Gene mutation affecting dog size was described in an article that appeared in Nature News on 27 January 2022:
Gene mutation affecting dog size was found by researchers as a variation in the region around the IGF1 gene in a stretch of DNA that encodes a long non-coding RNA. This long non-coding RNA is involved in controlling levels of the IGF1 protein (Insulin-like growth factor-1, a hormone that functions as the major mediator of growth hormone).
The researchers identified two versions, or alleles, of the variant. Across all breeds, dogs with two copies of one allele tended to weigh less than 15 kilograms, whereas two copies of the other version were more common in dogs weighing more than 25 kilograms. Dogs with one copy of each allele tended to be intermediate in size.
The researchers think that the allele linked to small bodies is, evolutionarily, much older than the large-bodied version.
Researchers also caution that the story of dog size is far from complete. Still they need to know how the gene mutation influence the levels of the IGF1. The IGF1 gene itself accounts for only about 15% of variation between breeds.