Unit II Assignment Genetics Worksheet
Solving Punnett Squares
Reginald Punnett was a British geneticist who developed the Punnett square to explain how the chromosomes of parents cross and produce offspring. To solve genetics problems using a Punnett square, it is necessary to a) understand the associated vocabulary and b) understand some of the rules for solving the problems.
Before you continue with the problems below, review the meaning of the terms allele, dominant, recessive, homozygous, heterozygous, genotype and phenotype.
You also should review the Punnett Square Basics video linked in the unit lesson.
Instructions: Answer the Punnett square questions below by placing the letter of the correct answer in the blank provided.
The genotypic ratio will be expressed using the alleles given in each scenario (e.g., 50% Gg : 50% gg). The phenotypic ratio will use descriptive terms, e.g., 3 (Green) : 1 (clear), 2 (Green) : 2 (clear), depending on the results of your cross. Be sure not to confuse the two ratios.
Punnett Square Problems
Question 1. In corn plants, the allele for green kernels (G) is dominant over clear kernels (g). A homozygous dominant plant is crossed with a homozygous recessive plant. The parental genotypes are provided for you.
Determine the genotype and phenotype of the F1 generation by filling in the Punnett square, then answer the questions by placing the letter of your answer in the answer blank to the right of the question.
Key: G = green kernels, g = clear kernels
Genotype of parents: _GG_ x _gg_
a) Complete the table below. (4 pts.)
g g
G
G
b) What is the genotypic ratio of the offspring in Question 1? (4 pts.) Answer:
a) 50% Gg : 50% gg
b) 100% Gg
c) 25% GG : 50% Gg : 25% gg
d) 100% Green
c) What is the phenotypic ratio of the offspring in Question 1? (4 pts.) Answer:
a) 50% Green : 50% Yellow
b) 100% Light Green
c) 25% GG : 50% Gg : 25% gg
d) 100% Green
d) The F1 generation of the cross in Question 1 are all . (4 pts.) Answer:
a) Homozygous dominant
b) Heterozygous dominant
c) Heterozygous
d) Recessive
Question 2: Green seeds are dominant over yellow seeds in pea plants. Cross a heterozygous (green seeded) plant with a yellow seeded plant. The parental genotypes are provided for you.
Determine the genotype and phenotype of the F1 generation by filling in the Punnett square, then answer the questions below.
Key: G = green seeds and g = yellow seeds
Genotype of parents: Gg x gg
a) Complete the table below. (4 pts.)
g g
G
g
b) What is the genotypic ratio of the offspring in Question 2? (4 pts.) Answer:
a) 75% Yellow : 25 % Green
b) 75% Gg : 25% gg
c) 50% Gg : 50% gg
d) 35% Green : 65% Yellow
c) What is the phenotypic ratio of the offspring in Question 2? (4 pts.) Answer:
a) 75% Yellow : 25 % Green
b) 75% Gg : 25% gg
c) 50% Green : 50% Yellow
d) 35% Green : 65% Yellow
d) The yellow-seeded offspring from the cross in Question 2 are (4 pts.). Answer:
a) Homozygous dominant
b) Dominant
c) Homozygous recessive
d) Heterozygous
Question 3: Now cross two of the heterozygous F1 offspring from Question 2.
a) Complete the table below. (4 pts.)
G g
G
g
b) What is the genotypic ratio of the offspring in Question 3? (4 pts.) Answer:
a) 75% Yellow : 25 % Green
b) 75% Gg : 25% gg
c) 50 Gg : 50% gg
d) 25% GG : 50% Gg : 25% gg
c) What is the phenotypic ratio of the offspring in Question 3? (4 pts.) Answer:
a) 75% Green : 25% Yellow
b) 75% Yellow : 25% Green
c) 50% Green : 50% Yellow
d) 100% Green
Question 4: Consider the resulting genotypic ratio of crossing the two heterozygous pea plants in Question 3. We will use this ratio in a short activity exploring probability. Keep in mind crossing two individuals that are heterozygous for a certain trait is similar to flipping two coins. Each coin has two sides (we might think of each side as an “allele”) and the chances of flipping heads/heads, heads/tails, or tails/tails should be similar to the ratio we see when crossing two heterozygotes.
For this simple activity, you will need two coins (pennies, nickels, dimes, quarters, or a mix). Alternatively, you may use Google to find a coin-flipper simulator that will allow you to flip two coins at once. You also will need a piece of scratch paper and a pen or pencil or a blank electronic document.
Directions: Flip the two coins simultaneously at least 50 times. For each flip of the pair of coins, you will record the results on a piece of scratch paper or electronic document. You might set up a table like the one below to record your results. Once you have flipped the coins at least 50 times, enter the number of heads/heads, heads/tails, and tails/tails in Table 1 below, which is part “a” of question 4.
Now determine the ratio for your results. You will do this by dividing the number for each result by the total number of flips, and then multiply by 100. Don’t forget to show your work!
(Example: If the number of heads/heads is 9 then 9/50 = 0.18, 0.18×100 = 18%), Repeat this mathematical procedure for heads/tails and tails/tails); be sure to include your math in the table!
a.) Table 1 (12 pts.)
Heads/heads (hh)
Head/tails (ht)
Tails/tails (tt)
Ratio (hh:ht:tt)
b.) Compare the resulting ratio from the Question 3 cross of two heterozygous parents to the ratio from the coin flipping exercise. Are there similarities? If so, what are they? Enter answer in the box below. (4 pts.)
Cancer Risk Factors
This question discusses cancer and risk factors. Begin by going to the website http://www.cancer.org/
Click “Cancer A-Z” in the upper-left corner. The page that comes up will provide links to information on breast cancer, colon and rectal cancer, lung cancer, prostate cancer, and skin cancer. Review the information for each these cancers.
Next, write an essay that discusses your own risk factors for each type of cancer and steps you might take to decrease those risk factors. Be sure to address all five types of cancer.