Final answer:
Homozygosity for the apolipoprotein E4 (APOE E4) allele is believed to increase the risk of developing late-onset Alzheimer's disease, which is the most common form of the disease.
Step-by-step explanation:
Homozygosity for the apolipoprotein E4 (APOE E4) variant is believed to predispose patients to Alzheimer-type dementia. The APOE gene, particularly its E4 allele, increases a person's likelihood of developing late-onset Alzheimer's disease, which is the more prevalent form of the disease. While other genes such as those coding for amyloid precursor protein (APP), presenilin, and tau protein are associated with the disease, the focus is on APOE E4 because of its significant correlation with increased risk in the more common sporadic cases.