Humans usually have 23 pairs of chromosomes or 46 total chromosomes. Occasionally someone may have three copies of a chromosome instead of two. They would then have 47 chromosomes. This condition is called trisomy.
Trisomy can occur in any chromosome, but the most common type is Trisomy 21, also known as Down Syndrome. Trisomy 21 is caused by an extra copy of chromosome 21 and is the most common cause of developmental disability in humans.
Trisomy 21 is usually caused by a random genetic error that occurs in the egg or sperm cell, although it can occasionally be inherited from a parent. The error results in an extra copy of chromosome 21, leading to three copies instead of the usual two. This extra copy affects the development of the embryo, resulting in physical and intellectual disabilities.
Individuals with Trisomy 21 typically have characteristic facial features, are shorter than average, and have a single deep crease across the palm of their hands.
Individuals with Trisomy 21 tend to have an IQ that is lower than average. They may speak later than other children. They may have difficulty with motor skills, communication, and socialization.
The severity of Trisomy 21 varies from individual to individual and is often determined by the presence of physical and developmental delays. Although there is no cure for Trisomy 21, there are treatments available that can help individuals with this condition reach their full potential.
Early intervention is an important part of helping individuals with Trisomy 21 reach their goals. Early intervention often includes physical, occupational, and speech therapy. Other treatments may include special education, medications, and assistive technology.
Trisomy 13 and 18
Trisomy 13, also known as Patau Syndrome, is a rare form of trisomy in which individuals have an extra copy of chromosome 13.
This condition is often marked by abnormalities of the skull and brain and sometimes extra toes and fingers. People born with this condition do not usually survive beyond infancy.
Trisomy 18, also known as Edward Syndrome, is also quite rare. It is a condition in which there is an extra copy of chromosome 18. Trisomy 18 causes many birth defects, and like Trisomy 13, people with this condition do not usually survive.
Testing for trisomy
There may be signs of trisomy during pregnancy. More than normal amounts of amniotic fluid, small size, or unusually low levels of activity can be warning signs. Ultrasound may also pick up abnormalities in development. There is also a blood test that may identify trisomy.
When you have concerns about trisomy, talk with your ob-gyn or family doctor.