We seem to hear about new discoveries in genetics almost every day. So why aren't new gene therapies flooding the market? What do these breakthroughs really mean?

Sometimes, a visible trait, called a phenotype, is caused by an easily-identified gene. If that certain gene is present, then the phenotype is visible in the person. The brown-eye gene creates a brown-eyed person.

This situation is actually quite rare. Many traits, like right or left-handedness, or even diseases, have a genetic basis but do not have a single gene that can be identified as the cause. Scientists believe that these phenotypes are caused by more than one gene working together.

Multiple gene phenotypes are very complex because small variations in the number, location, and linkage between the genes can cause different phenotypes. Say, for example, that nine genes are involved in a certain disease. Five genes working together cause the disease and three offer protection from it.

Scientists would have to figure out which genes did what and which phenotype or visible trait went with which combination of genes. For example, if you inherited three causal genes and two protective genes would you get the disease? What about five and one? or one and three? On top of this, many phenotypes have an environmental component and depend in some way on the person's circumstances. So, when you hear about new discoveries in genetics, you'll know that discovering a single gene is often only the beginning.