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Chromosome Count Changes
Next Generation: HS.LS3.2 HS.LS3.2

Chromosome Count Changes

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This lesson will explore what types of events can change the number of chromosomes an individual has and the physical outcomes of having an abnormal number of chromosomes.

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Tutorial

What's Covered

This lesson will cover changes in chromosome number and the effects this has by looking at:

  1. Nondisjunction
  2. Changes in the Number of Sex Chromosomes

1. Nondisjunction

Chromosome numbers can change during mitosis or meiosis, and are generally the result of nondisjunction. During the metaphase portion of mitosis, sister chromatids are being pulled towards the poles of the cell. Usually these sister chromatids would separate, but sometimes they do not separate correctly. Nondisjunction is the failure of one or more pairs of chromosomes to separate during cell division, and this results in some cells ending up with too many or too few chromosomes and can lead to several different disorders.

Term to Know

    • Nondisjuction
    • When chromosomes don’t separate during meiosis causing gametes to have abnormal numbers, Down syndrome is an example of this.

Down Syndrome, also called trisomy 21, is an example of nondisjunction. This disorder occurs when a person ends up with an extra chromosome on pair 21. Instead of having two chromosomes on pair number 21, they have three chromosomes on pair number 21.

Term to Know

    • Down Syndrome
    • A genetic disorder in which the 21st pair of chromosomes have three chromosomes vs. the normal pair, this is called a trisomy.

2. Changes in the Number of Sex Chromosomes

Turner syndrome, Klinefelter syndrome, and XYY condition are examples where a change in a chromosome number affects sex cells. Turner syndrome is when an X chromosome is missing. Generally, a person who is missing an X chromosome would be miscarried by the mother, but anyone who does survive is always going to be a female. That female is going to end up with sexual abnormalities and, generally, a shortened life span.

Term to Know

    • Turner Syndrome
    • A genetic condition in which an X chromosome is missing, if the child is not miscarried, the effects can range from non-life threatening to life threatening. People with Turner syndrome are female due to the presence of only an X chromosome, are almost universally infertile and typically have cardiovascular issues along with other organ system problems.

Klinefelter syndrome occurs when sex chromosomes have an extra copy. The person’s genotype on their sex chromosomes would be XXY and the individual would be a male with an extra X chromosome. A person with this disorder is going to have low fertility, possibility of mental retardation, and small testes. They will also have physical abnormalities because of the presence of this extra X chromosome.

Term to Know

    • Klinefelter Syndrome
    • A condition in which a person has an XXY chromosomal pattern; males that inherit this are often infertile, may develop breasts and other female secondary characteristics and possible mental retardation.

XYY syndrome is another condition that affects the sex chromosome. This person would be male because of the presence of the Y chromosome. The only real result of this is that the male is probably going to be a little bit taller than normal. There are no serious abnormalities caused by having an extra Y chromosome.

Term to Know

    • XYY Syndrome
    • A condition in which a male inherits an extra Y chromosome; XYY syndrome doesn’t express any cognitive or life threatening abnormalities, it just tends to make males taller than average.

Summary

Nondisjunction is the failure of one or more pairs of chromosomes to separate during cell division, and can lead to a change in chromosome number. This will result in cells that end up with too many or too few chromosomes. Down Syndrome is an example of a disorder caused by this. Changes in the number of sex chromosomes can also cause problems. Turner syndrome is caused when an X chromosome is missing, and Klinefelter syndrome is caused when an extra X is present in a male. Both of these disorders can cause long term problems for the person they affect. XYY condition causes no serious abnormalities.

Keep up the learning and have a great day!

Source: SOURCE: THIS WORK IS ADAPTED FROM SOPHIA AUTHOR AMANDA SODERLIND

Terms to Know
  • Down Syndrome

    A genetic disorder in which the 21st pair of chromosomes have three chromosomes vs. the normal pair, this is called a trisomy.

  • Klinefelter Syndrome

    A condition in which a person has an XXY chromosomal pattern; males that inherit this are often infertile, may develop breasts and other female secondary characteristics and possible mental retardation.

  • Nondisjuction

    When chromosomes don’t separate during meiosis causing gametes to have abnormal numbers, Down syndrome is an example of this.

  • Turner Syndrome

    A genetic condition in which an X chromosome is missing, if the child is not miscarried, the effects can range from non-life threatening to life threatening. People with Turner syndrome are female due to the presence of only an X chromosome, are almost universally infertile and typically have cardiovascular issues along with other organ system problems.

  • XYY Syndrome

    A condition in which a male inherits an extra Y chromosome; XYY syndrome doesn’t express any cognitive or life threatening abnormalities, it just tends to make males taller than average.