Because it defies the laws of Mendelian inheritance, the callipyge trait observed in certain sheep is of great interest due to its mode of transmission. Only heterozygous individuals carrying the mutation on the paternal allele display the characteristics associated with this phenotype: muscular hypertrophy. This phenomenon has been studied for many years in the Unit of Animal Genomics of GIGA at the University of Liege by Michel Georges’ team. New results have been published in PLOS ONE.

The callipyge phenomenon is a mystery that has fascinated genetics experts. Observed for the first time in 1983 in the US, this phenotype can be found in sheep and is characterized by a muscular hypertrophy. Animals with the callipyge phenotype – which means “beautiful buttocks” in Greek – have an increased muscle mass of 30%! This phenomenon appears in both sexes when the lambs are around one month old and results in a larger proportion and greater diameter of fast-twitch muscle fibers. These fast-twitch muscle fibers have a higher level of contraction but a weak fatigue resistance, in contrast with slow-twitch muscle fibers which are mobilized for weak-level contractions but which are very fatigue resistant. “Callipyge sheep would probably be very good sprinters”, says a smiling Haruko Takeda, a post-doctoral student in the Unit of Animal Genomics of GIGA at the University of Liege, directed by Professor . You might reasonably think that animals affected by this phenomenon represent an advantage for sheep breeders in view of the larger amounts of meat they offer. “Unfortunately, the meat from callipyge sheep is not tender and is therefore not a popular choice”, explains Haruko Takeda. The callipyge phenomenon is therefore not interesting from an economic point of view even though it is a scientific curiosity. 

From the phenotype to the genotype

At the beginning of the 1990s, a long and fruitful collaboration began. Michel Georges, who was in Genmark in the US at that time, was approached by Dr. Noelle Cockett of Utah State University who enlisted his help in finding the gene responsible for the callipyge phenotype. Since then, Michel Georges has returned to Belgium but the collaboration has continued to this day. The reason for this collaboration: identifying the genetic mechanisms behind the callipyge phenomenon is no mean feat. First of all, the mutation which is responsible is discretely located in a large non-coding section of the genome. This mutation is called CLPG and was independently highlighted in 2002 by an American team as well as Michel George’s team. But, in addition to the difficulty of identifying this mutation, the callipyge phenomenon has a very singular particularity: its unique mode of heredity transmission. This is what makes it so interesting in the eyes of geneticists.

In order to properly understand this particularity, we first need to return to the common way genetic transmission takes place, known as the Mendelian laws of inheritance. We can greatly simplify Mendel’s laws as follows: each individual possesses two versions or alleles of each gene: the maternal version and the paternal version. According to whether or not the parents are carriers of a gene mutation, four possible types of scenarios exist for their offspring. Imagine a gene “A” which we will call “a” in its mutated version. Four combinations (or genotypes) are possible: “AA”, “Aa”, “aA” or “aa”. In “Aa” individuals, the mutated allele “a” is transmitted by the father; in “aA” individuals, the mutated allele has been transmitted by the mother.  The mutation is therefore absent in individuals who are carriers of the “AA” genotype and present in the three other cases.  If the mutation ‘a’ is dominant, the individuals of the “Aa”, “aA” and “aa” genotypes will express the associated phenotype. If the mutation “a” is recessive, only the “aa” individuals will express the phenotype.