with other major conformational genes, and in at least one case, breeding records have shown an indication of genes passed on together. There are size genes on all 39 chromosomes, 17 classified as "major" genes. 7 of those are identified as being of key importance and each results in ~2x difference in body weight. IGF1 (Insulin-like growth factor 1), SMAD2 (Mothers against decapentaplegic homolog 2), STC2 (Stanniocalcin-2) and GHR(1) (Growth hormone receptor one) are dose-dependent with compact dwarfs vs leaner large dogs and heterozygotes of intermediate size and shape. IGF1R (Insulin-like growth factor 1 receptor) and HMGA2 (High-mobility group AT-hook 2) are incomplete dominant with delicate dwarfs vs compact large dogs and heterozygotes closer to the homozygous dwarfed phenotypes. GHR(2) (Growth hormone receptor two) is completely dominant, homozygous and heterozygous dwarfs equally small, larger dogs with a broader flatter skull and larger muzzle. It is believed that the PMEL/SILV merle gene is linked to the HMGA2 size gene, meaning that alleles are most often inherited together, accounting for size differences in merle vs non-merle litter mates, such as in the Chihuahua and the Great Dane (merles usually larger) and Shetland Sheepdog (merles frequently smaller). See also Labrador Retriever coat colour genetics Cat coat genetics Equine coat color genetics Farm-Fox Experiment Notes References External links Dog anatomy Animal hair Animal coat colors