 11th, 2011

John Grenham


A reader has pointed out that the proportions asserted so confidently in last week's column are way off. The number of hits per user on the church records site, irishgenealogy.ie, is only three times that on census.nationalarchives.ie, not 30 times. My recurring decimal point problem strikes again. The general argument stands, however: church records demand more sustained chewing on than census returns, even though it's only 300% more, not 3000%. (Is that right? No. Three times more = 200% more ...)

For the most part, genealogical research does not demand special mathematical skills, thank God. However, there is one area where decent mental arithmetic is essential. Most people know of the exponential growth in the number of ancestors with each generation back: 2 parents, 4 grand-parents, 8 great-grandparents, then 16, 32, 64, 128 . Even reckoning a conservative 3 generations per century, 1000 years ago you should have more than a billion ancestors. Of course every marriage where the couple are close or distant cousins, with at least some forebears in common, reduces that number significantly and, like it or not, the vast majority of marriages have always been between such cousins. But there is still an immense genetic diversity implied by the way these figures balloon once you get past the last two centuries. And you have to keep this in mind when trying to assess the meaning of much so-called "genetic genealogy". The most common genealogical tests involve the Y-chromosome, whose father-to-son transmission mimics European surname inheritance. You and someone else may indeed have both inherited your surname (and a tiny Y-chromosome mutation) from one individual who lived ten centuries ago. But the proportion of your genetic inheritance that you share from this common ancestor is vanishingly small. Even if the most recent common ancestor lived "only" 300 years ago, he constitutes just one part in 512 of your genetic make-up. Not much of a basis for tribal membership.


Comments and suggestions are welcome, to irishrootsatirishtimes.com.


Current column

2014

Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
6
3
3
7
5
2
7
4
1
6
3
1
13
10
10
14
12
9
14
11
8
13
10
8
20
17
17
21
19
16
21
18
15
20
17
15
27
24
24
28
26
23
28
25
22
27
24
 
 
31
 
 
30
 
 
29


2013

Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
7
4
4
1
6
3
1
5
2
7
4
2
14
11
11
8
13
10
8
12
9
14
11
9
21
18
18
15
20
17
15
19
16
21
18
16
28
25
25
22
27
 
22
26
23
28
25
23
 
 
 
29
 
 
29
 
30

 
30


2012

Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
2
6
5
2
7
4
2
6
3
1
5
3
9
13
12
9
14
11
9
13
10
8
12
10
16
20
19
16
21
18
16
20
17
15
19
17
23
27
26
23
28
25
23
27
24
22
26
24
30
 
 
30
 
 
30
 
 
29
 
31


2011

Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
3
7
7
4
2
6
4
1
5
3
7
5
10
14
14
11
9
13
11
8
12
10
14
12
17
21
21
18
16
20
18
15
19
17
21
19
24
28
28
25
23
27
25
22
26
24
28
26
31
 
 
 
30
 
 
29
 
31
 
 


2010

Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
4
1
1
5
3
7
5
2
6
4
1
6
11
8
8
12
10
14
12
9
13
11
8
13
18
15
15
19
17
21
19
16
20
18
15
20
25
22
22
26
25
28
26
23
27
25
22
27
 
 
 
 
31
 
 
30
 
 
29
 


2009

Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
6
2
2
6
4
1
6
3
7
5
2
7
13
9
9
13
11
8
13
10
14
12
9
14
20
16
16
20
18
15
20
17
21
19
16
21
27
23
23
27
25
22
27
24
28
26
23
28
 
 
 
 
 
29
 
31
 
 
30