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. If 18 pairs of sister chromatids segregate normally during meiosis II in cats (n=19) but we have non-disjunction of 1 pair, then at the end of meiosis II we will have

Question

A.  3 cells with 20 chromosomes and 1 cell with 18.
B.  2 cells with 20 chromosomes and 2 cells with 18.
C.  2 cells with 19 chromosomes, 1 with 20, and 1 with 18.
D.  3 cells with 18 chromosomes and 1 cell with 20.

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
  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 cells with 19 chromosomes, 1 with 20, and 1 with 18.

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 If 18 pairs of sister chromatids segregate normally during meiosis II in cats (n=19) but we have non-disjunction of 1 pair – if n=19 and only 18 pairs of sister chromatids segregate normally then one pair must go through non-disjunction leading to extra or missing 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=19 then a normally segregating cell would have 19 chromosomes and those with an extra or missing chromosome would have 20 or 18 chromosomes respectively.

  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 normal haploid cell has 19 chromosomes and the non-disjunction happens in meiosis II, after meiosis I both daughter cells will have 19 chromosomes with chromatid pairs.  After meiosis II one cell would go through normal separation of the sister chromatids producing two haploid cells with 19 chromosomes.  In the second cell that goes through non-disjunction, 18 of the sister chromatids would separate normally while one sister chromatid pair would not separate.  Thus one gamete would end up with one fewer chromosome for 18 total and the other would end up with an extra chromosome with 20 total.

  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 how many chromosomes a cell would have 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 meiosis II half of the gametes will have the normal number of chromosomes and half will have extra or missing chromosomes?  Did you think that three of the cells would have the normal number of chromosomes and only one would end up with an abnormal number?