haplotype

A haplotype is a set of DNA variants that sit close together on the same chromosome and tend to be inherited as a single package. Because nearby variants usually travel together from parent to child, they form a recognizable signature — like a string of beads passed down unbroken.

Variants stay grouped this way because crossing over during meiosis only occasionally splits a chromosome at any given spot. Stretches of DNA between those break points are handed on intact, so the particular combination of letters along a segment — its haplotype — is shared across relatives and, more broadly, among people with common ancestry.

Haplotypes are extremely useful in practice. Because variants within a haplotype are correlated, measuring just a few markers can stand in for many, which makes association studies more efficient. They also serve as fingerprints of ancestry and were the basis of large mapping efforts. A caveat: haplotype patterns differ between populations, so a block defined in one group may not hold in another.