The 21st chromosome is unique in the gene: it’s the only numbered pair that someone can survive having three of rather than the standard two. Three of any of the other chromosomes, aside from the X and Y that are primarily involved in sex characteristics and determination, will prevent the zygote from developing into an embryo and the baby from being born. In cases of trisomy 21, the baby can survive, albeit with Down syndrome, the most common birth defect.
Down syndrome used to be associated with a short lifespan—an average of 25 years for babies born with the condition in 1983—but that has changed. A person with the condition will exhibit stunted growth, profound cognitive deficits, a flat face with a protruding tongue, creased palms, small hands and feet, and often short fingers and poor muscle tone. These children, and later adults, are at heightened risk for hearing impairment, leukemia, and heart problems, and they are more prone to Alzheimer’s disease.
Today, Down syndrome can frequently be detected in utero with prenatal testing. Now a new testing procedure has been developed that is more accurate and better for women. When initial tests find signs of Down syndrome in the fetus, the next step is to confirm it with amniocentesis or a different test called chorionic villus sampling. However, these procedures are physically taxing for the mother-to-be and have a danger of miscarriage, which is why obstetricians try to avoid doing them. The new approach, a simple blood test, has a much lower false positive rate, meaning that it seldom find signs of Down syndrome in a fetus that doesn’t have the condition. That means the riskier procedures can be avoided in more cases.
After the child is born, new treatments may soon be available. Scientists have found a way to silence the extra chromosome in the lab. If this can be done in a living fetus, it would mean that the effects of the extra chromosome would be nullified, so that a child born with the genetic cause of Down syndrome would avoid the condition. Another research team is testing medications that, it is hoped, will enhance the ability of neurons to form connections, thereby facilitating learning and forming memories. This would enable students with Down syndrome to keep pace with their non-Down peers academically and socially.