Out of all the mammals on the planet, dogs differ in size the most. The biggest dog breeds are around 40 times bigger than the smallest breeds. A recent study has shown that this occurs because of a gene mutation that lies near a gene called IGF1. This gene was originally flagged 15 years ago as playing a major role in the variations of dog sizes. Ancient dogs that were domesticated from wolves in the past 30,000 years differ very little in size, however, in the past 200 years the largest difference in breed size has been recorded as people began to breed the more modern dog breeds during this time.
The IGF1 gene was studied comparing to body size of dogs and wild canids. There was one variant that stood out to researchers; this gene mutation was found in a stretch of DNA that works to encode a molecule called a long non-coding RNA. Long non-coding RNAs are a type of mammalian genome that lack protein coding capabilities. Specifically, the long non-coding RNA that was found to affect the size of dog breeds is involved with the levels of the IGF1 protein in the dogs bloodstream. As we learned in AP Biology, mutations in genes occur during the DNA replication phase of mitosis. Mitosis is the division of one mother cell into two daughter cells. DNA replication happens during the S phase of interphase. During this phase, the single stranded chromosome will duplicate and turn into two identical sister chromatids. The mutation will occur when copying the DNA, which would cause the sister chromatids to not be identical.
This study identified that there are two alleles of this variant. Dogs carrying two copies of the small-bodied allele were most likely to weigh 15 kilograms or less, meanwhile, dogs carrying two copies of the large-bodied allele were most likely to weigh more than 25 kilograms. Dogs that carry one copy of each allele tend to be of an intermediate size. Additionally, dogs containing the larger-bodied allele contain higher levels of the IGF1 proteins in their bloodstream compared to dogs who carry the smaller-bodied allele. Researchers also recorded a similar relationship in wild canids.
Prior to this study, researchers believed that certain dog breeds were smaller-bodied because of relatively new genetic changes. However, scientists now believe that the smaller-bodied allele is evolutionary and is actually much older than the bigger-bodied allele. They believe this to be true because the smaller-bodied allele was found in coyotes, foxes, jackals, and other smaller canids; this leads us to believe that this allele was present in one common predecessor. More studies must be done to truly determine how these variants impact the levels of IGF1 proteins in a mammals bloodstream. The IGF1 gene only accounts for about 15% of size variation in dogs, so there is still much more research do be done. This study is just the beginning to really figuring out how we came to have dogs as large as German Shepherds and as small as TeaCup Poodles. Which allele do you think your dog has?
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