Alzheimer’s Heritability, Selection, and Affects of Inbreeding

Heritability


The role of heritability in Alzheimer’s disease is a question that is heavily contemplated and explored by researchers. A study published in the Archives of General Psychiatry worked to quantify the role that genetics played in the onset of the disease. Researchers from the University of Southern California looked at 392 sets of elderly twins over the age of 65. The use of twins allowed scientists to look at both environmental and genetic factors of the disease. In the case of identical (monozygotic) twins, all of their genes are shared, so if the disease is highly genetic than it should be observed in both or neither of the twins. If only one individual had it, it is likely to be more influenced by environmental factors. It was found that identical twins were more likely to both have the disease when compared to non-identical twins, which suggested that genetic factors are relatively significant. The estimated heritability was found to be between 58% and 79%. This study confirmed the Alzheimer’s appears to be heritable and that the genetic factors influence both male and female patients, but there are also some environmental factors that affect the development of the disease. However, what all of the environmental factors maybe are still not completely known.


Alzheimer's risk 'is 80% genetic'. BBC. news.bbc.co.uk/1/hi/health/4686806.stm.


Selection


The onset of Alzheimer’s typically occurs later in life, so it seems that selection would have very little to no effect since the phenotype is not exhibited until after the usual reproductive age. It also does not seem plausible that the history of Alzheimer’s in a person’s family history would limit their reproductive prospects either.


Inbreeding


In most cases, only dominate genes have been associated as a possible cause of Alzheimer’s disease. However a study published in Neurology exposed the idea that a recessive gene may play a role in the development of the disease. 821 elderly residents of an Arab community called Wadi Ara in Israel were screened for the disease. It was found that 20% of individuals over 65 had the disease and 60% of individuals over 85 did as well. This was compared to an overall 40% occurrence in the general population. The dominate gene (APOE-4) which is commonly linked with Alzheimer’s was found to be of low occurrence in this population (only 15%). However, the Wadi Ara community is known for its large family size and high frequency of inbreeding (about 44%), which leads researchers to look for a recessive gene that would be exposed by the inbreeding of the population.


This is not an isolated case. Other populations, such as the people of Sangueany in Canada also have a reasonably high rate of intermarriage and a higher occurrence of Alzheimer’s disease. Unfortunately, Americans often refuse to participate in such studies, up to 30%, which limits the research that can be done in the United States regarding the connection between Alzheimer’s and a possibly dangerous recessive allele.


Buckles, Julie. First evidence recessive gene plays role in Alzheimer’s disease. Genome News Network. 2000.
genomenewsnetwork.org/articles/09_00/Alzheimer_recessive.shtml.