In human DNA, cells are supposed to have chromosomal pairs, but when there are too many or too few (and therefore incomplete pairs), a great number of abnormalities can take place. Perhaps the most famous of these is Trisomy 21, when there is an extra number 21 chromosome, otherwise known as Down syndrome.
When Down syndrome was first beginning to be understood, people with Trisomy 21 usually didn’t live to their teens. They usually died from an array of complications involving the heart, gastrointestinal system, and thyroid, as well as many other organs. Throughout the 20th century, life expectancy increased to almost 30 in the 1980s, and now the norm is well past 50. A curious development is that Alzheimer’s disease onset is almost always around 40 years of age (which is why many of the early genetic studies on AD started with examining chromosome 21).
For better or worse, because medical advances now allow almost all people with Down syndrome to have a much longer life expectancy, an inevitable consequence is the expectation of caring for a loved one with Alzheimer’s disease. It used to be that parents would outlive their children if the children had Down syndrome, but it is entirely more likely now to be the other way around, which means siblings will become the primary caregivers.
Parents of children with Down syndrome should prepare for this by educating those siblings and/or by pre-arranging and selecting a care facility that specialises in adult Down syndrome care with attention to Alzheimer’s disease. It may also be worth contributing to Alzheimer’s research, specifically within Down syndrome contexts.