Download Rules for Punnet Squares - Southington Public Schools

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Rules for Punnett Squares
1. When predicting the outcome of a cross of two organisms, it is important to know which phenotype is dominant
and which is recessive. Choose a two letter pair to represent the genotype of the phenotype you are testing.
Generally the letter chosen is the first letter of the dominant trait. Lowercase letters show the recessive allele.
2. Write down the genotypes for both parents involved in the cross, separated by an “X”.
For this example cross two hybrid pea plants that are tall. Write the letters left of the box below.
Note: If the capital and lowercase letters look the same when written (i.e. S), underline the capital letters. It is a
good idea to write a key to your letters to avoid confusion (i.e. S=short fur, s=long fur).
3. Draw a box with enough sections in it to hold the gene combinations for each parent. For a single trait cross, you
need a box with two sections of each side (because you have two possible genes from each parent) for a total of 4
sections. For the current example, the box would look like this:
X
4. Label the outside TOP of each box and the outside LEFT of each box with one letter from each of the possible
parent genes. Put only one letter for each trait at each section. BE SURE TO PUT THE GENES FROM ONE
PARENT ON THE SIDE OF THE BOX AND THE GENES FROM THE OTHER PARENT ON THE TOP OF
THE BOX. If you mix them up, your results will be incorrect.
5. Fill in the boxes with one letter from the top and one letter from the side for each trait. For a single trait cross, you
will end up with two letters in each box. Leave room at the bottom of the box for a label in the next step.
6. Label each box with the phenotype that is represented by each new gene combination that was just created. For
the above problem, the phenotype for TT and Tt are both tall, and tt is short.
You can now predict the odds or probability of getting a tall or short from the mating of two hybrid tall pea plants.
Practice problems
Write the parent genotypes out first, show all fully labeled Punnett squares and be sure to answer the questions.
1. Mendel found that round peas are dominant over wrinkled peas. Cross two hybrid round peas (Rr) and predict
the probability that plant grown from that cross would have wrinkled seeds.
2. Purple pea flowers are dominant to white flowers. Cross a hybrid purple flowered plant (Pp) with a white
flowered plant (pp). How many offspring would have purple flowers?
3. Pea seeds (the part you eat) can be green or yellow. Green seeds are actually recessive to yellow seeds in peas.
Cross a heterozygous yellow seeded plant with a green seeded plant and predict how many offspring plants
would have green seeds.
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4. You are a farmer. Green Giant (Ho, ho, ho) vegetable company will only buy green peas from you.
What type of pea plants should you plant to be sure that you would only get green seeds to sell to Green Giant?
Do a Punnett square to prove it.
5. In beans, brown seeds are dominant over white seeds. Cross a white seed plant with a heterozygous brown
seed plant. If 41 new bean plants are grown from this cross, how many are likely to have white seeds?
6. In squash, the allele for white fruit is dominant over the allele for yellow fruit. If a white-fruited squash is
crossed with a yellow-fruited squash and all of the offspring are yellow, what are the possible genotypes of the
parent plants? (use a Punnett square to help)