Day Five: Punnett Squares FTW/L?

Alright, long weekend involving buying a car (bye money) left me a bit overwhelmed. But let’s update from last Friday.

First, students were asked at the beginning of the hour to (1) label posterior, anterior, lateral, dorsal, ventral on a fly diagram and (2) the stages of the Drosophila life cycle along with time at each stage. For the most part they nailed it. A couple more days and it will be ingrained.

We then went to the dissecting scopes to check out day 5 of development and we certainly heard the most excitement of students so far. Their (relatively) tiny larva had grown to twice their size and now under 30x magnification they appeared to be monsters. (Which provided some entertainment when unsuspecting students peered first at them under 30x unknowingly). What did students see? Here’s a great student video:

What structures could students see? Trachea (lines running down the larva), mouthparts/head skeleton (dark spot-anterior), and spiracles (orangish spots-posterior).

30x magnification

After that, we asked students to remind us what traits they saw universally in these flies that looked abnormal from what you typically expect to see in fruit flies. The two mutations that at least a couple observant students caught was the orange eyes and curly wings. We then explained that these mutations were caused by the same gene, cyoYFP (cy = curly; o = orange; YFP = yellow fluorescent protein [not explained today what that was for]) Then we explained that this gene was on the 2nd (of 4) chromosome on the fruit fly. Furthermore, we asked how many chromosomes each fly has in each set (A: 2, like humans). So then we told them that the 2nd homolog of the 2nd chromosome (make sense?) was a balancer chromosome and that this chromosome contained the cyoYFP gene. Basically, this balancer chromosomes (AP Bio had to explaing TO US what these were through the magic of research [and Google]) does three unique things: 1) It prevents crossing over and therefore genetic variation, 2) It allows us to track the mutation through many generations, and 3) It only creates heterozygotes.

We also mentioned that the whole point of this research project was to characterize the function of the Tep3 protein and the gene for it was on the non-balancer homolog of the 2nd chromosome. So that led students to the question, “What would we only see heterozygotes?” What happens we we get Tep3/Tep3 or cyoYFP/cyoYFP individuals? To channel someone near and dear to all of us, “Sounds like an experiment.”*

This is when we had a boy and a girl (volunteered or otherwise) come up to the board to represent the mother and father fly. The father had the genotype Tep3/cyoYFP and the mother also had the genotype Tep3/cyoYFP. So what genotype combinations would we expect to see in the offspring? We asked the freshman students how many chromosomes get passed on from each parent. Predictably they answered 1 (or half). So they wrote the genes they could pass. Either Tep3 from the 1st homolog, or cyoYFP from the other. Then we asked, “What combination of offspring could we get?” The students wrote those on the board (pic below).

Mendelian Genetics in 2 seconds

Oh yeah, did I mention Punnett Squares? Some (clever?) students realized this combinations could have found out using Punnett Squares. Some of course when seeing this made them, but others were confused by using gene names and not big-A and little-a. But those same students who were confused by the gene name abbreviation were able to generate the combinations without using Punnett Squares. So we were still left at the crossroads of a timeless debate: To Punnett or not to Punnett? (COMMENTS SECTION!!)

But after this we asked students how we would in real life set up these combinations. They determined we probably needed male and female flies in the same vicinity. I asked if a large jar would do. They said that that would not provide good data *tear drop* so we should make multiple iterations. We gave them each a vial and they were charged with the task of finding 6 females and 5 males to put in the vial.** After that they labeled their vials with the cross (Tep3/cyoYFP x Tep3/cyoYFP), their name (w/ partner), and hour.

AP had the added excitement of creating the control setup, because we still had to determine if cyoYFP was a dominant or recessive allele. They accomplished this by using a Gal4 gene.

Monday we will be transferring flies to new vials, same for Thursday (post-final fun) and then their little ones will incubate for the remainder of winter break when they’ll see the product of their crosses when they return.

*Yes, I really did say that to them. It felt odd, but cool.

**Now, unfortunately, no student (AP or freshman) picked up on the fact that these flies had been already cohabiting the same vials, therefore the females they chose most likely did not need a male, they were already pregnant. This exercise was to help them get used to crossing flies and predicting the outcomes, nevertheless I thought one student (and they still could) would catch this flaw.



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