OMIM ID:
Blue Cone Monochromacy
Alternate Names
Defective Genes
Clinical Characteristics
Ocular Features
This is usually a stationary cone dysfunction disorder in which the causative mechanism has yet to be worked out. Typical patients have severe visual impairment from birth and some have pendular nystagmus and photophobia similar to other achromatopsia disorders. Vision seems to be dependent solely on blue cones and rod photoreceptors. The ERG always shows relatively normal rod function whereas the cones are usually dysfunctional.
In some families, however, there is evidence of disease progression with macular RPE changes and myopia. This has led to the designation of ‘cone dystrophy 5’ for such cases even though the mutation locus impacts the same cone opsin genes at Xq28 that are implicated in the more typical BCM phenotype.
Systemic Features
None.
Genetics
Inheritance
This is an X-linked recessive form of colorblindness in which DNA changes in the vicinity of Xq28 alters the red and green visual pigment cluster genes via recombination or point mutations. Alternatively, the control locus adjacent to the cluster may be altered. In either case, the result may be a loss of function of these genes leaving blue-cone monochromacy.
The mutation for cone dystrophy 5 maps to Xq26.1-qter but the locus encompasses the opsin gene complex at Xq28 as well.
At least a quarter of individuals with blue cone monochromacy, however, do not have mutations in the vicinity of Xq28 suggesting that additional genetic heterogeneity remains.
Pedigree
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.
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'.