What Is Trisomy? - Definition & Symptoms

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  • 0:02 Trisomy
  • 0:28 How Does Trisomy Occur?
  • 1:58 Symptoms of Trisomy
  • 2:41 Trisomies 13, 18 & 21
  • 3:18 Sex Chromosome Trisomies
  • 4:37 Lesson Summary
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Lesson Transcript
Instructor: Katy Metzler

Katy teaches biology at the college level and did her Ph.D. work on infectious diseases and immunology.

When a diploid organism has an extra copy of one of its chromosomes, it's called trisomy. In this lesson, find out what causes trisomy and its symptoms in humans.


In a diploid organism, each cell normally has two copies of each chromosome - one from its mother and one from its father. But sometimes, a mistake can occur, and an embryo gets an extra copy of one of its chromosomes. Trisomy is when a diploid organism has three copies of one of its chromosomes instead of two. Trisomy is an example of aneuploidy, or an organism having an abnormal number of chromosomes.

How Does Trisomy Occur?

Trisomy often occurs because of errors during meiosis, which is the process by which gametes, or eggs and sperm, are formed. In meiosis, the replicated chromosomes are sorted into daughter cells in two steps, called meiosis I and meiosis II. As you can see, at the end of meiosis II, each gamete should be haploid, meaning that it has only one copy of each of its chromosomes. The math checks out perfectly during fertilization: two haploid gametes fuse to form a diploid embryo.

Normal meiosis versus chromosome nondisjunction.

However, just like any other cellular process, meiosis is error-prone, and sometimes the chromosomes don't separate normally. This is called chromosome nondisjunction, and it can happen either in meiosis I or meiosis II. When chromosome nondisjunction occurs, the chromosomes don't separate normally. This results in some gametes that have an extra copy of a chromosome, and other gametes that are completely missing a chromosome.

When these gametes fuse with normal gametes during fertilization, the result is an aneuploid embryo. The diagram shows how trisomy can occur when a normal gamete fuses with a gamete that has an extra chromosome.

Diagram of how a trisomic embryo can form.

The chance of trisomy increases with the age of the mother. For unknown reasons, the likelihood that a nondisjunction event happens during egg formation is greater as a woman gets older. This is one reason that women in their late 30s and 40s may have a harder time becoming pregnant and have a higher chance of having babies with birth defects.

Symptoms of Trisomy

Our cells are optimized to function with the dosage or amount of proteins that is normally made from two copies of each chromosome. Trisomies and other aneuploidies disrupt the normal balance of gene products inside cells.

In humans, most trisomies are not tolerated. Scientists estimate that aneuploidy occurs in 5-20% of human embryos. However, the correct dosage of gene products inside cells is so important that the vast majority of these embryos die before the mother even realizes that she is pregnant. However, some trisomic humans survive until birth. They have a wide variety of symptoms depending on which chromosome is trisomic.

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