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Tag: dog size

What is the Real Reason Dog Breeds Vary in Size?

Dog (Canis lupus familiaris) (6)

Do you like big dogs or small dogs? This question is frequently asked, but how did we even come about having this option? Ancient domesticated dogs in the past 30,000 years differed in size but nothing as extreme as the modern size differences. Dogs now can range from 40 times in size, and these drastic differences emerged just in the past 200 years as humans started establishing more and more breeds. In a study conducted by Ewen Callaway, he looked at why dogs differ so much in size and how a mutation could be the cause of this. The mutation behind all of this has been traced all the way back to ancient wolves. It lies near a gene called IGF1(insulin growth factor) which researchers found to have a major role in the size variations of domestic dogs. IGF1 is a hormone that manages the effects of growth hormones and is primarily produced by the liver so liver diseases can cause its levels to change.

One variant stood out when comparing the region around IGF1 and dog sizes. This variant “lies in the stretch of DNA that encodes a molecule called a long non-coding RNA which lives in controlling levels of the IGF1 protein.” A gene variant is a permanent change in the DNA sequence that makes up a gene. Variants can be inherited from a parent or can just occur during a person’s lifetime. If a variant is inherited from a parent they are present in pretty much every cell of the body while a variant that occurs during someone’s lifetime is present only in certain cells. Most variants that lead to disease are not common in the general population; however, some variants occur often enough in the general population to be considered common genetic variations. Examples of this would be eye color, hair color, and blood type. Even though DNA variants can be seen as a negative, as it is explained not all variants produce fatalistic effects.

There are two identified versions (alleles) of the variant, which Callaway identified. An allele is a form of a gene and each organism inherits two alleles, one from each parent. Dogs who have two copies of one allele typically weigh more than 55 pounds and have higher IGF1 protein levels in their blood. Whereas, dogs with two copies of the other allele tend to weigh less than 33 pounds. In addition, there are dogs with one copy of each version and they tend to be intermediate in size. Researchers determined that the same relationship was present in other canids as well, such as foxes, coyotes, and wolves.

Protein IGF1R PDB 1igr

IGF1 structure

The allele linked to small-bodied animals is seen to be much more evolutionary than alleles linked to large-bodied animals. Coyotes, jackals, foxes, and a lot of other candids have two copies of the small version, suggesting that this variation could have been present in their ancestors. However, it is not as clear as to when the large-bodied allele formed. It has been traced back 53,000 years ago to an ancient wolf living in Siberia, and since then has been found in other ancient wolves. Robert Wayne, an evolutionary biologist at UCLA, states that the view used to be that animals that have a small body size can be linked to genetic changes that could be unique to domestic dogs. This study could be a sign that dogs were domesticated from smaller wolves rather than present-day gray wolves.

Overall researchers have discovered a big part as to why dogs vary so much in size, however, the story of dog size is far from complete as IGF1 proteins only make up 15% of the difference in dog size. Even though this is such a small percentage, we are 85% closer to finding the whole meaning.

Is the Difference in Size of a German Shepherd and a TeaCup Poodle Due to a Gene Mutation?

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. 

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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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