It has been observed that if a woman gives birth to a child with Down syndrome before the age of 35, she has an increased risk of developing Alzheimer's disease. Which of the following best explains this observation?

a. The same gene underlies both Down syndrome and Alzheimer's disease.
b. Women under 35 are not emotionally mature enough to manage a child with mental retardation.
c. Multiple copies of the APOE-4 allele increase the likelihood of both conditions.
d. A genetic mutation that increases susceptibility to Alzheimer's disease also increases the likelihood of passing on chromosomal abnormalities.

d. A genetic mutation that increases susceptibility to Alzheimer's disease also increases the likelihood of passing on chromosomal abnormalities.