Nuclear DNA comprises most of the genome of an organism and in sexually reproducing organisms is inherited from both parents, in contrast to mitochondrial DNA's maternal pattern of inheritance. Leigh syndrome caused by nuclear DNA mutations is inherited in an autosomal recessive pattern. This means that two copies of the mutated gene are required to cause the disease, so two unaffected parents, each of whom carries one mutant allele, can have an affected child if that child inherits the mutant allele from both parents.