Alleles - different forms of the same gene. Two or more alleles can shape each human trait. Each person receives two alleles of a gene, one from each parent. This combination is one factor among many that influences a variety of processes in the body. On chromosome 19, the apolipoprotein E (APOE) gene has three common forms or alleles: e2, e3, and e4. Thus, the possible combinations in one person are e2/2, e2/3, e2/4, e3/3, e3/4, or e4/4.
ApoE Gene - a gene on chromosome 19 involved in making ApoE, a substance that helps carry cholesterol in the bloodstream. The APOE e4 gene is considered a "risk factor" gene for AD and appears to influence the age of onset of the disease.
Chromosomes - thread-like structures in every cell of the human body. Chromosomes carry genes. All healthy people have 46 chromosomes in 23 pairs. Usually, people receive one chromosome in each pair from each parent.
Genes - basic units of heredity that direct almost every aspect of the construction, operation, and repair of living organisms. Each gene is a set of biochemical instructions that tells a cell how to assemble one of many different proteins. Each protein has its own highly specialized role to play in the body.
Genetic Mutations - permanent changes to genes. Once such change occurs, it can be passed on to children. The relatively rare, early-onset familial AD is associated with mutations in genes on chromosomes 1, 14, and 21.
Human Genome - the total genetic information found on the 23 chromosomes inherited from a parent. Through research decoding the human genome scientists believe humans have between 30,000 to 35,000 genes.
Proteins - Cells translate genetic information into specific proteins. Proteins determine the physical and chemical characteristics of cells and therefore organisms. Proteins are essential to all life processes.
WebMD Public Information from the U.S. National Institutes of Health