Table of Contents

• Punnett Squares in Theory
• 1: Introduction
• 2: The Punnett Menu
• 3: Creating combinations chez Punnett
• 4: Insults by the dozen
• 5: Punnett menu revisited
• Punnett Squares in Practice
• 6: Onwards to Genetics
• 7: Recap of genetics vocab
• 8: Can this marriage be saved?
• 9: Vampire Fangs
• 10: Example: Spock and Jax
• 11: Example: Spock and Nurse Chapel
• Bigger Punnett Squares
• 12: And you thought we were done!
• 13: The Dreaded Double Hybrid
• 14: Another Dihybrid Cross
• 15: Is the ratio exact? (Hint: NO!)
• 16: Review

Vampire Fangs

In the module on Laws of Probability, we figured out the probability of flipping 2 coins and getting 2 tails:

P(coin1=tails AND coin2=tails) =

P(coin1=tails) * P(coin2=tails) = 0.5 * 0.5 = 0.25

Or, you could do this as a Punnett Square. This time the mother's possible egg types go on the top, and the father's possible sperm types on the side. The kid gets one egg from Mom and one sperm from Dad.

Here is a Punnett Square. See if you can fill it in -- remember, both Mom's and Dad's genotypes are Tt. Click the "check" buttons at the bottom will tell you if you've filled it in correctly. (Also, notice that the top and left cells are color-coded: pink for Mom and baby-blue for Dad!)

Punnett Square
Mom = Tt, Dad = Tt

  

 

 

If you count, you will find 3 normal mice and 1 with fangs. This leads to a classic ratio, namely 3:1, or three to one. Applied to our mice, that means, if Mr. and Mrs. Mouse have a copy of each allele, they will produce (approximately) 3 normal looking mice for every 1 vampiric baby. And, looking at the Punnett Square , you can also deduce that, on average, 2 of the normal-looking offspring will have the vampire allele but not show it, while one will have no vampire allele at all.

It's very important to realize that these numbers are approximate. The actual production of eggs and sperm and their union are random processes, so the exact number of each phenotype is only approximate.