Excerpt:
A father passes down a near-exact copy of his chromosome Y (chrY) to his son. Imperfections during replication result in slight differences (e.g. single nucleotide polymorphisms; SNPs) that may be unique to the son. Genealogists use these patterns of similarities and differences to reconstruct paternal ancestry (the Y-tree). The FamilyTreeDNA team (FTDNA) released Big Y in 2013 to provide these patterns directly to customers. At that time, there were thousands of SNPs on the Y-tree...