Tuesday, January 26, 2010
Genetic Counseling Helps to Answer Many Tough Questions
Recently I was asked what would be the odds of having both a husband and wife with the defective gene. When I responded that it is possible, but highly unlikely, the person asked another question about how the 50% rule of passing along a defective gene is determined. I pulled the following from the Kennedy's Disease Association web site to use as a starting point:
"Are you wondering if your children are or will be affected by this Disease? Since the defective gene is in the "X" chromosome, if the mother is the only carrier of the defective gene: there is a 50 % chance of passing the affected gene on to male children (in which they would develop symptoms in adulthood). The chance of passing the defective gene on to female children is also 50 % (in which case they would be carriers of the gene, but usually do not ever develop any severe symptoms).
If the father is the only carrier of the defective gene, your chances of passing the defective gene to a male child are 0 %. However, it is 100 % certain that you would pass this gene on to your female child, and she will be a carrier."
The 50% rule is not a perfect one out of every two rule
Another example using coins might explain it better. A coin comes up either heads or tails when flipped (50-50 chance). If you have 100 people flipping a coin not every person flipping the coin will have a heads one time and a tails the next. Some might have three heads come up in a row before a tail. Conversely, another might have four or five tails come up before a heads. However, when you average out all of the heads and tails that come up for the 100 people, it should be close to 50% for each.
Using my family, for example, where my mother was a carrier three out of seven boys have the defect (43%). Out of three girls, none has the defective gene (0%). For our family, three defective "X" chromosomes (30%) and seven normal "X" chromosomes were passed to the ten children. And, as many times as I heard that all the males in someone's family have the defective chromosome, I also heard as many stories that only one male has it. In poker terms, it could be called the "luck of the draw."
Was it Adam or Eve?
Reference another question that I am often asked; I do not think anyone knows what actually caused the first defect in the gene and when or where it happened. It is very difficult to trace the lineage of the disorder since for many years Kennedy's Disease had been misdiagnosed. It was not until the 1960s that Dr. William R. Kennedy first named the disease. And, all the way to the 1990s it was still commonly misdiagnosed (most often as ALS). Once a simple DNA blood test was discovered, early detection and correct diagnosis has improved substantially.
Many qualified genetic counselors are familiar with Kennedy's Disease. If you feel there is Kennedy's Disease in your family, I would recommend talking with a counselor. They can help answer your questions regarding how the defective gene is passed through a family. They are also there to discuss the benefits and possible ramifications of having children tested. The National Institute of Health (NIH), for example, has genetic counselors on their staff that are fully versed in Kennedy's Disease. Use these services to help you better understand Kennedy's Disease and the impact it might have on your family.