On the trail of the world's smallest dog.

Across all confirmed males in the SIPGS dataset, the Y-chromosome lineage is identical. A single founder line, repeated across every male in five countries.

That kind of paternal uniformity is the genetic signature of a small founding population in a relatively short historical window. It tells us something specific about how the breed was assembled, and when.

A widely cited 2013 study reported a particular maternal haplotype in samples described as Chihuahuas, and used that result to argue for pre-Columbian Mexican continuity. The marker appeared in roughly a third of the dogs in that earlier paper. Their breed identity was not independently confirmed.

In the SIPGS dataset of 123 confirmed Chihuahuas across four continents, that same marker has not turned up yet. Not in any of them.

Long before any of these dogs reached Mexico, Roman writers were already documenting small lap dogs by the name Melitaeus catulus, the Maltese pocket dog. Roman trade reached from the Mediterranean east along the Silk Road, and where Roman trade went, small companion dogs traveled with it.

Wherever humans moved for trade, war, marriage, or escape, they carried their dogs. The maternal lines in the SIPGS dataset run through territory humans were crossing for centuries. The Chihuahua's family tree is also a human migration story.