Applying the Concept of Non-Disjunction to Trisomy

Written by Kelly A. Hogan at University of North Carolina at Chapel Hill

Learning Outcomes:

– To understand how non-disjunction is abnormal meiosis

– To see how non-disjunction leads to trisomy

– To show students how to think through an application-based question

Activity Description: Students are given a question via PowerPoint as a clicker question and asked to do the problem alone. After collecting initial answers (and not telling them the correct one), students are then directed to work through the same problem using a helpful worksheet and neighbors. Students are then asked the same clicker question.

Time Needed: Approximately 15-20 minutes

Materials Needed: Worksheet or blank paper for students (if showing everything via PowerPoint)

Activity Instructions: You can insert this question into PowerPoint or use as a worksheet with the skeleton images. Consider using it as a clicker question if you are using clickers. This will likely be a tough one that students would benefit from a discussion with neighbors after trying it on their own and before being asked the same question again via clicker.

Question 1:

Do you really UNDERSTAND meiosis and non-disjunction? Try this question:

If an individual has the genotype XXY, did non-disjunction occur in their mother or their father (or both) and at which division(s), meiosis I or meiosis II?

A. Mother in meiosis I or II, Father in meiosis I or II

B. Mother in meiosis I or II, cannot be a non-disjunction in father

C. Cannot be a non-disjunction in mother, Father in meiosis I or II

D. Mother in meiosis I or II, Father only meiosis I

E. Mother only in meiosis I, Father only in meiosis I

*Choice D is correct. Students will have the most trouble with what happens after non-disjunction at Meiosis I. What they fail to understand is that the sister chromatids will still line up at meiosis II and separate. This is especially easy to spot with the father’s nondisjunction at Meiosis I. If they don’t understand this concept they will have a XX and a YY cell forming, instead of two XY cells.

Question 2:

If an individual has a genotype XYY, did non-disjunction occur in their mother or their father (or both) and at which division(s), meiosis I or meiosis II?

A. Mother in meiosis I or II, Father in meiosis I or II

B. Cannot be a non-disjunction in mother, Father in meiosis I or II

C. Mother in meiosis I or II, Father only meiosis I

D. Cannot be a non-disjunction in mother, Father in meiosis I only

E. Cannot be a non-disjunction in mother, Father in meiosis II only

*Choice E is correct. A sperm that was YY was fertilized with a normal egg.

Worksheet: Meiosis and Non-Disjunction Worksheet