These genes were thought to lead to blindness 100% of the time. They don't.

Genetic variants believed to cause blindness in nearly everyone who carries them actually lead to vision loss less than 30% of the time, new research finds.

The study challenges the concept of Mendelian diseases, or diseases and disorders attributed to a single genetic mutation. The idea is that Mendelian diseases — such as the neurological disease Huntington's and the bleeding disorder hemophilia — are passed down in predictable ways in families, and if a given person carries a disease-causing mutation, they will have it.

These diseases stand in contrast to those caused by multiple genes and environmental factors, which makes their occurence harder to predict in family lines.

"What we suggest is that there is overlap there," senior study author Dr. Eric Pierce, director of the Ocular Genomics Institute at Mass Eye and Ear and an ophthalmologist at Harvard Medical School, told Live Science. In other words, many diseases thought to have simple, Mendelian causes might be a lot more complex than previously thought.

And this doesn't only apply to inherited blindness. Similar results have been found for other...