Non-disjunction is the failure of homologous chromosomes to separate during meiosis I, or the failure of sister chromatids to separate during meiosis II or mitosis. As a result, both homologous chromosomes or both sister chromatids migrate to the same pole of the cell. This produces daughter cells with an imbalance of chromosomes. A cell biologist examines the final products of meiosis in an earthworm (2n=36) and finds 2 cells with 20 chromosomes, and 2 cells with 16 chromosomes. Most likely this was because

Question

A.  2 pairs of sister chromatids failed to separate during meiosis II.
B.  1 pair of sister chromatids failed to separate during meiosis II.
C.  2 pairs of homologous chromosomes failed to separate during meiosis I.
D.  1 pair of homologous chromosomes failed to separate during meiosis I.

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 non-disjunction? 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.  2 pairs of homologous chromosomes failed to separate during meiosis I.


Clarify Question
  · What is the key concept addressed by the question?
        o The question asks about the effect of non-disjunction on the number of chromosomes in a gamete.
· What type of thinking is required?
        o You are being asked to analyze different statements about non-disjunction to explain how many chromosomes would be present in a gamete.
· What key words does the question contain and what do they mean?
        o (2n=36) and finds 2 cells with 20 chromosomes, and 2 cells with 16 chromosomes – If a cell has 2n=36, then n=18.  The haploid cells produced have either 20 or 16 chromosomes, so plus or minus 2 chromosomes.

  Gather Content
  · What do you know about non-disjunction? How does it relate to the question?
        o In non-disjunction, chromosomes fail to segregate, leading to gametes with extra or missing chromosomes.  If n=18, and the haploid gametes have either 16 or 20 chromosomes, then both cells either gained or lost two additional chromosomes.

  Consider Possibilities    · What other information is related to the question? Which information is most useful?
        o If the non-disjunction happens during meiosis I, then the two daughter cells will have either missing or extra chromosomes.  When these daughter cells go through a second division in meiosis II then both of their daughter cells would inherit the same number of chromosomes.  So the four daughter cells would all contain extra or missing chromosomes.
        o If the non-disjunction happens during meiosis II, one of the original daughter cells will divide normally producing two new daughter cells with the correct number of chromosomes.  The cell that goes through non-disjunction will produce two daughter cells with abnormal chromosomes.  So in this case two gametes would end up with a normal number of chromosomes and two would have extra or missing chromosomes.

  Choose Answer
  · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?  
        o Because the original diploid cell contains (2n=36), the haploid cell would end up with 18 chromosomes.  Because there are 2 cells with 20 chromosomes, and 2 cells with 16 chromosomes the non-disjunction must have happened in the first round of meiosis.  Non-disjunction would have led to two cells, one with 20 chromosomes and one with 16, each of these chromosomes would contain two sister chromatids.  In meiosis II the two cells would divide again producing two haploid cells with 20 chromosomes and two with 16 chromosomes.
   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 analyze the results of an experiment to predict when non-disjunction occurred. If you got the correct answer, great job! If you got an incorrect answer, where did the process break down? Did you recall -that if non-disjunction occurs in the first round of meiosis I the two daughter cells would then divide producing identical gametes?  Did you realize that if non-disjunction occurred in meiosis II that one daughter cell would produce two gametes with the correct number of chromosomes and the second undergoing non-disjunction would produce gametes with extra or missing chromosomes?