Generally speaking, people have 23 pairs of chromosomes for a total of 46 chromosomes. Chromosomes are rod-like structures on DNA that store your genes, the material that carries the instructions for inherited traits such as hair color, skin color, and height. For each pair of chromosomes, you get one from your mother and one from your father.
A person with Down syndrome has a third copy of one chromosome, chromosome 21, instead of the usual pair. This extra copy of chromosome 21 disrupts the course of normal brain and body development. Those with Down syndrome typically have distinct physical features and often experience physical and mental delays and disabilities.
Down syndrome is the most common chromosomal condition identified in the United States. Each year, approximately 6,000 American babies are diagnosed with Down syndrome. It occurs in about one in every 700 babies. According to the National Down Syndrome Society, there are more than 350,000 people living with Down syndrome in the US.
Certain physical features are common in people with Down syndrome. These features include a slightly flattened face, particularly the bridge of the nose, almond-shaped eyes that slant upward, a short neck, small ears, bulging tongue, small hands and feet, short stature, and poor muscle tone. People with Down syndrome may have some or all of these features to varying degrees.
People with Down syndrome typically have some degree of developmental disability, but it’s generally mild to moderate. They may be slow to reach developmental milestones such as crawling, walking, and talking. Mental and social delays may result in impulsive behavior, poor judgment, short attention spans, and slow learning.
There are also certain medical problems that are common in people with Down syndrome. These conditions include congenital (meaning present at birth) heart defects; obstructive sleep apnea, a condition in which breathing temporarily stops while the person is sleeping; hearing loss; ear infections; eye disorders, and thyroid problems. People with Down syndrome must be routinely monitored for these conditions by a health care provider.
There are three types of Down syndrome: trisomy 21, translocation Down syndrome, and mosaic Down syndrome. Trisomy 21 is the most common, occurring in about 95 percent of Down syndrome cases. With trisomy 21, each cell in the body has three separate copies of chromosome 21.
Translocation Down syndrome occurs in about 3 percent of cases. With this type, an additional full or partial copy of chromosome 21 is present, but it attaches, or translocates, to another chromosome, usually chromosome 14, instead of existing on its own.
People with translocation Down syndrome can inherit the condition from an unaffected parent who is a carrier of the genetic rearrangement. In one-third of cases of Down syndrome resulting from translocation, there is a hereditary component, accounting for about 1 percent of all cases of Down syndrome.
Mosaic means mixture or combination, so people with mosaic Down syndrome have a mixture of cells with three copies of chromosome 21 and cells with the normal two copies. People with this type of Down syndrome, which occurs in about 2 percent of cases, may have milder symptoms and effects because they have fewer cells with an extra chromosome 21.
The cause of Down syndrome is unknown, but certain factors increase your risk of having a baby with the syndrome. One factor is the mother’s age. Your risk of having a baby with Down syndrome increases as you get older, especially if you are age 35 or older. Still, the majority of babies with Down syndrome are born to mothers younger than 35 because there are many more births in younger women.
In addition, if you or your partner are a carrier of the translocation Down syndrome gene, you can pass it on to your baby. And if you already have a baby with Down syndrome, your risk of having another baby with the same syndrome is increased.
If you want to know if your baby has Down syndrome before it is born, there are two types of tests that can be done: screening tests and diagnostic tests. Screening tests can reveal if your baby has a higher risk, but cannot determine if your baby definitely has Down syndrome. Screening tests typically include various blood tests and ultrasound.
Diagnostic tests can determine if your child has Down syndrome, but there’s a slightly higher risk for miscarriage associated with these tests. One diagnostic test is chorionic villus sampling, which analyzes a tissue sample removed from the placenta to look for genetic disorders.
Other diagnostic tests include amniocentesis, which examines some of the amniotic fluid surrounding your baby in the uterus, and percutaneous umbilical cord sampling, which involves taking a small amount of blood from the baby’s umbilical cord to check for chromosomal defects.
If you choose not to have these prenatal tests, Down syndrome will be diagnosed when your baby is born. If your doctor suspects Down syndrome, he or she will take a sample of your baby’s blood for karyotype testing. This test is done to identify and evaluate the size, shape, and number of chromosomes. It can detect three copies of chromosome 21.
Many problems associated with Down syndrome are lifelong, but early intervention, beginning in infancy, can help people overcome the challenges and live long lives. In 1960, babies born with Down syndrome often didn’t see their 10th birthday. Today, the average life expectancy of individuals with Down syndrome is 60 years, with many living into their 60s and 70s.
What’s more, many people with Down syndrome are able to work and fully participate in social activities. These people live full and meaningful lives.