1. Probability and Punnett Squares

2. Probability
Probability is the likelihood that an event will occur. What chance does it have of happening?
Punnett squares are a useful tool for predicting what the offspring will look like when mating plants or animals.
Punnett squares only show the probability (chance) of the offspring, NOT exactly what will happen.
Reginald Crundall Punnett, a mathematician, came up with these in 1905, long after Mendel's experiments.

3. G = grey fur
g = white fur
3 crosses have a dominant allele (GG, Gg, and Gg) and 1 cross has 2 recessive alleles (gg) so 75% have the chance to have gray fur and 25% have the chance to have white fur.

4. Punnett Squares
Punnett Square Basics and Practice – 6:00 Grab a dry erase marker and get ready to try these on your desk.

5. Yeah, more practice!
What percentage of offspring would be expected to have short whiskers from the cross of two long- whiskered seals if both are heterozygous?

6. P-squARe prActICE QueSTioN #1a
What percentage of offspring would be expected to have short whiskers from the cross of two long-whiskered seals if both are heterozygous?
W w W
W = allele for long whiskers w = allele for short whiskers homozygous dominant seal = WW = 1/4 heterozygous seal =Ww = 2/4
homozygous recessive – ¼ w ANSWER: 25% WW Ww ww

7. P-squARe prActICE QueSTioN #2
In purple people eaters, one-horn is dominant and no horns is recessive. Draw a Punnett Square showing the cross of a purple people eater that is hybrid for horns with a purple people eater that does not have horns. Use H and h for your alleles. Summarize the genotypes & phenotypes of the possible offspring.

8. P-squARe prActICE QueSTioN #2
In purple people eaters, one-horn is dominant and no horns is recessive. Draw a Punnet Square showing the cross of a purple people eater that is hybrid for horns with a purple people eater that does not have horns. Use H and h for you alleles. Summarize the genotypes & phenotypes of the possible offspring.
What are the alleles for the hybrid parent? Hh
What are the alleles for the parent without horns?
Both recessive, so hh
What are the genotypes?
50% hybrid (Hh), 50% homozygous recessive (hh)
What are the phenotypes?
50% have 1 horn and 50% do not.
H h
Hh hh h

9. Punnett Square Practice Page
You can try these at home if you need more practice!

10. Now it is your turn with some sample problems. Have fun
Now it is your turn with some sample problems. Have fun! Use the back of this sheet to answer the questions below. Don’t forget to determine your alleles first and then make your Punnett Square.
One unicorn carries heterozygous dominant, straight horned traits, and its mate carries homozygous spiral horned traits. Use a Punnett Square to determine the probability (what percentage) of one of their offspring having a spiral horn. Use any letter you’d like to represent the alleles.

11. One Sasquatch carries homozygous, long-haired traits, which is dominant and its mate carries homozygous short-haired traits. Use a Punnett Square to determine the genotypes & phenotypes of the possible offspring. Use any letter you’d like for the alleles. Then summarize the phenotypes and genotypes.