Von Willebrand disease is a hereditary disorder. It is caused by an abnormal gene.
Each cell of the body contains structures called chromosomes. A chromosome is a long chain of chemicals known as DNA. This DNA is arranged into about 25,000 units called genes which determine such things as the colour of a person’s eyes.
Each cell contains 46 of these chromosomes arranged in 23 pairs. Von Willebrand disease is caused by a defect on chromosome 12. The defect could be on the chromosome 12 from the mother, on the chromosome 12 from the father, or on both.
There are two ways of getting the hereditary form of VWD.
• It can be passed from a parent who has the defective gene (even if this person has no symptoms of VWD) to a child at the time of conception.
• One of the baby’s genes can undergo a change. This is called a genetic mutation. The baby’s parents do not carry this defective gene, and the parents' other children would not inherit it.
Male and female children have an equal chance of inheriting VWD.
Because it is a hereditary disorder, VWD often affects several members of the same family. One can often trace VWD through a family tree. Figure 3 shows three generations of a family with VWD.
In the first generation: The grandfather (John) has mild Type 1 VWD and the grandmother (Mary) is unaffected by VWD.
In the second generation: There is a 50% chance that each of John and Mary’s children will be born with the VWD gene. The diagram shows one of the two daughters (Ann) and one of the two sons (Peter) inheriting the defective gene. They both have mild Type 1 VWD.
In the third and fourth generations: The daughter with VWD (Ann) marries a man who does not have VWD (Charles). Their children have the same 50 % chance of inheriting the disease. In the diagram one daughter (Julia) gets VWD.
The son with VWD (Peter) marries a woman (Isabel) who also carries the abnormal chromosome 12. Their children have a 25% chance of being unaffected (Claire), a 50% chance of inheriting the defective gene from one of the parents and thus having mild Type 1 VWD (Helen and David), and a 25% chance of inheriting the defective gene from both parents (Robert) and getting severe Type 3 VWD.