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Changes in Chromosome Number
Next Generation: HS.LS3.2 HS.LS3.2

Changes in Chromosome Number


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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Source: Video and Images Created by Amanda Soderlind

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Welcome to this lesson today on changes in chromosome number. Today, we will be identifying how changes in chromosome number can have an effect on an individual. So chromosome numbers can change during mitosis or meiosis and are generally the result of nondisjunction. So nondisjunction is the failure of one or more pair of chromosomes to separate during cell division. And so this results in some cells ending up with too many or too few chromosomes.

So during the phase of mitosis, if we have our cell here, we have all the cells in metaphase that are being pulled towards the poles of the cell. Now usually, these sister chromatids would separate, and we would end up with something like this. So we have the individual sister chromatids being pulled toward the separate poles of the cell.

But sometimes they won't separate correctly. So it might look something like this. So you can see that this chromosome here didn't separate properly from its sister chromatid. So this would be an example of nondisjunction, improper separation during cell division. So, as I mentioned, you can end up with too many or too few chromosomes in a cell. So these are disorders here that can be caused by changes in chromosome number.

So Down Syndrome is the first example we're going to talk about now. And Down Syndrome is also called trisomy 21. So what this means is that a person ends up with an extra chromosome number 21. So instead of having two chromosomes on pair number 21, they have three chromosomes on pair number 21. So they have three copies of that chromosome rather than two.

So this is an example of a change in chromosome number that affects somatic cells or your body cells. So Down Syndrome is an example of something that affects somatic cells. Whereas Turner syndrome, Klinefelter syndrome, and XYY condition are examples where a change in a chromosome number affects sex cells.

So Turner syndrome is when an X chromosome is missing. So an X chromosome is missing. So, as I mentioned, it's affecting the sex cells. So 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. And that female is going to end up with sexual abnormalities and generally a shortened life span. So that's Turner syndrome.

Klinefelter syndrome is when the sex chromosomes have an extra copy. So you actually end up with XXY. So this individual is actually going to be a male. A presence of a Y chromosome signifies a male. So it's a male that has an extra X chromosome. So basically, what this is going to do is it's-- that male is going to have low fertility, possibility of mental retardation, small testes. So they're going to have some definite physical abnormalities because of the presence of this extra X chromosome.

And then XYY is another condition that affects the sex chromosome. And again, it's going to be a male because we have a Y chromosome here. So this, again, is due to nondisjunction during meiosis. But really the only result of this is that the male is probably going to be a little bit taller than normal. But otherwise, there's not really going to be any real physical abnormalities as a result of this condition, because they just have that extra Y chromosome. So this lesson has been an overview on how changes in chromosome number can have an effect on an individual.

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.


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.