The numbers and terms

The most important thing in doing a  DNA test on the ancestry side, is to find your long lost relatives, knew or unknown. So the prediction is the core. According to 23andme, their “ability to detect first cousin relationships or closer is approximately 100%.” The following is the relationships and the approximate percentages:

  1. 100%: identical twin
  2. 50%: father, mother, son daughter, brother, sister  
  3. 25%: half-bro/sis, grandpa/ma, grandson/daughter, uncle, aunt, nephew niece
  4. 12.5%: 1st cousin, great-grandpa/ma, great-grandson/daughter, great-uncle/aunt, great-nephew/niece
  5. 3.125%: second cousin – same great-grandparents
  6. 0.781%: third cousin – great-great-grandparents
  7. 0.195%: fourth cousin – great-great-great-grandparents
  8. 0.05%: fifth cousin – gggggrandparents
  9. 0.0122%: sixth cousin – ggggggrandparents

Finding genetic relatives link here 

Cousin relationship Probability of detecting
1st Cousin or closer ~100%
2nd Cousin >99%
3rd Cousin ~90%
4th Cousin ~45%
5th Cousin ~15%
6th Cousin and beyond <5%

Percent DNA Shared by Relationship link here 

Relationship Average % DNA Shared Range [1]
Identical Twin 100% N/A
Parent / Child
Full Sibling
50% Varies by specific relationship
Grandparent / Grandchild
Aunt / Uncle
Niece / Nephew
Half Sibling
25% Varies by specific relationship
1st Cousin 12.5% 7.31% – 13.8%
1st Cousin once removed 6.25% 3.3% – 8.51%
2nd Cousin 3.13% 2.85% – 5.04%
2nd Cousin once removed 1.5% 0.57% – 2.54%
3rd Cousin 0.78% 0.3% – 2.0%
4th Cousin 0.20% 0.07% – 0.5%
5th Cousin 0.05% Variable
6th Cousin 0.01% Variable

Once removed means a generation difference, and twice removed is a two-generation difference. It does NOT to describe your relationship:

  • Peter is my mother’s first cousin, who is my first cousin, once removed. It emphases on the relationship between Peter to my mom.
  • Jane is your grandfather’s first cousin, she is my first cousins, twice removed.

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