Wonder no more. This week's issue of Science sees a study published that gives us a clue to how a mutation in a single locus regulates size in these creatures.
A team of more than 20 scientists from eight institutions (United States and the U.K.) toured the dog show circuit and collected blood from 3,241 canines of all sizes and shapes. They found that the reduced growth mapped to a locus, near the IGF1 gene. In human IGF1 plays a role in the growth from birth till adolescence.
In all the "toy" dogs tested (distantly related and reproductively isolated), there were mutations in the region next to IGF1. This is intriguing to see a single change map to all the small breeds, indicating that the change probably maps to the time when dogs evolved from their wolf-like ancestors. The same "small dog marker" however, is not found in the modern wolves.
Why is this so fascinating? Mostly because the entire study pinpoints a single allele to be responsible for the phenotypic variation (small size) and the result was achieved without doing a single cross!!