These factors are ubiquitously expressed and it is proposed that defects in a ubiquitous factor (a protein expressed everywhere) should only cause disease in the retina because the retinal photoreceptor cells have a far greater requirement for protein processing (rhodopsin) than any other cell type.The somatic, or X-linked inheritance patterns of RP are currently identified with the mutations of six genes, the most common occurring at specific loci in the RPGR and RP2 genes.Types include:

Pathophysiology
A variety of retinal molecular pathway defects have been matched to multiple known RP gene mutations. Mutations in the rhodopsin gene (RHO), which is responsible for the majority of autosomal-dominantly inherited RP cases, disrupts the rhodopsin protein essential for translating light into decipherable electrical signals within the phototransduction cascade of the central nervous system. Defects in the activity of this G-protein-coupled receptor are classified into distinct classes that depend on the specific folding abnormality and the resulting molecular pathway defects. The Class I mutant proteins activity is compromised as specific point mutations in the protein-coding amino acid sequence affect the pigment proteins transport to the outer segment of the eye, where the phototransduction cascade is localized. Additionally, the misfolding of Class II rhodopsin gene mutations disrupts the proteins conjunction with 11-cis-retinal to induce proper chromophore formation.
Combined with cross-sectional imagery of optical coherence tomography, which provides clues into photoreceptor thickness, retinal layer morphology, and retinal pigment epithelium physiology, fundus imagery can help determine the state of RP progression.While visual field and acuity test results combined with retinal imagery support the diagnosis of retinitis pigmentosa, additional testing is necessary to confirm other pathological features of this disease. Electroretinography (ERG) confirms the RP diagnosis by evaluating functional aspects associated with photoreceptor degeneration, and can detect physiological abnormalities before the initial manifestation of symptoms. An electrode lens is applied to the eye as photoreceptor response to varying degrees of quick light pulses is measured. Patients exhibiting the retinitis pigmentosa phenotype would show decreased or delayed electrical response in the rod photoreceptors, as well as possibly compromised cone photoreceptor cell response. The patients family history is also considered when determining a diagnosis due to the genetic mode of inheritance of retinitis pigmentosa. At least 35 different genes or loci are known to cause "nonsyndromic RP" (RP that is not the result of another disease or part of a wider syndrome). Indications of the RP mutation type can be determine through DNA testing, which is available on a clinical basis for:

RLBP1 (autosomal recessive, Bothnia type RP)
RP1 (autosomal dominant, RP1)
RHO (autosomal dominant, RP4)
RDS (autosomal dominant, RP7)
PRPF8 (autosomal dominant, RP13)
PRPF3 (autosomal dominant, RP18)
CRB1 (autosomal recessive, RP12)
ABCA4 (autosomal recessive, RP19)
RPE65 (autosomal recessive, RP20)For all other genes (e.g. DHDDS), molecular genetic testing is available on a research basis only.