DNA Terms in Plain English
The words you meet in match lists and Y-DNA results, each in a sentence or two.
Matches and shared DNA
Centimorgan (cM)
The unit for how much DNA two people share. More cM usually means a closer relationship: a parent and child share about 3,400 cM, distant cousins often under 30.
Segment
One unbroken stretch of DNA that two people share. A match's total is all of their segments added together.
Longest segment
The biggest single stretch you share with a match. One long segment is far harder to get by chance than a pile of small ones, so it's often the best quick test of a real connection.
Unweighted shared DNA
Ancestry's total for a match before its Timber adjustment. You'll find it by clicking into the match.
Timber
Ancestry's adjustment for matches under 90 cM. It discounts DNA that many people in a population share, so the figure on your match list (weighted cM) can be much lower than the unweighted total.
Endogamy
Endogamy
Marrying within the same community over many generations, as Ashkenazi Jews did for centuries. Everyone ends up related many times over, so matches look closer than they really are.
Background match
A match no closer than two random people from the same community would be. The DNA is real, but it's shared through so many distant ancestors that it can't point you to one.
Y-DNA and mtDNA
Y-DNA
DNA passed from father to son almost unchanged. It follows one line only: your father, his father, and so on. Only men carry it.
mtDNA
Mitochondrial DNA, passed from a mother to all her children. It follows your mother's mother's line back.
Haplogroup
A named branch of the human family tree on the paternal (Y-DNA) or maternal (mtDNA) line, such as R-Y2619. Branches nest inside each other, from very old to very recent.
STR
Short tandem repeat: a spot where a short bit of DNA repeats several times in a row. Y-37 and Y-111 tests count the repeats at 37 or 111 spots. Good for finding matches, but the counts change quickly and can mislead.
SNP
Single nucleotide polymorphism, said "snip": a change to one letter of DNA. New ones are rare and get passed down, so a shared SNP marks a shared branch.
Genetic distance
FamilyTreeDNA's count of how many steps apart two men's STR results are. Zero means identical on the markers compared. It counts steps, not generations.
Big Y
FamilyTreeDNA's test that reads SNPs across much of the Y chromosome. It tests which branch you're on, rather than predicting it, and can discover new branches.
Private variant
A SNP found in only one tester so far. When someone else turns up with it, it can become a new branch of the tree.
Plain-English summaries, simplified on purpose. Each testing company has its own help pages with the full detail.