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IBDP Biology- Karyotyping and Nondisjunction

Topic 3- Genetics

· IBDP BIOLOGY,IB,genetics

How do mothers find out what gender their babies are, and what are the possible genetic mutations? In this IBDP Biology blog we will find out what karyotyping is and what non-disjunction is, let's go!

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Karyotyping is the process of pairing and ordering all the chromosomes of an organism, which provides a genome-wide snapshot of an individual's chromosomes.

  • Extra or missing chromosomes, or abnormal positions of chromosome pieces, can cause problems with a person's growth, development, and body functions.

Karyotyping is performed using:

Amniocentesis: a sample of amniotic fluid is removed from the amniotic sac around the fetus using a hypodermic needle. Contains cells from the fetus.

Chorionic villus sampling: Cells are removed from fetal tissues in the placenta called chorionic villi, by a  needle. 

Non-disjunction and Down Syndrome

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The pairing up of homologous chromosomes during Prophase I is called synapsis. Each resulting pair is called tetrad or bivalent.

The separation of the tetrads during Anaphase I is called disjunction. Failure of chromosomes to separate can lead to aneuploidy which means having one extra chromosome or missing one chromosome.

In extreme cases all homologous pairs fail to separate – total disjunction.

Best known example of aneuploidy: 

Down syndrome - If non-disjunction occurs in either parent, one of the two gametes will carry two copies of chromosome 21. When the zygote is formed, it will contain 3 copies of c21. The results are varying degrees of disability.

Unlike the production of sperm cells (begins at puberty) female gametes develop before birth. They remain the same until ovulation. This means for a 20 year old woman, her egg cells have collected 20 years of damage. For a 40 year old woman, cells have 40 years damage, meaning a higher chance of non-disjunction and Down syndrome.

That's all!

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References:

  • https://www.nature.com/scitable/topicpage/karyotyping-for-chromosomal-abnormalities-298/
  • https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.ucsfbenioffchildrens.org%2Fmedical-tests%2Fkaryotyping&psig=AOvVaw3HT5lPaFevKkwNPui9sjJW&ust=1625722142286000&source=images&cd=vfe&ved=0CAoQjRxqFwoTCPiD7oid0PECFQAAAAAdAAAAABAa
  • https://www.google.com/url?sa=i&url=https%3A%2F%2Fknowledgeclass.blogspot.com%2F2015%2F09%2Fso-what-is-non-disjunction.html&psig=AOvVaw3rtuS6_8BgsPylxd6UznY6&ust=1625722214964000&source=images&cd=vfe&ved=0CAoQjRxqFwoTCOiP5Kud0PECFQAAAAAdAAAAABAm
  • https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.pinterest.com%2Fpin%2F489555422001292678%2F&psig=AOvVaw2NqXKKojcn2drPV308lRC9&ust=1625727268067000&source=images&cd=vfe&ved=0CAoQjRxqFwoTCICF_JSw0PECFQAAAAAdAAAAABAO

Drafted by Venetia (Biology)

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