To understand how HAE is inherited, it’s helpful to look at genetics. People receive half their genetic code from their mother and half from their father. The genetic code is carried in units called ‘genes’.
In some diseases, such as cystic fibrosis or sickle-cell anaemia, a child will only get the disease if they receive an abnormal, disease-causing gene from each of their parents. This type of gene, that usually only causes a disease when a person has two copies, is called a ‘recessive’ gene. This means that although the parents of the affected child each have an abnormal gene, they may not have the disease themselves. These parents are considered ‘carriers’ of the disease.
In HAE, having just one abnormal gene will cause the disease. Biologists call these types of genes ‘dominant’. This means that there is a 50/50 chance that a child who has one parent with HAE will also have HAE themselves.
Children born without the abnormal gene will effectively put an end to the occurrence of HAE in that branch of the family tree. All following generations (that child’s children and grandchildren etc.) will be free of HAE.
Occasionally, HAE can occur in a person without a previous family history of HAE. This is due to a spontaneous change in the genetic code for the C1-INH protein, creating an abnormal version of the gene. Not only will this person have the disease, but he or she will also have the potential to pass on this changed gene (and therefore the disease) to his or her children.