Understanding guilt as it applies to families with genetic diseases is also helping researchers better understand the emotion, which has classically been explained as an internalized sense of morality.
"Psychoanalytically, we look at guilt as an internal process of punishment. You've done something bad, and your superego says you have done something bad and need to be punished. It is an internal way of keeping us moral," says Dr. Philip R. Muskin, a psychiatry professor at Columbia University.
"Guilt can be warranted when you've really done something wrong, but you can also feel it without having done anything," he says.
Advances in medicine have only intensified potential feelings of guilt surrounding inherited diseases. Illnesses that were once chalked up to God's will or a combination of factors can, in some cases, be absolutely attributed to a single bad gene inherited from one parent.
Huntington's disease, a degenerative illness that causes certain nerves in the brain to waste away and results in uncontrolled movements and mental deterioration, is caused by a single defective gene.
If one parent has the gene, there is a 50 percent chance the child will have the defect. Everyone who has the gene eventually develops Huntington's, but because the symptoms do not develop until middle age, parents often do not know they have passed down the disease.
With Huntington's, either parent may have passed down the gene, but in some cases like the degenerative eye disease retinoschisis, which is passed only from unaffected mothers to their sons, there is no question whose genes are responsible and who must shoulder the guilt.
"It is common for parents to feel this is my fault, and in the strictest sense it is their fault. In the strictest sense of who is to blame, the mother is at fault," Muskin says.
"But should she feel guilty? No, though many people in those circumstances would feel guilty," he says.
Because feeling guilty about a genetic disease is an irrational feeling, the best thing to do is think about it rationally through talking with friends, family, a psychologist or a religious adviser, Muskin says.
"A parent can't control the fact that her child has a disease, but she can emotionally take on the burden. Feeling guilty is a sly, emotional way to take control. Ideally, the parent comes to the conclusion that 'this isn't my fault' and accepts there are things out of our control. Can people reach that point? They can. Is it easy? No," he says.
Parents are not the only ones subject to feelings of guilt. Healthy siblings, too, report feeling ashamed that they were spared illness. Siblings feel guilty because they were embarrassed about a sick sibling or because they blame themselves for not doing enough to help.
"My wish was to go to the hospital where my brother was being treated and change places with him so he could be free," says Jay Neugeboren, an accomplished writer and advocate for the mentally ill who has been his brother Robert's caretaker for nearly 50 years.
"I felt I was unworthy. Robert was pure, noble and good and was being punished for being a better person than I was. I was succeeding in the world because I was selfish and he was punished for being loving," Jay Neugeboren says of Robert, who has schizophrenia.
Neugeboren says it took years of therapy for him to stop feeling guilty about his brother's illness and coming to accept that he was not to blame.