D7 Dihybrid Cross
Dihybrid Cross
INTRODUCTION
Corn kernel color and shape will be used to examine Dihybrid crosses. Hybrids are
individuals resulting from the mating of two pure bred individuals. Hybrids are heterogeneous
(possess one dominant allele and one recessive allele) for a trait. The prefix “di” means 2, so a
dihybrid is an individual that is heterogeneous for not one, but two traits.
The two trait possibilities for color are purple and yellow. Purple kernels are dominant.
The shorthand used to represent Purple allele will be the letter “P”. A lower case “p” will
represent yellow allele. The round allele is the dominant shape and will be represented by “R”.
The wrinkled allele is represented by “r”.
PURPOSE
1. Students will observe the expression of traits in a dihybrid cross and compare the values to
the theoretical ratios.
SAFETY
 This corn has been treated to inhibit pests. Wear gloves when handling the corn.
 The pins used to mark your place while counting the kernels are sharp. Be careful not to
stick yourself, your lab partners.
 Do not puncture or remove the protective plastic covering.
PROCEDURE
1. Complete the Punnett square on your data sheet.
2. Record the expected number of each phenotype in the blanks below the Punnett square.
3. Use a colored marking pin to mark the row of kernels where you will begin your count and
another pin to mark the row you are counting. If a kernel appears to have a small amount of
purple on it, count it as purple.
4. Each partner should record the data for at least 100 kernels.
5. Have one partner call out the phenotypes of each kernel and record the phenotypes on the
data sheet
6. Complete the formula to determine the ratio of the phenotypes from your data collected by
your group.
7. Add the numbers for each phenotype for the entire class and complete the ratio formula
to determine for the class data.
8. Answer the questions in the Data Analysis Section.
D7 Page 1 Revised 5/05
D7 Dihybrid Cross
STUDENT DATA SHEET
Name ________________________
Dihybrid Cross
Date _________________________
Parent One Genotype - P p
Parent Two Genotype - P p
Data
Possible Gametes PR
Possible Gametes PR
Rr
Rr
Pr
Pr
pR
pR
pr
pr
Record a tally mark for each phenotype you observe.
Purple Round
Purple Wrinkled
Yellow Round
Yellow Wrinkled
Record the number of each phenotype here.
Purple Round _____, Purple Wrinkled _____, Yellow Round _____, Yellow Wrinkled _____
Calculate the Phenotypic Ratio for each of the 4 phenotypes and record the results here
Find the ratio by dividing the numbers for each phenotype by the number you recorded for the
smallest phenotype.
Purple Round ______, Purple Wrinkled_____, Yellow Round _____, Yellow Wrinkled _____
D7 Page 2 Revised 5/05
D7 Dihybrid Cross
Record the number of each phenotype for the entire class.
Purple Round _____, Purple Wrinkled _____, Yellow Round _____, Yellow Wrinkled _____
Calculate the Phenotypic Ratio for each of the 4 phenotypes using data from the entire
class and record the results here.
Find the ratio by dividing the numbers for each phenotype by the number that is recorded for the
smallest phenotype.
Purple Round _____, Purple Wrinkled_____, Yellow Round ______, Yellow Wrinkled _____
Data Analysis
How did the results for your group compare to the expected ratio? Were the ratios matched
perfectly?
How did the results for the entire class compare to the expected ratio? Were the ratios matched
perfectly? Were they more or less accurate than the group sample?
When Mendel observed peas for shape and color, he used a very large sample size.
Compare a small sample size (the group data) and a large sample size (the class sample). Did
you find that sample size made a difference in the outcome of this experiment?
The two genes you examined today are located on different chromosomes. If the genes
were located on the same chromosomes, would you expect the results to be the same or
different? Why? Remember that the allele you get in offspring depend on the duplication and
division of chromosomes during meiosis. Four copies of each chromosome are packaged into
four separate sex cells.
D7 Page 3 Revised 5/05
D7 Dihybrid Cross
TEACHER HANDBOOK
TEACHER NOTES
This lab requires a regular class period.
BACKGROUND
The students will observe 2 different traits in each kernel of corn. They will not only observe the
color, but also the shape.
CORRELATIONS
COURSE OF STUDY-SCIENCE
COS
Biology Content Standards: 1,7
Genetics 2,3,5
ALABAMA HIGH SCHOOL GRADUATION EXAM
I-The student will understand concepts dealing with the nature of science.
1. Analyze the methods of science used to identify an solve problems.
IV-The student will understand concepts of heredity.
1. Recognize heritable characteristics of organisms.
PRELAB
Make sure that you have your classes divided into groups so that each group will have one ear.
SAFETY
The students should wear gloves. They should keep the corn in the plastic wrap. Watch the
students as they use the pins.
TROUBLESHOOTING
The numbers will not be close to 9:3:3:1 until you have a very large sample. Have the students
repeat the lab and save numbers all day. It is helpful to let them see how close they can get as
the sample size grows.
ANTICIPATED RESULTS
The phenotypic ratios will vary but are usually close to 9:3:3:1. If the student correctly
calculates the ratios, the last ratio will always be one. Here is an example:
Purple Round = 69 Purple Wrinkled = 25 Yellow Round = 30 Yellow Wrinkled = 8
D7 Page 4 Revised 5/05
D7 Dihybrid Cross
Purple Round = 69 Purple Wrinkled = 25 Yellow Round = 30 Yellow Wrinkled = 8
8
8
8
8
= 8.6 (9)
= 3.1 (3)
= 3.7 (4)
= 1
1) Question: The ratios are not usually perfectly matched. Some groups will get closer that
others
2) Question: The ratios should approach the predicted values with the increase of sample
size.
3) Question: If the genes were located on the same chromosome, they would not assort
themselves independently. They would be more likely to stay linked together and the
Mendalian ratio would not be expressed.
D7 Page 5 Revised 5/05