Mutations in the ryanodine receptor 1 (RYR1) gene are associated with several human congenital myopathies including the dominantly inherited central core disease and exercise- induced rhabdomyolysis and the more severe recessive phenotypes including multiminicore disease, centronuclear myopathy and congenital fiber type disproportion. Within the latter group, those carrying a hypomorphic mutation in one allele and a missense mutation in the other are the most severely affected. Because of nonsense-mediated decay, most hypomorphic alleles are not expressed, resulting in homozygous expression of the missense mutation allele. This should result in 50% reduced expression of the ryanodine receptor in skeletal muscle, but its observed content is even lower. To study in more detail the biochemistry and pathophysiology of recessive RYR1 myopathies here we investigated a mouse model we recently generated, by analyzing the effect of bi-allelic versus mono-allelic expression of the RyR1 p.A4329D mutation. Our results revealed that expression of two alleles carrying the same mutation or of one allele with the mutation in combination with a hypomorphic allele does not result in functionally equal outcomes and impacts skeletal muscles differently. In particular, the bi-allelic RyR1 p.A4329D mutation caused a milder phenotype than its mono-allelic expression, leading to changes in the biochemical properties and physiological function only of slow twitch muscles and largely sparing fast twitch muscles. In summary, bi-allelic expression of the RyR1 p.A4329D mutation phenotypically differs from monoallelic expression of this mutation in a compound heterozygous carrier.