Myopia 1, X-linked, Nonsyndromal
Patient Information
Characteristics
Background and History
Refractive errors, in the absence of other eye disease, for which corrective lenses are prescribed, have been known since antiquity. It has also been recognized that they are often familial although the transmission pattern does not follow the usual patterns of simple, single gene inheritance. This condition of simple myopia (nearsightedness) is one of the few exceptions.
Clinical Correlations
At least three families have been reported in which only males had marked nearsightedness. The onset was reported to be late in the first decade of life. No other ocular abnormalities were present and specifically reports do not mention retinal thinning or detachments.
Genetics
Inheritance
In reported families only males were highly nearsighted although some females had low-grade myopia. In these families the inheritance pattern was consistent with X-linked inheritance and an area purported to contain a gene mutation was found to be located on the X-chromosome.
Pedigree
X-linked recessive, carrier mother
X-linked disorders are caused by a mutation on the X chromosome and both sexes can pass this to their children. If the mutation is in a recessive gene and carried by the mother, she usually does not have the disease since the normal X chromosome without the mutation neutralizes the mutation in the abnormal X chromosome. However, half her sons will inherit the mutation-containing X chromosome and therefore have the X-linked disease. Half the daughters will inherit the mutation-bearing X chromosome and are usuallly healthy 'carriers'.
X-linked disorders are caused by a mutation on the X chromosome and both sexes can pass this to their children. If the mutation is in a recessive gene and carried by the mother, she usually does not have the disease since the normal X chromosome without the mutation neutralizes the mutation in the abnormal X chromosome. However, half her sons will inherit the mutation-containing X chromosome and therefore have the X-linked disease. Half the daughters will inherit the mutation-bearing X chromosome and are usuallly healthy 'carriers'.
X-linked recessive, father affected
X-linked disorders are caused by a mutation on the X chromosome and both sexes can pass this to their children. If the mutation is in a recessive gene and carried by the father, he has the disease since his only X chromosome is mutant and he has no normal X to blunt the effects of the abnormal gene. His sons only receive his Y chromosome and thus are all normal. However, all his daughters receive his one and only X chromosome and will be healthy 'carriers'. Thus such males will have no affected children but half their grandsons from those daughters will have the same disease as he does.
X-linked disorders are caused by a mutation on the X chromosome and both sexes can pass this to their children. If the mutation is in a recessive gene and carried by the father, he has the disease since his only X chromosome is mutant and he has no normal X to blunt the effects of the abnormal gene. His sons only receive his Y chromosome and thus are all normal. However, all his daughters receive his one and only X chromosome and will be healthy 'carriers'. Thus such males will have no affected children but half their grandsons from those daughters will have the same disease as he does.
Diagnosis and Prognosis
The diagnosis of nearsightedness is usually made late in the first decade of life by ophthalmologists. The initial evaluation should include a detailed retina examination as myopia is a risk factor for retinal degeneration and detachment. Moreover, since myopia occurs in many heritable diseases, it is important that a complete general physical examination be done to ensure that no other condition is present.
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