Dog breed size is caused by a single gene
First off, I wish to note that the source is a scientific/ medical paper that has jargon and statistics well beyond anyone not practicing in these fields. This means that rather than providing the proof by way of graphs and tables, that an analysis of their discussion points will be made.
As a base, you should be aware that " Size variation in the domestic dog is extreme and surpasses that of all other living and extinct species in the dog family,"
The reason why this is so special is that knowing that the domestic dog descended from the gray wolf approximately 15,000 years ago AND the vast majority of dog breeds have been created by man over the past few hundred years - MEANS that the rate of evolution is extraordinary.
SPOLIER ALERT: It was found that "A single IGF1 single-nucleotide polymorphism haplotype is common to all small breeds and nearly absent from giant breeds."
1 - Through a genome-wide scan, (they) identified a major quantitative trait locus (QTL) on chromosome 15 influencing size variation within a single breed.
2 - They "examined genetic variation in the 15-megabase interval surrounding the QTL in small and giant breeds."
One of the main dogs they analysed was the Portuguese water dog (PWD), because as a breed it is allowed to have a significant variation in size by the American Kennel Club (AKC).
The researchers received full marks for thoroughness as they made 92 radiographic skeletal measurements for size and shape of the PWD and "found that two QTL (FH2017 at 37.9 Mb and FH2295 at 43.5 Mb) within this region were strongly associated with body size in Four Hundred and sixty three (463) PWDs. That is, common genetic markers were found that related specific to size within that ONE breed.
For comparison between breeds genetic samples were taken from another one hundred and forty three breeds!
DOG SIZE ANALYSIS RESULTS
Because first archaeological record of dogs, (12,000 to 15,000 years ago) shows size diversity was present early in the history of domestication, the gene responsible for size variation was most likely present at the genesis of the domestic dog.
Domestic dogs evolved from wolves 15,000 years ago, so you would expect that the early dogs were all large, yet "The early and widespread appearance of small size suggests that an ancestral small dog IGF1 haplo-type was readily spread over a large geographic area by trade and human migration and was maintained in local gene pools by selection. Such early selection on dogs may have been manifest as intentional artificial selection exercised by early humans or as an adaptive trait for coexistence with humans in the more crowded confines of developing villages and cities."
DOG SIZE ANALYSIS CONCLUSION
The reason that this research is considered break-though is that previously it was thought that major differences in dog size between breeds was due to " elevated mutation or recombination rates." Something that no other mammalian species experience. HOWEVER, this research shows that " a single IGF1 allele is a major determinant of small size in dogs and that intense artificial selection has left a signature in the proximity of IGF1 that can readily be found by genomic scans of breeds sharing a common phenotype."
Thus without this specific gene, some breeds were not easily 'bred down'. For instance most breeds start with a large single breed (such as poodle or schnauzer) and because these breeds have the IGFI allele gene, they are able moderately easily have smaller dogs in the litter bred together until a whole new breed size is created. Such as the poodle Standard TO miniature TO toy TO tea cup.
Article by Bruce Dwyer. If you wish to use any of this information please use a LINK reference to http://www.dogwalkersmelbourne.com.au
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A Single IGF1 Allele Is a Major Determinant of Small Size in Dogs. Nathan B. Sutter et al (2007)