What is Down syndrome?

Down syndrome occurs when a person is born with extra genetic material from chromosome 21. Usually, this is due to an extra chromosome. Chromosomes are groups of genes, the material that tells the body how to grow, develop, and function.

People typically have 46 chromosomes when they are born. Most often, people with Down syndrome have an extra copy of chromosome 21, for a total of 47 chromosomes.

Down syndrome affects the development of the brain and body. People with Down syndrome have a wide range of abilities and disabilities. There are several medical and mental health conditions that occur at higher rates in people with Down syndrome.

What are the types of Down syndrome?

Doctors categorize Down syndrome into three categories. They include:

  • Trisomy 21: The most common type of Down syndrome, trisomy 21 occurs when a developing baby has 3 copies of chromosome 21 in every cell instead of the typical 2 copies. This form is present 95% of the time.
  • Translocation: In this type of Down syndrome, there is an extra full or partial amount of chromosome 21 attached to another chromosome.
  • Mosaicism: In the rarest type of Down syndrome, some cells contain the usual 46 chromosomes, and some contain 47. The extra chromosome in these cases is chromosome 21.

How common is Down syndrome?

Down syndrome is the most common chromosome-related condition in the United States. Roughly 6,000 babies are born with the condition in the U.S. every year (about 1 in every 700 babies). More than 300,000 people are living with the condition in the U.S. today. The chance of giving birth to a child with Down syndrome increases as women grow older.

What causes Down syndrome?

Each human cell typically contains 23 pairs of chromosomes. Down syndrome occurs because of changes in the way cells in chromosome 21 divide. Every person with Down syndrome has an extra amount of this chromosome in some or all of their cells.

Down syndrome is a genetic condition. In the most common type of Down syndrome, the condition occurs sporadically and is not inherited. However, when translocation or mosaicism is the cause of Down syndrome, hereditary (passed down among family members) should be looked at as a cause.

What are the signs and symptoms of Down syndrome?

Different people with Down syndrome have variable symptoms. The condition causes physical, cognitive and behavioral symptoms.

Physical signs of Down syndrome can include:

  • Small physical size
  • Weak muscle tone
  • Flattened face
  • Eyes that slant upward
  • Single crease in the palm of the hand

Common learning and behavioral symptoms of Down syndrome include:

  • Delays in speech and language development
  • Attention problems
  • Sleep difficulties
  • Stubbornness and tantrums
  • Delays in cognition
  • Delayed toilet training

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