It is believed that uveal coloboma is primarily genetic in origin. "Genetic" means that coloboma is caused by a gene that is not working properly when the eye is forming. Sometimes, coloboma is part of a specific genetic syndrome for which the genetics are known. For instance, coloboma is one feature of CHARGE syndrome, which is associated with a change in, or the complete deletion of, a gene called CHD7.
Researchers have found genes associated with coloboma in a few cases. To date, however, we still do not know which genes explain most cases of coloboma.
Some researchers have proposed that certain environmental factors may contribute to the development of coloboma, either in humans or in animals. These findings have been published over time in the research literature, but there have been no systematic analysis of possible links. For instance, it is known that babies exposed to alcohol during pregnancy can develop coloboma-but they also have other anomalies. There are no known strong links between environmental exposures and isolated coloboma.
It is always possible that coloboma happens strictly by chance. In summary, there is little data to presently say why coloboma happens to a person in a family where no one else is affected.
This answer is based on source information from the National Institute of Diabetes and Digestive and Kidney Diseases.