Saturday, December 17, 2011

A modern genetics problem

This problem was on the final exam of our new Fundamentals of Genetics course.  It's an example of what I'd like our students to be able to do.

(10 points)  The ideogram above shows a normal child’s genome, with her chromosomes coloured by 23andMe to show the results of genotyping her DNA and the DNAs of her maternal grandparents.  Blue segments indicate blocks of alleles shared with her maternal grandmother, and white segments indicate blocks of alleles shared with her maternal grandfather.  Hatched segments could not be analyzed because they have too few SNPs.

a. (1 point)  What genetic process is responsible for these blocks of alleles?


b. (2 points)  When and where did this process occur?


c. (2 points)  What property of the child’s maternal chromosomes 11 and 14 is unexpected?  Why is this property unexpected?


d. (4 points)  Suggest two different kinds of events that could explain this unexpected property.  Give rough estimates of the probabilities of the events you propose.


e. (1 point)  The black triangles above some chromosomes show the locations of SNPs linked to effects on nose shape.  What do these predict about the child’s appearance?

8 comments:

chuck b. said...

I only see a tiny bit of gray, like at the tips of #1, and a fleck in #8. Is that correct? Is the blue paternal?

chuck b. said...

Anyway, I got a B in genetics in 1994.

I'm guessing the answers have something to do with a) crossing over, b) some phase of meiosis in the mother when she was still in maternal grandmother's womb (because females are born with all their eggs, right?), c) no blue in 11 or 14...but I'm still not sure what blue is. d) Independent assortment? e) she resembles her grandfather.

Russell Neches said...

I think it should be noted that "blue" == "gray", unless the department springs for color printing for its exams.

Rosie Redfield said...

Thanks Russell - I've now changed 'grey' to 'blue' for this colour version.

Sandra Porter said...

I like it. Only problem is many of my students have been brainwashed to think genetic testing is bad and shouldn't be done without counseling.

Alex said...

This is an excellent question, but it is also one that I hope my high school students can mostly answer by the time they are through with my Biology classes (well, at least the better ones who are interested in Biology).

Do you mind if I borrow this and offer it as extra credit to my students?

And, just to make sure I'm understanding it correctly, are these answers correct?

a.) crossing over during meiosis
b.) in the germ cells during meiosis prophase I
c.) the child seems to have inherited these chromosomes complete from her maternal grandfather with no crossing over. It's unexpected because crossing over is very common during synapsis. (I think there must be a better answer than this, but I don't know it)
d.) I actually don't know... perhaps something to do with meiotic drive? Or, perhaps both the maternal G'mother and G'father had the same alleles there, and so are indistinguishable? Actually, I'd really love to know the answer to this one.
e.) She'll probably have her Grandfather's nose

Anyway, as a high school biology teacher (in Thailand), I just want to say that as someone who likes to keep up with modern issues in biology and get my students learning the latest information and techniques, I'll be taking your thoughts in the previous post and this one to heart, in the hopes that they are better prepared if they do eventually study biology in university. :)

Rosie Redfield said...

b. in meiosis I in the mother

d. random segregation after no crossing over, or crossover in other chromatids, or crossovers in hatched regions. (all fairly high probability)

e. Resembles her maternal grandfather more than her maternal grandmother. But she has another set of alleles from her father which will also affect her appearance.

Paduan Warna Cat Rumah Yang Bagus said...

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