Abnormal Chromosome Number & Structure

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  • 0:01 Chromosomes
  • 0:31 Abnormal Chromosome Number
  • 2:55 Abnormal Chromosome Structure
  • 4:20 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
The chromosomes inside of a person's cells can undergo structural changes or changes in number, causing severe and fatal disorders. In this lesson, we'll outline what these changes are and discuss examples of some of these disorders.


A chromosome is a packaged structure located in a cell's nucleus that contains DNA. It's like a zip file of really important information. Normally, people have 23 pairs of chromosomes, for a total of 46. However, there are times when people have an abnormal number of chromosomes or their structure is abnormal. The terms for these conditions and their associated disorders are the topic of this lesson.

Abnormal Chromosome Number

Any deviation in the number of chromosomes that involves individual chromosomes as opposed to entire sets of chromosomes, be it more or less than the expected 46, is called aneuploidy.

If there are fewer than the expected numbers of chromosomes, we call this monosomy, the presence of only one complete chromosome, instead of a pair. Here mono- means one, like a monorail uses one rail. An example of a disorder resulting from this abnormality is found in girls: Turner syndrome, a condition where one X chromosome is missing or is altered in structure. Problems resulting from Turner's include a short stature, learning disabilities, an inability to normally conceive a child and many other issues.

In opposition to monosomy is trisomy, the presence of an additional (third) chromosome of one particular type. Here, tri- means three, like a tricycle has three wheels. Trisomy 21, otherwise known as Down syndrome, refers to the presence of all or part of a third copy of the 21st chromosome. Problems associated with Down syndrome include a small head, short neck and intellectual disabilities, among lots of other potential signs and symptoms.

Now, if a person has more than two sets of homologous chromosomes, we term this polyploidy, where poly- means many. Please note the important distinction between polyploidy and aneuploidy, which we discussed earlier. Aneuploidy refers to a change in part of the chromosome set, while polyploidy refers to a change in the entire set of chromosomes. Aneuploidy would be like you having one set of colored balls, plus one extra ball of a color present in your set. Polyploidy would be like having three sets of all of the balls you had originally.

One example of polyploidy is Triploidy, where a fetus has an extra set of chromosomes in their cells. This means they have 69 as opposed to 46 chromosomes owing to an additional entire set of 23 chromosomes. This is a lethal condition and either results in a miscarriage, stillbirth or death not long after birth.

Abnormal Chromosome Structure

Sometimes, a chromosome is present in normal quantities, but its structure is altered as a result of something like damage from radiation or errors during cell division. This can lead to one of four types of changes to the chromosome's structure:

  1. Deletion, which is the loss of a chromosomal segment from a chromosome set
  2. Duplication, which is when there's more than one copy of a specific chromosomal segment
  3. Inversion, which is the removal of a chromosomal segment, followed by its 180-degree rotation and the reinsertion of this reversed segment into the same location
  4. Translocation, or the relocation and attachment of a chromosomal segment to a nonhomologous chromosome. This means that chromosome segment is moved to a completely different chromosome

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