Congenital Myasthenic Syndrome, more commonly known as CMS is caused by a deletion of the homozygous base pair 20 (470del20), the function of this base pair is to code for the epsilon subunit of the acetylcholine receptor at the neuromuscular junction. The neuromuscular junction allows the motor neuron to transmit a signal to the muscle fibre, causing a muscle contraction. In layman’s terms, animals affected with CMS do not transmit signals from the brain to the muscles as effectively as animals who are free of the condition. Hence, CMS causes progressive muscular weakness and effects muscle development from birth. In some instances, farmers have observed weak calves at birth, who were unable to rise, and when helped to their feet the calf would be able to stand or walk for about 30-45 minutes before collapsing again, not getting sufficient chance to suckle from their dams. Gradually these calves’ strength declined and they died at about 3 months of age.
Similarly, to Pompes, CMS is inherited in a simple recessive manner which implies that animals that are homozygous normal (NN), are free of the mutation. Animals that are heterozygous carriers (Nn), will have one copy of the CMS mutation on the recessive allele. Heterozygous carriers of CMS do not express the symptoms of CMS, they are only expressed in the affected (nn) animals, who have two copies of the mutation on the recessive alleles (homozygous recessive). CMS can be managed to keep allele frequencies of the mutant gene low, in order to do this, however, the disease status of the animal needs to be known. If you mate two normal (non-carrier) parents, all the progeny will be free of the disease. If you mate one normal parent (NN) with a carrier parent (Nn) the outcome of the mating will on average present 50% normal progeny and 50% carrier progeny. If you mate two carrier parents (Nn) with each other, will produce, on average 25% normal progeny (NN), 50% carrier progeny (Nn) and 25% affected progeny (nn).

