Genetic diseases have been traced back to an error in a gene. Many times, a genetic disease can result from errors or mutations in one specific gene, but the mutations that people carry in that gene may not be the same. Sometimes that can result in variability in how the disease looks in different people.
With modern technologies, doctors can find out exactly what error has occurred in a patient’s genetic sequence that causes their disease; this video from Howard Hughes Medical Institute’s (HHMI) biointeractive outlines in only one minute how clinicians find that mutation.
The video uses the disease retinitis pigmentosa (RP) and a patient that suffers from it as an example. In RP, there are several genes that have already been associated with the disease, so a doctor would sequence those genes first when looking for the mutation carried by the RP patient. If the error wasn’t identified after that, other genes might then have to be sequenced. The patient’s mutation might also be modeled in a research organism to observe the impacts.
Source: HHMI