Reaction to halothane anaesthesia among heterozygotes at the halothane locus in British Landrace pigs

Growth and carcass performance of British Landrace pigs heterozygous at the halothane locus

The effects of the halothane gene on growth and performance were estimated from British Landrace positive and negative selection lines and crosses between the lines. A total of 214 full-sib pairs were performance tested from 25 to 85 kg live weight on either ad libitum or scale (0·69 g/g ad libitum food intake) feeding. Half-sides of 45 carcasses were fully dissected.

In the crosses, growth and performance did not depend on whether the sire or the dam was from the positive line. Carcass length, conformation score, eye muscle area and incidence of pale, soft, exudative meat (PSE) were all intermediate in the crosses. Given food ad libitum, the crosses had increased appetite and growth rates which resulted in increased backfat depths. However, in the fully dissected carcasses, the crosses were intermediate for lean proportion. Although the crosses gave a net economic advantage, the incidence of PSE may be unacceptably high for commercial exploitation.

Cosegregation of porcine malignant hyperthermia and a probable causal mutation in the skeletal muscle ryanodine receptor gene in backcross families

A study of the inheritance of malignant hyperthermia (MH) in the British Landrace breed revealed the same substitution of T for C at nucleotide 1843 in the ryanodine receptor (RYR1) gene that was previously shown to be correlated with MG in five Canadian swine breeds. Cosegregation of the mutation with MH in 338 informative meioses led to a lod score of 101.75 for linkage at Omax = 0.0. The substitution was also associated with a HinPI- BanII+ RsaI- haplotype in this breed, as in the five breeds tested earlier, suggesting its origin in a common founder animal. DNA-based detection of the MH status in 376 MH-susceptible heterozygous (N/n) and homozygous (n/n) pigs was shown to be accurate, eliminating the 5% diagnostic error that is associated with the halothane challenge test and flanking marker haplotyping procedures in current diagnostic use. These results strongly support the view that the substitution of T for C at nucleotide 1843 is the causative mutation in porcine MH and demonstrate the feasibility of rapid, accurate, noninvasive, large-scale testing for porcine MH status using DNA-based tests for the mutation.

Method to estimate genotype probabilities at individual loci in farm livestock

A Bayesian method to estimate genotype probabilities at a single locus using information on the individual and all its relatives and their mates has been developed. The method uses data over several generations, can deal with large numbers of individuals in large livestock families and allows for missing information. It can be extended to multiple alleles and can be used for autosomal or sex-linked loci. The allele frequencies and the form of expression (dominance, penetrance) must be specified. An algorithm using the method and involving an iterative procedure has been developed to calculate the genotype probabilities for practical use in livestock breeding. The method and algorithm were used to determine the accuracy of estimating genotype probabilities of sires for a female sex-limited trait, such as genetic variants of milk proteins. Data were similated and genotype probabilities estimated for 100 sires (20 replicates) with 3, 6 and 12 female offspring per sire, for different population frequencies, for additive and dominance gene action and for variable genotypic expression. Such simulation is useful in the design of testing systems for the use of information on specific genetic loci in selection.

Halothane-sensitivity gene and muscle contractile properties in malignant hyperthermia

Malignant hyperthermia (MH) results from the presence of the halothane-sensitivity gene and is characterized by abnormalities in muscle function. Populations of genetically defined pigs were used to determine the in vivo and in vitro expression of this gene in both the homozygous and the heterozygous condition. On exposure to halothane, isolated muscle bundles from the homozygous halothane-sensitive pigs exhibited decreased tetanus tension and increased tetanus half-relaxation time and contracture and were clearly distinguished from homozygous normal muscles. The heterozygous and homozygous normal muscles were similar in contractile responses except for the occurrence of halothane-induced contractures in the heterozygotes. The heterozygous halothane-negative pigs did not exhibit the characteristic signs of an MH episode in response to halothane succinylcholine, although some metabolic responses were significantly altered (e.g., increased venous partial pressure of CO2 and arterial and venous K+ concentration). Thus the heterozygous pigs were not MH susceptible but did represent a phenotype distinct from the homozygous normal pigs both in vitro and in vivo. These data provide the first convincing evidence for the expression of the halothane-sensitivity gene in heterozygotes.