Autosomal DNA statistics

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An understanding of autosomal DNA statistics is helpful when trying to understand results from a Family Finder test from Family Tree DNA or a Relative Finder test from 23andMe. Autosomal DNA is inherited from both parents. It is randomly shuffled up in a process called recombination and the percentage of autosomal DNA is diluted with each new generation.

Contents

Simple mathematical average of sharing

The following figures show the average percentage of autosomal DNA shared with relatives assuming that every child gets 50% from his mother and 50% from his father and the average number of shared centiMorgans:

Percentage centiMorgans Relationship Notes
50% 3400.00 Mother, father, siblings The actual half-identical value is 3384.31 cMs but has been rounded up here for convenience
25% 1700.00 Grandfathers, grandmothers, aunts, uncles, half-siblings, double first cousins
12.5% 850.00 Great-grandparents, first cousins, great-uncles, great-aunts, half-aunts/uncles, half-nephews/nieces
6.25% 425.00 First cousins once removed, half first cousins
3.125% 212.50 Second cousins, first cousins twice removed
1.563% 106.25 Second cousins once removed, half second cousins
0.781% 53.13 Third cousins, second cousins twice removed
0.391% 26.56 Third cousins once removed
0.195% 13.28 Fourth cousins
0.0977% 6.64 Fourth cousins once removed
0.0488% 3.32 Fifth cousins
0.0244 1.66 Fifth cousins once removed
0.0122% 0.83 Sixth cousins
0.0061% 0.42 Sixth cousins once removed
0.00305% 0.21 Seventh cousins ca. 92,000 base pairs
0.001525% 0.10 Seventh cousins once removed
0.000763% 0.05 Eighth cousins ca 23,000 base pairs

The percentages and the number of centiMorgans can vary. For example, a brother might share 53% of his DNA with one sibling and 47% with another sibling. Because of the random way that autosomal DNA is inherited third, fourth and more distant cousins will not necessarily match you with a Family Finder or Relative Finder DNA test. According to Family Tree DNA's figures the Family Finder test has a greater than 90% chance of detecting a match with a third cousin, but just over a 50% chance of detecting a match with a fourth cousin. In contrast the test will sometimes pick up traces of autosomal DNA from your more distant cousins (for example, fifth cousins and beyond). The chart below (courtesy Dimario, Wikimedia Commons) shows the average amount of autosomal DNA inherited by all close relations up to the third cousin level.

Cousin tree (with genetic kinship).png

Ranges of sharing percentage

Figures from 23andMe's Relative Finder:

  • Parent/child: 47.54 (for father/son pairs, who do not share the X chromosome) to ~50%
  • 1st cousins: 7.31-13.8
  • 1st cousins once removed: 3.3-8.51
  • 2nd cousins: 2.85-5.04
  • 2nd cousins once removed: .57-2.54
  • 3rd cousins: ca .3-2.0
  • 3rd cousins once removed: .11-1.32
  • 4th and more distant cousins: .07-.5

Shared SNPs

Figures from 23andMe Compare Genes function (from Tim Janzen's data):

  • Parent-child pairs share between 83.94% and 84.20% of SNPs (50% of DNA in common)
  • Siblings share between 83.81% and 87.47% of SNPs (50% of DNA in common)
  • Uncle/aunt-niece/nephew pairs share between 78.48% and 79.57% of SNPs (25% of DNA in common)
  • Grandparent-grandchild pairs share between 77.96% and 80.59% of SNPs (25% of DNA in common)
  • First cousins and great uncle/great aunt-grandniece/grandnephew pairs share 75.78% and 77.03% of SNPs (12.5% of DNA in common)
  • First cousins once removed share ca 75.5% of SNPs (6.25% of DNA in common)
  • Second cousins and first cousins twice removed share ca 75% of SNPs (3.125% of DNA in common)
  • Unrelated people of European descent share 73-74.6% of SNPs

Identical by Descent segments

The recombinant DNA has a limit of breakdown, meaning we can not inherit DNA pieces from every biological ancestor. In other words only a small part of the biological ancestors can be detected in the DNA of an individual. Luke Jostins calculated that on average a human has no more then ca. 125 genetic ancestors from the same ancestral generation. This means that only up to the seventh ancestor generation (128 ancestors) segments of those ancestors are detectable in the personal DNA. Therefore it is important to identify Identical By Descent (IBD) segments versus Identical By State (IBS) segments. John Walden's research on IBD and IBS segments can be used as a guideline:[1][2]

cM  % IBD  % IBS
10 99 1
9 80 20
8 50 50
7 30 70
6 20 80
5 5 95

Resources

Charts and tools

Statistics categorized by genealogical relationship

In order to help people who have taken an autosomal DNA test gain greater insight into the genealogical relationships implied by the resultant data Tim Janzen has created three charts that provide statistical information in various categories. The charts provide statistics on close relatives, distant endogamous relatives and distant non-endogamous relatives. The charts were originally designed for use with 23andMe data but now also incorporate data from FTDNA's Family Finder test. The charts are organized by the degree of relationship, with the most closely related people (parents and children, full siblings) being listed at the top and more distant cousins being listed at the bottom. The statistics are based on information from real people who have been tested by 23andMe and Family Tree DNA and who have a known genealogical relationship to someone else who has also been tested by the same company. The charts also include information on the median and the average number of shared cMs for people who are related to each other from the first cousin once removed level of relationship to the 5th cousin level of relationship. The charts can be downloaded from Anabaptist Genetic Genealogy website.

An unidentified author has also provided a spreadsheet on DNA Inheritance Statistics to which anyone can add their data. The spreadsheet can be found here.

Resources

Blog posts

References

  1. John Walden's research reported in a message to the Autosomal DNA Rootsweb list by Tim Janzen, 6 January 2012.
  2. See also the files on John Walden's website.

See also