Figure 1. Principle of HAPPY mapping. The example shows the mapping of just three sequence-tagged site (STS) markers (A, B and Z), although typically many hundreds are mapped in parallel. Genomic deoxyribonucleic acid (DNA) is prepared from cells (a), and is broken randomly to give a pool of fragments (b). The fragments are dispensed into a series of samples (c) constituting the mapping panel; each sample contains less than a genome's worth of fragments and hence only a subset of the markers. Polymerase chain reaction (PCR) is used to ascertain which markers are present in each member of the panel (d). Linked markers such as A and B, because they tend to lie on the same DNA fragments after breakage, tend to cosegregate (as in samples 1, 3 and 96 of this mapping panel). Cosegregation frequencies reflect the distances between markers, which allows a map (e) to be deduced.