Scientists recognize that Alzheimer's Disease has two forms.
- Familial autosomal dominant (FAD) Alzheimer's Disease (i.e. disease clearly passed from generation to generation)
- Sporadic Alzheimer's Disease
The hereditary component is different for each.
- FAD is known to be entirely inherited. It is estimated that only 5 to 10% of all cases are of this type.
- Recent discoveries suggest there is a link between genetics and sporadic Alzheimer's Disease.
- Although discoveries have brought scientists closer to a possible test for Alzheimer's Disease, no reliable diagnostic or predictive genetic test is available at this time.
- The role of heredity in the various forms of Alzheimer's Disease is not clearly understood and continues to be the subject of much research.
How is a disease or characteristic passed from one generation to the next?
The relationship between Alzheimer's Disease and heredity is complicated. In understanding the role of heredity in Alzheimer's Disease, it is helpful to define some terms which explain how human characteristics are passed from generation to generation:
Chromosomes: Rod-like structures found in all cells of the body
Each individual has a total of 46 chromosomes in 23 pairs. Under normal circumstances, each person receives one chromosome of a pair from his/her mother and the other from his/her father.
Genes: The basic units of heredity that allow specific attributes or characteristics to be passed from one generation to the next.
Each chromosome carries genes which are like beads on a string.
Allele: Alternative forms of a gene
Each attribute or characteristic can have 2 or more alleles. For example, a given characteristic may have two forms of the gene, or two alleles, "A" and "a". The combination of alleles (e.g. AA, Aa, or aa), one of which comes from the mother and one from the father, determines what characteristics or attributes will be expressed and what alleles will be subsequently passed on to future generations.
Mutations: Genes that have undergone a process of change.
Once a mutation occurs, it can be passed on from generation to generation. Mutations can result in a disease or a disease susceptibility being expressed; mutations can also have not visible effects.
A family history of Alzheimer's Disease
Research indicates that individuals with a family history of Alzheimer's Disease (e.g. an affected parent or brother/sister) have a greater chance of developing the disease compared to persons having no affected relatives. Determination of the chance of risk of developing Alzheimer's Disease is difficult. A family history of Alzheimer's Disease does not imply the familial autosomal dominant type of the disease.
Separating the role of genes from that of other nongenetic factors such as toxic agents, viruses, head injuries, etc. is difficult. It may well be that in most families, inherited susceptibility and a particular "trigger" are both required before an individual develops the disease.
What are scientists doing in order to learn more about family history and Alzheimer's Disease?
In many parts of the world, medical and genetic records of families with Alzheimer's Disease are being collected. It is hoped that this information will increase knowledge about Alzheimer's Disease and heredity.
Large population surveys
Another method of investigation is to conduct large population-based surveys. These are important as they identify all Alzheimer's Disease cases within the population, as well as similarly aged individuals who do not have Alzheimer's Disease. The results of one such study, the Canadian Study of Health and Aging sponsored by Health Canada, confirmed that individuals with one or more relatives with Alzheimer's Disease have a somewhat greater chance of developing the disease compared with people with no affected relatives.