Mad Cows, Mad Sheep, and Punnett Squares (bonus post)

XKCD-date_634
See https://xkcd.com/634/ for the punch line

Since finishing graduate school, I’ve been working as a free-lance science tutor. I tutor a mix of college and high-school students, mostly in Biology, which means I get to know basic bio really, really well. The units also tend to run on roughly the same schedule, so it’s all either photosynthesis or DNA, or more recently, Mendelian Genetics.

The cornerstone tool of Mendelian Genetics is the Punnett Square, which works on the same principle as a multiplication table. The possible genes the mom can give are on one side, and the possible genes the dad can give are on the top (or vice versa). The student is given the parent’s phenotypes (visible traits) and asked to figure out their offsprings’ possible phenotype. For example, the Punnett Square below crosses a white guinea pig (mom) with a black guinea pig (dad), with the black (B) variant of the color gene dominant to the white (b) variant. This means any baby guinea pig with a B variant will be black, and in order to have a white coat, the baby must inherit a b variant from both mom and dad. Of course, for a bio course, the next question is what percentage of the babies will be black and what percentage will be white (50%).

Punnett Squares

Personally, I don’t care much about guinea pigs, so I prefer to focus on something nearer to my heart–namely sheep and infectious disease. See, in addition to raising chickens, my family raised sheep, and I spent a lot of time feeding them, watering them, helping treat their various ailments, and making Punnett Squares for Scrapie resistance/sensitivity in their offspring. Scrapie is the sheep form of “Mad Cow” Disease, and some sheep are naturally resistant to it. This resistance is caused by having a genotype (combination of gene variants) of RR. Sheep with one resistant and one susceptible variant (QR), tend to be resistant as well, while QQ sheep are completely susceptible to Scrapie (4). So we’d run Punnett Squares on each year’s crop of lambs to see which ones we needed to get tested for Scrapie or lab-typed before sale. You can see a typical Punnett Square for our sheep below and guess the results.

Our Punnett-square approach was successful, and we soon eliminated Scrapie sensitivity from our flock and were able to be certified as Scrapie-free sheep breeders enrolled in the National Scrapie Eradication Program. I also gained a valuable “hook” for curious kids struggling with genetics. It’s hard to beat brain-eating diseases and farmers running Punnett Squares for zaniness.

Scrapie

 

Scientific Note: both Scrapie and the cow version (BSE) are caused by rogue proteins which can trigger a chain reaction of protein misfolding in the brain they infect. This is inevitably fatal. Humans may be able to contract it from eating contaminated meat (beef has a much stronger link than lamb/mutton) (1,2,3). Deer also have a version of this (Chronic Wasting Disease), and hunters are advised to not eat sick-looking deer.

 

References and Further Resources:

(1) Bovine Spongiform Encephalopathy (BSE) Questions and Answers. 2015. https://www.fda.gov/BiologicsBloodVaccines/SafetyAvailability/ucm111482.htm

(2) Can Scrapie be Transmitted to Humans? (2014) http://www.thesheepsite.com/news/702/can-scrapie-be-transmitted-to-humans/

(3) Evidence for zoonotic potential of ovine scrapie prions. Nature Communications. (2014) https://www.nature.com/articles/ncomms6821

(4) Scrapie Genetics. 2003. https://www.aphis.usda.gov/animal_health/animal_diseases/scrapie/downloads/scrapie_genetics.pdf

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