A normal human karyotype has 46 chromosomes: 22 pairs of autosomes and 2 sex chromosomes.

What happens when a person has something different, such as:

  • Too many or too few chromosomes?
  • Missing pieces of chromosomes?
  • Mixed up pieces of chromosomes?

Too many or too few chromosomes

To understand how our cells might end up with too many or too few chromosomes, we need to know how the cells normally get 46 chromosomes.

First we need to understand meiosis. Meiosis is the cell division process that produces egg and sperm cells (gametes), which normally have 23 chromosomes each.

If eggs and sperm only have one set of chromosomes, then how do we end up with 46 chromosomes? During fertilization, when the egg and sperm fuse, the resulting zygote has two copies of each chromosome needed for proper development, for a total of 46.

How can cells end up with too many or too few chromosomes?

Sometimes chromosomes are incorrectly distributed into the egg or sperm cells during meiosis. When this happens, one cell may get two copies of a particular chromosome, while another cell gets none.

What happens if a sperm or egg cell with an abnormal number of chromosomes participates in fertilization? It depends on how many chromosomes the gamete has. For example, if a sperm with an extra chromosome fertilizes an egg with a normal chromosome number, the resulting zygote will have 3 copies of one chromosome. This is called trisomy (pronounced TRY-so-mi).

If a sperm that is missing a chromosome fertilizes an egg, then the resulting zygote will have only one copy of that chromosome. This is called monosomy (pronounced MA-no-so-mi).

People who are born with an abnormal number of chromosomes often have genetic disorders because their cells contain too much or
too little genetic information. Scientists can predict genetic disorders by looking for extra or missing chromosomes in a karyotype.

Missing pieces: deletions of chromosome sections

In some cases, genetic material is missing from a chromosome. Such chromosomes are said to have deletions. Deletions of the tips of chromosomes are called terminal deletions. Internal deletions, where a chromosome has broken, lost material, and rejoined, are called interstitial (pronounced in-ter-STIH-shul) deletions.

Deletions large enough to be seen in a karyotype result in the loss of many genes. In humans, these are less comman than deletions that remove small portions of a chromosome. As with abnormal chromosome numbers, genetic disorders can result from the imbalance in genetic material caused by deletions.

A translocation is a chromosome rearrangement in which part of a chromosome breaks off and then reattaches to a different chromosome. Different types of translocations that can occur include:

  • A reciprocal translocation is an even swap made between two chromosomes.
  • A Robertsonian translocation occurs when the long arms of two acrocentric chromosomes fuse at a centromere, and the two short arms are lost.

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