AncestryDNA Matches Among Siblings

Date: [unknown] [unknown]
Location: [unknown]
Surname/tag: DNA

This research report discusses how matches of siblings do and do not match to each other when tested on AncestryDNA. Data was gathered as of April 2023.

It's important to understand that each individual inherits 50% of their autosomal DNA (atDNA) from each parent - but does not inherit the other 50%. Every child inherits a randomly different 50% than any other. On average, we expect full siblings to share about 50% of their atDNA, but this can vary significantly due to the randomness of atDNA inheritance.

Visit Shared DNA Amounts and Relationship Probability to determine the most likely relationship between two matching people.

Scenario 1 - Two Brothers and their Parents Tested

Two Parents and Two Siblings

In this scenario, we examine AncestryDNA matches of two parents, GI and CL, and two of their sons, KI and EI. GI, CL, and KI were all tested prior to 2020 while AncestryDNA captured matches who had 6cM+ of shared DNA. EI was tested in 2022 after AncestryDNA stopped matching individuals who shared less that 8cM. Except where otherwise stated, this analysis will focus on matches of 8cM+ for all four individuals.

Confirm the Individual Relationships

GI and CL do not match on AncestryDNA and are not known to have any common ancestors within a meaningful timeframe; they are not related. They do have a small subset of people that they both match to, but they're believed to be related to those people on different lines.

	GI - Dad	CL - Mom	KI	EI
KI cM	3,455 cM	3,438 cM	-	2,614 cM
KI segs	27 segs	32 segs	-	46 segs
EI cM	3,489 cM	3,487 cM	2,615 cM	-
EI segs	25 segs	23 segs	46 segs	-

These values confirm that KI and EI are full siblings, and that GI and CL are their parents. Because EI inherited fewer and longer segments from each parent than his brother KI, this may explain why EI has more matches on each side of the family than KI. KIs segments were more broken up, presumably missing sections that would otherwise match to more relatives.

Examine Matches