HER - Errors in Meiosis [LESSON]

Errors in Meiosis

Some things just work well in pairs. Everyday examples include shoes, gloves, and the earbuds on a music player. If you're missing one member of a pair, it's likely to be a nuisance, and might even be a serious problem (for instance, if you're already late for school!).

Changes in a cell's genetic material are called mutations. In one form of mutation, cells may end up with an extra or missing chromosome.

Each species has a characteristic chromosome number, such as 46 chromosomes for a typical human body cell. In organisms with two full chromosomes sets, such as humans, this number is given the name 2n.

If a cell has an incorrect number of chromosomes, it is said to be aneuploid. 

If a cell has an incorrect number of chromosomes, it is said to be aneuploid. There are two common types: monosomy – when an organism has one copy of a chromosome instead of two and trisomy – when an organism has three copies of a chromosome instead of two.

Nondisjunction

Disorders of chromosome number are caused by nondisjunction, which occurs when pairs of homologous chromosomes or sister chromatids fail to separate during meiosis I or II. When nondisjunction occurs for a set of chromosomes during meiosis, some gametes end up with two copies of that chromosome while the other gametes receive no copies of that chromosome. When this happens, it usually only affects one set of chromosomes, with the other chromosomes being distributed normally. If a zygote is formed by one of the gametes affected by nondisjunction, the resulting human will have cells with too many or too few chromosomes. This can result in a variety of genetic disorders, depending on the affected chromosome. For example, an extra chromosome 21 is the most common cause of Down syndrome. An extra X chromosome in a male causes a disorder known as Klinefelter syndrome. Here is a karyotype of an individual with Down syndrome.

The image shows three copies of chromosome 21 and two copies of all others.

The diagram below shows how nondisjunction can take place during meiosis I if homologous chromosomes don't separate and how this can lead to the production of aneuploid gametes (eggs or sperm):

The diagram shows how nondisjunction can take place during meiosis I

Nondisjunction can also happen in meiosis II, with sister chromatids (instead of homologous chromosomes) failing to separate. Again, some gametes contain extra or missing chromosomes:

Nondisjunction can also happen in meiosis II

Chromosomal Rearrangements

In another class of large-scale mutations, big chunks of chromosomes (but not entire chromosomes) are affected. Such changes are called chromosomal rearrangements. They include:

  • A duplication, where part of a chromosome is copied.
  • A deletion, where part of a chromosome is removed.
  • An inversion, where chromosomal region is flipped around so that it points in the opposite direction.
  • A translocation, where a piece of one chromosome gets attached to another.

The image below shows examples of the first three types.

This image shows deletion, duplication, and inversion of chromosomes.

Use the Meiosis Practice Quiz below to test your knowledge from this lesson.

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