The numbers and terms

One of the main reasons to perform a DNA test is to confirm and find long lost relatives, whether you knew they existed or not. According to 23andme, their “ability to detect first cousin relationships or closer is approximately 100%.” The following are the types of relationships and the approximate predication rates:

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:

• 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.