What is trisomy?

chromosomes1Trisomy is a condition in which individuals are born with an extra copy of a specific chromosome in most or all of their cells. This means that they have three copies of a given chromosome in each cell rather than the typical number, which is two.

Chromosomes are the structures in cells that contain genes. Each person normally has 23 pairs of chromosomes, or 46 in all. An individual inherits one chromosome from the mother’s egg and one from the father’s sperm. When an egg and sperm join together, they normally form a fertilized egg with 46 chromosomes.

Sometimes something goes wrong before fertilization. A developing egg or sperm cell may divide incorrectly, causing that egg or sperm to have an extra chromosome. When this cell joins with a normal egg or sperm cell, the resulting embryo has 47 chromosomes instead of 46.

There are a variety of health conditions that may be associated with trisomy. Common physical problems for individuals with a trisomy include heart defects, vision or hearing problems, and intellectual and developmental disabilities.

Although trisomy can occur with any chromosome, there are three conditions that are most often associated with an extra chromosome. They are:

  • Trisomy 21 or Down syndrome: Down syndrome is one of the most common birth defects. In the US, about 6,000 babies (or 1 in 700) are born with Down syndrome each year. Most affected individuals have intellectual disabilities within the mild to moderate range. Although health conditions such as heart defects and vision and hearing problems are associated, most of these can be treated, and life expectancy is now about 60 years
  • Trisomy 18 is also called Edward syndrome. Trisomy 18 occurs in about 1 in 5,000 live births each year. Affected individuals may have heart defects, significant intellectual and developmental delay, and other life-threatening medical problems.
  • Trisomy 13, also known as Patau syndrome, occurs in about 1 in 10,000 to 16,000 live births each year worldwide. Individuals with trisomy 13 often have heart defects, brain or spinal cord abnormalities, severe intellectual and developmental disabilities, and multiple physical problems in many parts of the body.

It is important to understand that every individual with a trisomy is unique and not all of them will have the same symptoms. The problems depend on which chromosome is duplicated and how much of the extra chromosome is present.

March of Dimes grantees are studying basic biological processes of development to better understand the process of early cell division and how a trisomy may occur. In the past five years, the March of Dimes has invested $5,274,554 in trisomy research.

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