Alzheimer’s disease is an irreversible, progressive brain disease that is characterized by the development of brain lesions (known as plaques) and tangles in the delicate fibers of the nerve tissue. This causes the loss of connections between neurons, the specialized cells of the nervous system, and the eventual death of these cells. Scientists have long known that neurons are highly specialized cells. They have a streamlined design, giving them the ability to quickly conduct electrical messages as the body responds to changes in its environment. This specialization comes at a price, however, as these neurons can only make repairs to themselves if the damage is slight. Once damaged beyond repair, or dead, these cells can’t regenerate, leaving the victim lacking a sufficient number of vital neurons.
There are two types of Alzheimer’s disease: early onset, and late onset. Early onset occurs in people age 30 to 60. It is the rarer form of the disease, representing less than 5% of the people who contract it. Most cases of Alzheimer’s are of the late onset form, which develops after age 60. The causes of both diseases are still being worked out. Much research has been done, including a hunt for a possible genetic connection that may be the root cause of the disease. Scientists are looking at genes, sequences of DNA which code for the making of proteins, as possible contributors to both varieties of Alzheimer’s. Researchers first asked if perhaps victims have faulty DNA which could be causing their condition. Scientists pinpointed a specific neuron protein on Chromosome #21 that may lead to Alzheimer’s. It is called amyloid precursor protein (APP).
Researchers studied APP DNA sequences, making comparisons between people who have both types of Alzheimer’s, as well as people who don’t. The results of the sequence analysis are shown in the table below:
|Patient Type||Age||Partial AAP Gene Sequence From Chromosome #21|
Which explanation would best account for the differences found in DNA sequences among the non-Alzheimer’s men and women?