Inheritance

There are several types of inheritance of defective genes that are commonly seen causing some eye diseases. However it is not always possible to reliably determine which mode of inheritance is responsible in a given individual or family, particularly when only one person is affected.

Genes are bits of DNA found on chromosomes in each cell of your body are analagous to small bits of computer code. Genes tell the cell to do certain things, typically to make proteins which have specific functions within the cell.

An altered gene can have its effect in several ways, e.g. absence of production of that particular protein causing loss of that function, production of defective protein causing misdirection of that function, and then secondary events which flow on from these defects.

In many cases, eye diseases can be caused by many altered genes. Retinitis pigmentosa is known to be caused by at least 70 genetic mistakes!

The common inheritance patterns are:

1. autosomal dominant
2. autosomal recessive
3. x-linked recessive

Autosomal Dominant Inheritance

In this form of gene transmission, only one copy of the altered gene is required to cause the disease to appear.

The risk to a subsequent generation is that about 50% of offspring will be affected by the condition. Males are affected roughly equally to females.

Autosomal Recessive Inheritance

In this form of inheritance, two copies of the altered gene are required to produce the disease. Unions that bring two copies of the altered gene together are usually just bad luck (genetic lottery) or occasionally the result of marriage within the same family (consanguinity).

The chance of an affected offspring of such a union is 25% but note that 50% of offspring will be carriers once again. Carriers usually do not manifest the disease, or only to very limited extent.

X-linked Recessive Inheritance - female

Some altered genes are carried on the x-chromosome. In the normal female there are two x-chromosomes and typically an altered gene on one is over-ridden by the normal gene in the other (sometimes to varying extent). Thus a carrier female is usually not affected or only minimally affected by an x-linked recessive disease. However in the normal male there is only one x-chromosome and any gene alteration will be manifest as disease.

In the illustration here, a carrier female transmits the altered gene to 50% of her male offspring and to 50% of her female offspring. A male with the gene will be affected by disease whilst the females will be carriers once again.

X-linked Recessive Inheritance - male

Some altered genes are carried on the x-chromosome. In the normal female there are two x-chromosomes and typically an altered gene on one is over-ridden by the normal gene in the other (sometimes to varying extent). Thus a carrier female is usually not affected or only minimally affected by an x-linked recessive disease. However in the normal male there is only one x-chromosome and any gene alteration will be manifest as disease.

In this illlustration an affected male transmits the gene to none of his male offspring but all his female offspring are carriers.