Types of Down Syndrome

Every year, doctors diagnose more than 6,000 babies in the U.S. with Down syndrome, affecting 1 in 700 infants. (1) And while nearly all children born with Down syndrome share some common challenges and characteristics, the condition is a wide-ranging one that comes in several types and can present in different ways. Here are some helpful basics when it comes to understanding the disorder.

By definition, Down syndrome is a genetic condition whereby an individual has a surplus of DNA material, says Mary Pipan, MD, a behavioral pediatrician and director of the Trisomy 21 Program at Children’s Hospital of Philadelphia.

A quick genetics lesson is probably in order here.

Every cell in a person’s body contains their genetic code, or DNA, which typically consists of 23 pairs of chromosomes — with one chromosome in each pair coming from the child’s mother and the other chromosome in each pair coming from the child’s father. (2) Those 46 chromosomes are a person’s genetic blueprint — the stuff that makes them who they are, determining things like eye color and inherited risk of different diseases.

When someone has Down syndrome, they have all 23 pairs of their chromosomes, but they are also born with an extra full or partial copy of the 21st chromosome, Dr. Pipan explains. The condition is sometimes referred to as “chromosome 21 Down syndrome” or “trisomy 21 Down syndrome” because of this third copy of the 21st chromosome.

Every chromosome carries dozens, hundreds, or even thousands of specific genes. Pipan points out that many disorders stem from a single gene abnormality or mutation. Cystic fibrosis and muscular dystrophy are two examples of these single-gene disorders.

The 21st chromosome has 300 genes, and at least two of them — and potentially more — contribute to Down syndrome. These genes are located in the so-called Down syndrome critical region, a chromosome 21 segment purported to contain genes responsible for many features of Down syndrome, including craniofacial dysmorphology.

“So it’s a pretty complicated disorder,” Pipan says.

The extra genetic material is present from the moment of conception, so Down syndrome is not something a person can develop after birth or later in life: You’re either born with it or you’re not. Also, because every cell in the body has the extra 21st chromosome, children with Down syndrome “may have medical conditions that affect every part of the body,” Pipan adds.

While all forms of Down syndrome are associated with an extra full or partial copy of the 21st chromosome, there are a few different types of the condition. Those types are:

1. Trisomy 21 Down Syndrome The most common form of Down syndrome — accounting for roughly 95 percent of all cases — is trisomy 21 Down syndrome. The condition stems from an error in cell division known as “nondisjunction.” (4) At some point leading up to or at conception, in either the sperm or the egg, one of the parents’ pairs of chromosome 21 failed to separate. So instead of getting one chromosome each from the mother and father, the embryo’s DNA ends up with an extra chromosome in the 21st pair. As the embryo matures and its cells continue to replicate, the extra copy of that 21st chromosome is replicated over and over in every cell.
2. Translocation Down Syndrome The genetic science gets even more complicated here. Translocation Down syndrome results when an extra full or partial copy of chromosome 21 is present — just like in trisomy 21. But in these individuals, the extra full or partial chromosome 21 is attached to one of the other 23 chromosomes. (5) So a person with translocation Down syndrome has the usual 46 chromosomes, but one of them is bound to an additional copy of chromosome 21. Just 3 to 4 percent of Down syndrome patients have this type.
3. Mosaic Down Syndrome Again, the genetic science is tricky. But for those with mosaic Down syndrome (also called “mosaicism”), some — but not all — of their cells contain an extra copy of chromosome 21. This is the rarest type of Down syndrome, accounting for 1 to 2 percent of cases, and people with this type may have fewer or less-severe symptoms than those with the two more common types of Down syndrome.

Every person with Down syndrome is unique, and that uniqueness extends to the ways their form of the condition is expressed, says H. Craig Heller, PhD, a professor of biology and director of the Center for Down Syndrome Research at Stanford University in Palo Alto, California. While many people with Down syndrome will experience similar intellectual, medical, and physical challenges, those challenges can differ in severity, Heller explains.

“Facial-cranial features, for example, are nearly always present,” says Heller, referring to the distinctive appearance of individuals with Down syndrome.

A flattened face — especially across the nose — and upward-slanted, almond-shaped eyes are two of the common physical features of Down syndrome. Some of the other common physical characteristics include a short neck, small hands and feet, small stature, and a lack of muscle tone.

While some individuals with Down syndrome do not experience any other effects from the disease, Heller says problems with digestive function, immune function, and heart function are all common. So are issues with bone density, weight control, and speech.

Intellectual disabilities are also common. “There can be effects on learning and memory, and specifically the ability to form long-term memories,” Heller says.

But he emphasizes that every individual with Down syndrome is different.

“We certainly see some kids who are less affected than others, but we shy away from terms like ‘low-functioning’ and ‘high-functioning,’” Pipan explains. Some children with Down syndrome have a hard time learning to walk, and others have no problem running and jumping and riding a bike. Likewise, some individuals with Down syndrome may struggle to learn to read while others don’t, she says.

Pipan emphasizes that there are also positive characteristics associated with Down syndrome. “Children with Down syndrome are generally really social and outgoing, for example,” she says.

“Every individual with Down syndrome has their own strengths and weaknesses,” she adds. “Given the right support, they can accomplish a lot.”