Klinefelter and XYY Syndrome: Types of Sex Chromosome Aneuploidy

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  • 0:04 Aneuploidy
  • 0:56 Klinefelter Syndrome
  • 2:37 XYY Syndrome
  • 4:10 Lesson Summary
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Instructor: Natalie Boyd

Natalie is a teacher and holds an MA in English Education and is in progress on her PhD in psychology.

Most boys and men have two sex chromosomes. But what happens when a male has more than two sex chromosomes? In this lesson, we'll explore two examples of male aneuploidy: Klinefelter Syndrome and XYY Syndrome.


Christie is worried about her son, Zack. He is different from other teenage boys: he hasn't started puberty yet. He also has softer, more full breasts than boys usually do. He struggles in school and has been diagnosed with a learning disability. Christie is worried that something might be wrong.

Zack's differences might be caused by his genes. Usually, people have two sex chromosomes in their cells. For males, there is usually one X chromosome and one Y chromosome. But some people have additional chromosomes. Sex aneuploidy is a condition in which a person has more or less than two sex chromosomes. This can cause developmental issues, like those that Zack is displaying. Let's look closer at two examples of male aneuploidy: Klinefelter syndrome and XYY syndrome.

Klinefelter Syndrome

Remember Zack? He's a teenager, but hasn't started puberty. He has breasts that are more like girl's breasts than boys. In addition, he has a learning disability. Zack might have Klinefelter syndrome, which occurs when a male has one or more extra X chromosomes. As we mentioned before, most boys have one X and one Y chromosome, but boys with Klinefelter syndrome usually have two X chromosomes and one Y chromosome, though sometimes they can have three or more X chromosomes.

The symptoms of Klinefelter syndrome involve developmental delays. There might be delays in speech and language, and there might be learning disabilities, like Zack has. But there are also delays or struggles with physical development. The Y chromosome is in charge of male sexual development. But in Klinefelter syndrome, the extra X chromosome makes it hard for the Y chromosome to do its job.

As a result, boys with the syndrome often have less testosterone in their systems. Because testosterone is the hormone that is in charge of male physical development, Klinefelter syndrome often results in delayed or incomplete puberty. Some boys also develop breasts like females, like Zack has. Boys with Klinefelter syndrome might have less facial and body hair than other men, and they might end up being unable to have children.

Christie, Zack's mom, is worried that she might have done something to cause Zack's condition. But Zack's doctor reassures Christie that it's not her fault. Klinefelter syndrome is usually a result of a random event early during pregnancy, and there's nothing Christie could have done to cause it.

XYY Syndrome

To help Zack deal with Klinefelter syndrome, his doctor suggests he meets with other boys who have sex aneuploidy. In his support group, Zack meets Charlie, who is both different and similar to Zack. Charlie is a lot taller than other boys his age, but unlike Zack, Charlie doesn't have any other physical abnormalities compared to other boys. But he does have a learning disability, like Zack, and some other developmental issues.

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