Choroideremia (CHM) is genetically passed through families by an X-linked pattern of inheritance. In this type of inheritance, the gene for choroideremia is located on the X chromosome. Females have two X chromosomes, but generally, only one of the X chromosomes will carry the defective gene. Because females have a healthy version of the gene on their other X chromosome that produces REP-1 (Rab escort protein-1), they normally do not suffer the full effects of CHM.
However, female carriers of choroideremia will also develop alterations to they eyes ranging from mild retinal changes to extensive retinal degeneration, visual field loss and decreased visual acuity in a substantial number of patients, especially later in life. Reduced levels of wild-type CHM RNA expression, albeit measured in blood, may be associated with phenotype severity. Males have only one X chromosome (paired with one Y chromosome) and are therefore genetically susceptible to inherit X-linked diseases.
Males cannot be carriers of X-linked diseases, but they will pass the gene on their X chromosome to their daughters, who then become carriers. Affected males never pass an X-linked disease gene to their sons because fathers pass only the Y chromosome to their sons. Female carriers have a 50 percent chance (or 1 chance in 2) of passing the X-linked disease gene to their daughters, who as a result will become carriers