Monosomy: Definition, Examples & Disorders

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  • 0:00 Monosomy Defined
  • 0:50 Meiotic Nondisjunction…
  • 3:20 Chromosome…
  • 5:00 Disorders Caused by Monosomy
  • 6:50 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 is missing a copy of one of its chromosomes, it's called monosomy. In this lesson, discover the way in which monosomy occurs some of the disorders it causes in humans.

Monosomy Defined

Diploid organisms, like humans, have two copies of each one of their chromosomes, one from their mother and one from their father. Since the human genome is made up of 23 different chromosomes, that brings the grand total to 46 chromosomes packed into the nucleus of each one of our cells.

However, sometimes an organism is missing a chromosome. Monosomy is when a diploid organism has only one copy of one of its chromosomes instead of two. 'Mono' means 'one,' and 'somy' refers to 'chromosomes. 'Monosomy is an example of aneuploidy, or having an abnormal number of chromosomes. A human with a monosomy would only have 45 chromosomes instead of 46.

Meiotic Nondisjunction and Complete Monosomy

We need to talk about the relationship between meiotic nondisjunction and complete monosomy, but before we do, it's important we first review meiosis. Meiosis is the process of gamete, or sperm and egg, formation. During meiosis, the chromosomes are separated into daughter cells in two rounds called meiosis I and meiosis II. At the end of meiosis, the result should be gamete cells that each have one copy of each chromosome and are thus haploid instead of diploid. In humans, egg and sperm cells each have 23 chromosomes, and they fuse during fertilization to create an embryo with 46 chromosomes, the normal number a human should have!

The top part of this diagram shows normal meiosis. To simplify the drawing, only two chromosomes are shown, a purple one and a blue one.

Meiotic nondisjunction occurs when a chromosome doesn't separate properly during meiosis I or II. In the bottom half of the diagram, you can see what happens when nondisjunction happens in meiosis II. Both copies of the blue chromosome went into one gamete, and the other gamete didn't get any copies of the blue chromosome.

If these gametes fuse with normal gametes during fertilization, the embryo will be aneuploid. Instead of having two copies of the blue chromosome, one from the mother and one from the father, the embryo will have either one copy or three copies of the blue chromosome, depending on which abnormal gamete is involved.

This diagram shows how monosomy happens when a normal gamete fuses with a gamete that has no copies of the blue chromosome.

As you can see, the embryo gets two copies of the purple chromosome, which is exactly what is supposed to happen. Yay! But because of meiotic nondisjunction that happened in one of the gametes, the embryo only gets one copy of the blue chromosome. That means the embryo has a complete monosomy, because one copy of the blue chromosome is completely missing.

Monosomies that happen because of meiotic nondisjunction are not inherited because nondisjunction is a random error that just happens sometimes. However, the chances of nondisjunction during egg formation increase as a woman gets older. This is one reason that older women have more trouble carrying a pregnancy to term and are at a higher risk of having babies with birth defects.

Chromosome Translocation and Partial Monosomy

Partial monosomy is when part of a chromosome is missing. One way this can happen is if part of a chromosome is accidentally deleted during meiosis or during the early development of the embryo. This can happen if a chromosome breaks and part of it is not transferred into the daughter cells. If a chunk of a chromosome is missing, then the individual only has one copy of all of the genetic material that was lost. Partial monosomies that occur because of deletion are not inherited because deletion is a random event.

Chromosome translocation is another way that partial monosomy can happen. This kind of partial monosomy can be inherited, and here's why. Let's say that one parent has a balanced chromosome translocation, which is a kind of chromosome rearrangement where a piece of a chromosome ends up stuck onto another chromosome instead. It's called a 'balanced' translocation because the genetic material is all still there, it's just rearranged a bit. The parent is most likely totally fine and doesn't even know they have a translocation.

However, if the parent transfers the rearranged chromosome to its child, that child could have an unbalanced chromosome translocation. If the child gets a chromosome with an extra chunk of genetic material attached, plus all of the normal chromosomes, he or she will have a partial trisomy. And, on the other hand, if the child gets the shortened chromosome but not the longer chromosome that has the missing genetic material attached to it, he or she will have a partial monosomy.

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