Variant of the myostatin gene (MSTN, also known as GDF8), a negative regulator of muscle mass. Homozygous dogs present a "Bully Whippet" phenotype with extreme muscle hypertrophy and a conformation unsuitable for the breed standard; heterozygotes show greater muscle mass and superior athletic performance in racing. It is a paradigmatic example of a variant with a dose effect and of artificial selection in a racing line.

Whippet. The variant has been selected in racing lines for its athletic advantage; no reliable frequencies are published in large series (limited data).

- Homozygotes: generalised muscle hypertrophy, broad head, short limbs, non-standard conformation\n- Heterozygotes: increased musculature and better racing performance, without serious functional impairment\n- Possibly increased risk of musculoskeletal injuries in homozygotes

The mutation was characterised molecularly in the Whippet and was associated with the "Bully" phenotype in homozygosity and with greater racing performance in heterozygosity, in genome-wide association studies published in the 2000s (Mosher and colleagues, PLoS Genetics). The finding was considered the first example in dogs of a naturally occurring variant in MSTN with a clear phenotypic effect, analogous to that observed in "double-muscled" cattle. The causal variant is a deletion of two base pairs in exon 3 (c.939_940delTG), which generates a premature stop at codon 313.

- Genotype breeding animals before mating\n- Do not mate two carriers: 25% risk of "Bully" homozygotes unsuitable for the standard\n- A carrier may be mated to a clear animal\n- In racing lines, assess performance but ALWAYS inform the buyer of the genetic status\n- Avoid mating two homozygous "Bully" dogs\n- Document the status in the pedigree

The "Bully" phenotype must be distinguished from other causes of muscle hypertrophy (intense training, other genes). The molecular test specific for the Whippet MSTN variant confirms the status. The selection of heterozygotes in racing lines poses an ethical dilemma: it improves performance but perpetuates the allele that in homozygosity produces an unwanted phenotype.

1. Mosher DS et al. A mutation in the myostatin gene increases muscle mass and enhances racing performance in heterozygote dogs. PLoS Genet 3(5):e79, 2007. PMID: 17530926
2. OMIA:000683-9615. Muscular hypertrophy (double muscling) in Canis lupus familiaris. https://omia.org/OMIA000683/9615/