(MayoClinic.com) Genes control the function of every cell in your body. Some genes determine basic characteristics, such as the color of your eyes and hair. Other genes can make you more likely to develop certain diseases — including Alzheimer's.

Researchers have identified several genes that are associated with Alzheimer's disease. But genetic risk factors are just one part of the Alzheimer's story. According to the National Institutes of Health, 75 percent of the people who get Alzheimer's disease have no family history of the disorder.

While some rare forms of Alzheimer's occur before the age of 65, the most common variety of Alzheimer's usually begins after the age of 65. The most common gene associated with this late-onset Alzheimer's is called apolipoprotein E (APOE).

APOE has three common forms:

• APOE e2 is the least common. It appears to reduce the risk of Alzheimer's.
• APOE e4 is a little more common. It appears to increase the risk of Alzheimer's.
• APOE e3 is the most common. It doesn't seem to affect the risk of Alzheimer's in either direction.

Because you inherit one APOE gene from your mother and another from your father, you may have two different types of APOE genes — for example, one APOE e3 gene and one APOE e4 gene. Having at least one APOE e4 gene increases your risk of developing Alzheimer's. And if you have two APOE e4 genes, your risk is even higher.

But not everyone who has an APOE e4 gene — or even two APOE e4 genes — develops Alzheimer's. And the disease occurs in many people who have no APOE e4 gene. This indicates other factors are involved in the development of Alzheimer's.

Study results also show a link between late-onset Alzheimer's and a gene known as SORL1. This is the first gene, other than APOE, to be linked to the most common form of Alzheimer's. Some variations of SORL1 appear to increase the production of amyloid-beta fragments, which form structures called amyloid plaques in the brain. These plaques are one of the hallmarks of Alzheimer's disease.

Less than 10 percent of the people who develop Alzheimer's disease have the early-onset variety, which typically begins before the age of 65.

Scientists have identified three genes that cause early-onset Alzheimer's. If you inherit one of these mutated genes from either parent, you almost certainly will experience Alzheimer's symptoms before the age of 65. The genes involved are:

• Amyloid precursor protein (APP)
• Presenilin 1 (PS1)
• Presenilin 2 (PS2)

These abnormal genes cause the production of excessive amounts of a toxic protein fragment called amyloid-beta peptide. As these fragments proliferate in the brain, the brain cells start dying and the signs and symptoms of Alzheimer's begin.

However, at least half of the people who have early-onset Alzheimer's don't have any of these three gene mutations. That suggests that this aggressive form of Alzheimer's disease is linked to other genetic mutations that haven't been identified yet.

Most experts don't recommend genetic testing for late-onset Alzheimer's. In some instances of early-onset Alzheimer's, however, genetic testing may be appropriate.

In the case of APOE, knowing whether you have the e4 variety really doesn't tell you much. Although many people with APOE e4 develop Alzheimer's, many don't. Conversely, some people with no APOE e4 genes get Alzheimer's.

While testing for the mutant genes that have been linked to early-onset Alzheimer's — APP, PS1 and PS2 — may provide more certain results, the emotional consequences of having that information might be devastating. And the information could result in employment or health insurance discrimination.

Even without genetic testing, doctors can diagnose Alzheimer's with 90 percent accuracy.

Scientists study the genetics of Alzheimer's disease because understanding the basis of a disease may provide clues about how to combat it. Such information may prove vital in the development of new ways to treat, or even prevent, Alzheimer's disease in the future.

The Alzheimer's Disease Genetics Study, sponsored by the National Institute on Aging, is examining genetic information from families that have at least two siblings who have developed Alzheimer's after the age of 60. If your family is interested in participating in this study, call 800-526-2839.

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