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More Isn't Always Better
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More Isn't Always Better

One of these things is not like the other ones...one of these things does not belong...can you tell which one?

When Three Is Definitely Too Many

Two's company and three's a crowd - or so some people say. Usually, they say this when they are trying to get rid of someone, so it's kind of up to the individuals if three is a crowd in these cases. But, when the individuals are chromosomes, three is definitely a crowd. See, humans (Homo sapiens) normally have 46 chromosomes. They received 23 from their mother and 23 from their father. This pairing of chromosomes is a beautiful symmetry and part of the sophisticated system that controls the development and maintenance of humans. When a third copy of a chromosome shows up, the system doesn't work too well. When that chromosome is chromosome 18, you get Edward's Syndrome, and these are the challenges it creates:

Extension Investigation

Use the resources below to answer the following questions:

  1. What is "trisomy"? How does this condition arise?
  2. What are some of the symptoms of Edward's Syndrome? What complications can these symptoms cause?
  3. What is the survival rate of individuals with Edward's Syndrome? How does this compare to the survival rate for Down's Syndrome?
  4. What do the different survival rates for Edward's Syndrome (trisomy 18) and Down's Syndrome (trisomy 21) suggest about the genes carried on chromosome 18 versus the genes carried on chromosome 21? Can you think of a way to test your hypothesis? Don't over complicate your answer. Think in very basic terms as a starting point.
  5. How many types of trisomy 18 are there? How are they the same? How do the differ? What does this situation suggest to you about the way this condition develops?

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