Further evidence on the inheritance of halothane reaction in pigs.

One thousand six hundred forty-one Pietrain, 163 Minnesota No. 1 and 158 Pietrain X Minnesota No. 1 crosses and their reciprocals were tested for porcine stress syndrome susceptibility using halothane gas between September 1975 and July 1981. The frequency of reactors in the Pietrain breed was 93.9%. Matings of reactor males to reactor females within the Pietrain breed resulted in 632 reactor and 14 nonreactor offspring. A boar, judged to be nonpenetrant on the basis of the halothane reaction of his parents and littermates, was shown by progeny test to be homozygous for the halothane allele. These data indicate that halothane sensitivity is due to a single autosomal recessive gene with a penetrance of about 98% and a frequency of .98 in this Pietrain herd. No halothane reactors were found in the 163 Minnesota No. 1 pigs tested. Only one Pietrain X Minnesota No. 1 gilt reacted positively to halothane and later, the same gilt produced both positive and negative offspring, indicating that she was most likely heterozygous. Blood group typing of 107 crossbred pigs provided insufficient information to predict accurately the halothane reaction, although some associations were observed between the A and H loci and halothane sensitivity. The time taken by Pietrain pigs to react to halothane was measured and recorded. Analysis of these data showed that progeny of some sires had significantly faster reaction times than others and that reaction time had decreased over the years. These results as well as other data presented here indicate the existence of certain modifier genes that influence halothane reaction.