9.3: Using Punnett Squares
- Page ID
- 133680
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Now that we understand the relationship between genotype and phenotype, let’s learn how we can use this information to make predictions. We can use the genotypes of the parents to predict their offspring’s possible genotypes and phenotypes as well as their probabilities. The best way to chart these possible outcomes is to use the Punnett square.
"Punnett Square Explained" by Karen Marks, Reedley College is licensed under CC BY 4.0
Punnett squares work by crossing the alleles of each parent to determine all of the possible genotypes for their offspring. We separate each parent’s alleles to represent their possible gametes (sperm or egg) so that when we cross it with the other parent’s gametes, each offspring receives one allele from each parent.
When we fill in the Punnett square, we can then determine the probability of each genotype and phenotype. Since a Punnett square has four outcomes, each outcome has a 1-in-4 chance of happening. However, this does not mean that if the parents have 4 offspring that each outcome will be represented. For this exercise, we will practice using Punnett squares to find genotypic and phenotypic frequencies. Then we will conduct an experiment to see how close real results come to expected outcomes.
Punnett Squares
Complete the following crosses using the provided space.
1. tt x tt
Genotypic Ratio: ________________ Phenotypic Ratio :________________
2. NN x nn
Genotypic Ratio: ________________ Phenotypic Ratio :________________
3. Ee x ee
Genotypic Ratio: ________________ Phenotypic Ratio :________________
Expected Outcomes versus Real Results
For this exercise, we will determine the expected outcomes for a specific cross and then test to see if the real results resemble these expected outcomes. Each parent will be represented by a coin. Since coins have a heads and tails side, this means both parents will be heterozygous, with heads being the dominant allele.
Using the information above, determine the genotype for each parent using the letter R.
Mother: Father:
What are the expected genotypic and phenotypic ratios for this cross?
Genotypic Ratio: ________________ Phenotypic Ratio :________________
Now you will use the coins to determine real-world results. You will need two coins and a cup or small bag to complete this procedure.
Procedure:
- Place the coins in the cup or bag and shake it briefly.
- Pour the coins from the bag onto the table top and record the outcome below.
- Repeat this for a total of 40 times.
- Find the genotypic/phenotypic ratios by dividing the total by the lowest outcome.
| Outcome | Tally | Genotypic Total | Genotypic Ratio | Phenotypic Total | Phenotypic Ratio |
|---|---|---|---|---|---|
| Heads/Heads | |||||
| Heads/Tails | |||||
| Tails/Tails |
How do your real genotypic and phenotypic ratios compare to the expected ones?