Q27 - Dog Body Size and the IGF1 Locus

Theoretical 1 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

Domestic dogs (Canis lupus familiaris) are a subspecies of grey wolves (Canis lupus). Dogs are unique amongst mammal species for their extremely large range of body sizes.

Scientists sequenced a region of DNA around the IGF1 (insulin-like growth factor 1) gene in thousands of dogs. They found a common SNP (single nucleotide polymorphism); a C vs T mutation.

The genotype of the IGF1 SNP was plotted against dog body mass, and against levels of IGF1 in serum (blood).

Scatter plot with body mass (kg) on the x-axis and IGF-1 serum level (nmol/L) on the y-axis. Dogs are colored by genotype: red (CC), orange (CT), yellow (TT). A blue trend line shows a positive correlation between body mass and serum IGF1. TT dogs tend to be larger, CC dogs tend to be smaller, but there is substantial overlap between all three genotype groups across the full range of body sizes. X = Body mass (kg), Y = IGF-1 Serum level (nmol/L). Red = CC genotype, Orange = CT genotype, Yellow = TT genotype.

Next, scientists sequenced the same SNP in individuals from species across the wider canid (dog) family.

World map showing the geographic distribution of IGF1 SNP genotypes across 13 canid species (A through M). Each species is marked with symbols colored by genotype: yellow = TT, orange = CT, red = CC. Grey wolves (K) and domestic dogs show both T and C alleles. Most other canid species (foxes, jackals, wild dogs) are predominantly or exclusively CC (red). Distribution of IGF1 SNP genotypes across the canid family. A = Golden Wolf, B = Hunting Dog, C = Andean Fox, D = Black Jackal, E = Island Fox, F = Coyote, G = Dhole, H = Ethiopian Wolf, I = Golden Jackal, J = Grey Fox, K = Grey Wolf, L = Red Wolf, M = Striped Jackal. Yellow = TT, Orange = CT, Red = CC.

Q27.1.1. The T allele is fully dominant to the C allele, which is fully recessive, in controlling dog size.

Q27.1.2. IGF1 controls the majority of size variation of dogs.

Q27.1.3. The C/T SNP must only influence body size through changing levels of IGF1 in serum.

Q27.2. Based on the canid family SNP data, which hypothesis about the most recent common ancestors of domestic dogs is most likely?

Q27.3. Based on the canid family SNP data, which hypothesis about the allele history is most likely?

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Question reproduced from IBO 2023, Theoretical Paper 1, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF