MathBench > Probability

Mice with Fangs: Intro to Punnett Squares

But is the ratio exact? (hint: NO)

I'm willing to bet that you found the same ratio of phenotypes as before; namely, there were 9 normal kids, 3 with xray vision, 3 with flight, and 1 lone super-superhero. In fact, its not hard to intuit that ANY dihybrid cross will give you the same result – a 9:3:3:1 ratio.

Does that mean that if Mr and Mrs Normal have exactly 16 kids, they are guaranteed exactly and no more than one super-superhero? Of course not. But it is likely that they will have kids in approximately these ratios.

 

In the applet below, the Punnett Square shows a dihybrid cross. Each time you click on "make one kid", a single offspring will be generated; you can see the mother's and father's contributions on the Punnett Square, and the kid's phenotype will get recorded on the bar chart.

The hollow bars on the bar chart show the ideal 9:3:3:1 ratio for 160 kids. After you have made at least 5 kids, you can fill in the rest by clicking "add lots of kids".

Once you get 160 kids made ... are the proportions exactly 9:3:3:1? Why or why not? Try clearing and adding all the kids a few times. How close can you get to the ideal? Or how far?

 

Don't see the applet? If you are on a computer you own, please do the following

Go to http://java.com/en/download/index.jsp . There is a big flashing button that says Download Now. Look directly under and click on Manual Download. On the next screen, choose the first button, for Windows Online Installation (or go to the bottom for directions for macintosh installation). You might be asked to allow the program to run -- just say yes to whatever they ask, and request a typical installation. You will have to restart after you install.

If you are on a lab computer, ask the lab administrator.