In 95% of cases of Down's syndrome, there is one extra chromosome (number 21) in every cell. This aneuploid condition is most likely the result of

Question

A.  failure to suppress DNA replication between meiosis I and meiosis II.
B.  failure of the chromosomes to assort independently during meiosis.
C.  failure of 1 homologous pair to segregate during meiosis.
D.  failure of 2 homologous pairs to segregate during meiosis.
E.  failure of the cytoplasm to divide at the end of meiosis II.

Clarify Question
· What is the key concept addressed by the question?
· What type of thinking is required?
· What key words does the question contain and what do they mean?

  Gather Content
  · What do you know about meiosis? How does it relate to the question?

Consider Possibilities
  · What other information is related to the question? Which information is most useful?

  Choose Answer   · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?  

  Reflect on Process
  · Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

C.  failure of 1 homologous pair to segregate during meiosis.

Clarify Question
  · What is the key concept addressed by the question?
        o The question asks about origin of a trisomy.
· What type of thinking is required?
        o You are being asked to apply your knowledge of meiosis to predict when a third copy of a chromosome would occur.
· What key words does the question contain and what do they mean?
        o One extra chromosome in every cell – this means that the addition of the extra chromosome must have happened early in meiosis so that a gamete would contain an extra chromosome 21.

  Gather Content
  · What do you know about meiosis? How does it relate to the question?
        o During meiosis chromosome pairs separate during meiosis I.  If these do not segregate then a daughter cell would get an extra chromosome.

Consider Possibilities
  · What other information is related to the question? Which information is most useful?
        o The presence of an extra chromosome is due to a problem with segregation, not DNA replication or independent assortment, which would affect more than one chromosome.
        o If two homologous pairs of chromosomes did not segregate then the cell would end up with two extra chromosomes.
        o The division of cytoplasm will not affect the number of chromosomes in a cell.

  Choose Answer   · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?  
        o The best answer is that if one pair of chromosome does not segregate during meiosis then one daughter cell will obtain two copies of that chromosome.  When this gamete fuses with a normal gamete that contains one copy of each chromosome there will be three copies of that chromosome.

  Reflect on Process
  · Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        o This question asked you to apply your understanding of meiosis to explain how a trisomy arises. If you got the correct answer, great job! If you got an incorrect answer, where did the process break down? Did you recall that homologous chromosome pairs separate during meiosis?   Did you understand that if this does not happen one gamete will end up with an extra copy of that chromosome while the other will have no chromosome?  Did you think that the extra chromosome arose because of increased DNA replication or a failure of independent assortment?