Kinship DNA Relationship Analyzer
Investigate whether two people could be half-brothers, full siblings, first cousins, or unrelated. Based on empirical distributions from the Shared cM Project and IBD1 / IBD2 segment modeling.
Predicted Relationships & Likelihood
Understanding Genomic Kinship & Recombination
The Recombination Randomizer
Each sperm and egg cell inherits a shuffled mosaic of grandparents' chromosomes through crossing over (meiosis). Because crossovers occur somewhat randomly (~1–3 per chromosome arm), shared centiMorgan totals for any relationship follow a bell curve, not an exact percentage.
A parent gives exactly 50% of their DNA (approx 3,400 to 3,600 cM depending on laboratory platform threshold), but siblings can vary from ~2,200 cM to ~3,300 cM.
IBD1 vs IBD2: The Half vs Full Sibling Clincher
When two people share one common parent (half-siblings), they can at most share one copy of a chromosome segment from that shared parent: Identical By Descent 1 (IBD1).
Full siblings can independently inherit the same maternal chromosome segment and the same paternal segment in certain regions (~25% of their genome on average). This is IBD2 (Fully Identical). Detecting substantial IBD2 conclusively separates full siblings from half-siblings, aunts/uncles, or grandparents!
The Avuncular & 25% Overlap Dilemma
Half-siblings, uncle/niece (or aunt/nephew), and grandparent/grandchild all share roughly 25% DNA (~1,750 cM) and have zero IBD2.
To tell them apart without additional family tests, geneticists rely on: 1) Age/Generation gaps; 2) X-Chromosome inheritance patterns (e.g. half-brothers through a father share 0 X-DNA, while half-brothers through a mother share ~50% X-DNA); and 3) Segment count and length (grandparent matches have longer, fewer segments than half-siblings due to fewer recombination generations).
Frequently Asked Questions
Can McConaughey and Harrelson prove they are half-brothers with standard home DNA kits?
Yes. A simple commercial autosomal DNA test (like 23andMe, AncestryDNA, or MyHeritage) from both men would immediately yield an answer. If they are half-brothers, they will share ~1,300 to 2,200 cM (around 20% to 30% of their genome). If they are merely close friends with no biological kinship, they will share 0 cM (or under 15 cM of background noise).
What is a centiMorgan (cM)?
A centiMorgan is not a physical unit of base pairs, but a measure of genetic linkage and recombination frequency. One centiMorgan corresponds to a 1% chance that a marker at one position will be separated from a marker at a second position due to crossing-over in a single generation. The human autosomal genome is roughly 3,400 to 3,600 cM long.
Why do commercial test kits sometimes differ in total cM?
Different testing companies use different algorithms, SNP arrays, and minimum threshold filters (e.g., ignoring segments under 6 cM or 7 cM to prevent false-positive matching caused by population linkage disequilibrium). However, for 1st- and 2nd-degree relationships (like half-siblings), these minor differences are negligible.