Meiosis and Inheritance: Down Syndrome Case Study

Class began with an entry task intended to provide students with practice identifying the four allele combinations possible from two different genes.  A picture of the white board is shown below:

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Next, we transitioned in to the lesson about Down Syndrome.  We discussed the first three slides of the Down Syndrome Case Study slide deck and then detoured to the Wikipedia entry on aneuploidy.  We focused on the Types section of the entry, examining how, of the autosomal chromosomes, only Trisomy 21 will result in a viable fetus most of the time.  Students learned that when trisomy occurs in most of the other autosomal chromosomes, the result is an embryo that is non-viable, often resulting in miscarriage.  There was a lot of student interest in learning more about polyploidy in the sex chromosomes, and students are encouraged to keep learning outside of class!  There were also students curious about my use of Wikipedia.  I explained that in my opinion, Wikipedia is a powerful research tool as long as the information included in an article is properly cited.  Students can use a Wikipedia entry to locate primary source documents and then reference those documents directly once the information they contain has been verified.  I referenced scientific publications that have determined Wikipedia to be as accurate as more traditionally accepted credible sources.  Ironically, although Wikipedia is freely available, the publications determining the credibility of Wikipedia are not.  Students interested in learning more without paying for the publications can read the Live Science article from 2011 titled “How Accurate is Wikipedia” as a starting point and come up with their own conclusions.

Back to the Case Study!  To learn how Trisomy 21 occurs, students watched a short video illustrating non-disjunction.  To put a human face on Trisomy 21, or Down Syndrome, students watched another short video about two young twins with Down Sydrome.  Both videos are shown below:

The second video naturally leads to questions about twins, so slide 5 explains the difference between identical and fraternal (non-identical) twins.  In slide 6, students are introduced to an adorable set of fraternal twins whose skin-color genetics will be revisited in our next unit.  The embed feature of the video is deactivated, so here is the link to the ABC News video shown on YouTube.

Class concluded with a final look at the genetic phenomenon of X-inactivation.  USing calico cats as the model, Mr. Anderson of Bozeman Science explains how in females, only one X chromosome is active in a given cell.  We can visualize the result of this process in calico cats.  At the end of the video, we explains how male calico cats, although extremely rare, can arise from the XXY phenotype (connecting back with our learning about aneuploidy).

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