Human colour vision is based upon the functions of three types of cone cells(L,M and S cone) in eye. A defect (mutation or rearrangement of the gene) in any one or two of the gene producing these cones will result in the production an abnormal cone causing CVD in a person. This mutation passes from one generation to another i.e. a genetic condition which is inherited from parents to their children precisely known as ‘congenital’ colour vision deficiency. Most commonly the inheritance takes place from mother to her child.
How is CVD inherited?
Know these terms before you start reading:
- Sex linked(denoted as x-linked): The defective gene is encoded on the sex chromosome ‘X’; men only have one of them (XY) compared to women (XX).
- Autosomal: The defective gene is not encoded on the sex chromosome, equal for men and women.
- Dominant: if it is encoded on one chromosome, you really suffer from it.
- Recessive: if you have another healthy chromosome, it won’t show up.
The faulty gene for red-green colour vision defect is present on chromosome X in an X linked recessive pattern of inheritance. Males have one X chromosome and females have two X chromosomes. Females are usually carriers of the faulty gene, i.e., they have one chromosome with the faulty gene and another with a normal gene. For a female to be affected (rare), she needs to have a faulty gene in each chromosome which happens when both her parents carry the faulty gene. The males are affected when the X chromosome has the faulty gene. They receive the faulty gene only from the mother. The father passes the chromosome with the faulty gene to his daughter and not to his son.
The gene responsible for yellow-blue colour vision defect is present on chromosome 7 and is passed on in an autosomal dominant pattern of inheritance. The chromosomes are present in pairs. Each chromosome in the pair is received from each parent. For a person to be affected with yellow-blue colour vision defect, a single faulty gene is sufficient. This gene can be inherited from either one of the parent.
The faulty gene responsible for complete colour blindness is present on chromosome 2 and is inherited in an autosomal recessive pattern of inheritance. For a person to be affected with complete colour blindness, they should have two copies of the faulty genes (in each chromosome). A person is affected only when both the parents carry one or two copies of the faulty gene.
What are the genetic tests available and how will it be useful?
Analysing the sequence of the gene responsible will help in identifying the type of colour blindness and the severity of it. This result will be useful for a person to plan his/her future generations through genetic counselling.
Genetics of CVD – scientific facts
(Scientific Information on human colour vision deficiency genetics)
There are three genes (OPN1LW, OPN1MW, and OPN1SW) that are responsible for the production of the opsin in the cone. OPN1LW codes for the opsin that is sensitive to the yellow/orange part of the light spectrum and the cone with this pigment are called L cones.
The opsin produced by OPN1MW gene is sensitive to the yellow/green part of the light spectrum and the respective cones are called M cones. OPN1SW gene codes for opsin that is more sensitive to light in the blue/violet part of the visible spectrum, and the cones with this opsin are called S cones.
The blue pigment gene (OPN1SW) is located on chromosome 7 and mutation in this gene causes yellow-blue colour vision defects(rare). The red and green pigment genes (OPN1LW & OPN1MW) are located on chromosome X and genetic changes in any one of these genes, cause red-green colour vision defects.
When there is a mutation in both, OPN1LW and OPN1MW genes, it causes blue cone monochromacy. The red-green colour vision defects and blue cone monochromacy are inherited in an X-linked recessive pattern.
Since the OPN1LW and OPN1MW genes are located on the X chromosome, males (have only one X chromosome), are always affected when they have a mutated gene. The females (have two X chromosomes) are most often the carriers as they would require two copies of the mutated gene (in each X chromosome) for them to be affected.
Fathers cannot pass the X linked traits to their son(s) which is a characteristic feature of X-linked inheritance pattern. Yellow-blue colour vision defects (rare) are inherited in an autosomal dominant pattern, which means that one copy of the mutated OPN1SW gene in each cell is sufficient to cause the condition. In many cases, an affected person inherits the condition from an affected parent.
Rod monochromacy is a very rare type of CVD and is caused when all the three types of cones are defective. The gene responsible for this defect is named as CNGA3 which is present on chromosome 2. This is inherited in an autosomal recessive pattern of inheritance, which means that 2 copies of the mutated gene is necessary for the person to be affected.
Are there any treatments available?There are strategies available that will help the affected person to function better in a colour oriented environment. Various clinical trials are being conducted for treating complete colour vision defect (achromatopsia) through gene therapy, but currently, there is no cure for colour vision deficiency.
Source: Center for Medical Genetics, Chennai
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