Dihybrid Crosses Are Twice The Fun!
Lesson 11 of 13
Objective: SWBAT analyze key concepts in genetics as they utilize probability to determine the variations in the genetic outcomes of this activity. Students will create Punnett squares to predict the probability of the given trait variations expressed in a population.
In an effort to activate student prior knowledge, students will complete the Punnett Squares Practice Worksheet to reinforce their understanding of single-trait Punnett squares before this lesson dives into more complicated dihybrid crosses using Punnett squares to predict the probability of an outcome.
Students were first introduced to the concept of Punnett Squares in a prior lesson.
This anticipatory activity is a quick formative assessment to determine whether students are capable of advancing to the next step of rigor in regards to the study of genetics. I collected two samples of student work to exemplify the diverse levels of mastery among the students.
Student Work Sample #1: Punnett Squares - Mastery - This student has demonstrated a mastery of creating and evaluating Punnett squares to predict the probability of a trait. The work sample contains a few minor errors, but overall the student has exhibited full understanding of the genetics' concept.
Student Work Sample #2: Punnett Squares - Needs Intense Review - To be honest, I was shocked that these two samples of student work were taken from the same class. After reviewing this activity, this student was invited to lunchtime tutoring for remediation of the curriculum. After two tutoring sessions, the student was able to progress and work at the level of the rest of the class.
If you students are struggling with the concepts of Punnett Squares, this video clip does a great job making the multi-step process easier to complete:
Students will record the Dihybrid Crosses Lecture Notes as an introduction to dihybrid crosses. The students will examine two different traits at the same time and use a Punnett square to determine the probability of each outcome.
Possible Student Misconception: It is crucial to remind students that Punnett squares are not crystal balls and cannot predict the future . . . If so, teachers would use them each week to predict the winning lottery numbers! Punnett squares can only give the probability of a certain outcome occurring for the examined traits.
The FOIL Method: Students will struggle to organize their 16-square Punnett square. The best strategy to guide students to determine which alleles pair up is the F.O.I.L method.
F - "first" - select the first allele for each trait: T is the first "t" of the first trait and B is the first "b" of the second trait. The allele pair will be TB
O - "outer" - select the outer allele for each trait: T is the outside "t" of the first trait and b is the outside "b" of the second trait. The allele pair will be Tb
I - "inner" - select the inside allele for each trait: t is the inside "t" of the first trait and B is the inside "b" of the second trait. The allele pair will be tB
L - "last" -select the last allele for each trait: t is the last "t" of the first trait and b is the last "b" of the second trait. The allele pair will be tb
The allele pairs for this parent will be TB, Tb, tB, tb.
These four allele pairs will be placed across the top of the 16-square Punnett square and the second parent's genotype will be FOILed using the same method. The resulting four allele pairs will be organized along the side of the 16-square Punnett square.
Step-By-Step Handout: Students are encouraged to use the step-by-step handout to guide their progression through completing the dihybrid crosses. I make sure that each student has a copy of this handout before they leave class to use as a reference for the remainder of the Genetics Unit. It is an investment in paper, but the students seem to gain confidence due to the access of the handout.
Students will use their Dihybrid Crosses Lecture Notes as a reference to complete the Dihybrid Cross Practice worksheet. Students will need to complete their work on a separate sheet of paper. The most difficult aspect of successfully completing a dihybrid cross is keeping the alleles organized on the outside of the grid and being able to read your writing on the inside of the 16-square grid.
Students will work collaboratively with their partner to problem solve difficulties since this practice activity is the first time students will attempt to complete a dihybrid cross. The teacher will rotate around the room at this time to provide additional support to students who need the extra instruction.
Sample of Student Work - Dihybrid Crosses: The side-by-side comparison of the two samples of student work demonstrate the effectiveness in using highlighters to count up the number of each expressed phenotype for the two traits that are being examined. The use of highlighters to assist in student calculations helps to minimize student error when tabulating the number of offspring will express the two traits. The data is essential because it will used to support the NGSS LS3-3 which emphasizes the use of probability to explain the variations of a given trait in a population.
Activity Review: The students will select the practice question that seemed most difficult and will review in a whole group discussion. As the class is discussing specific difficulties they encountered, students who have mastered the concept, will be asked to go to the front board to demonstrate the steps for each of the practice problems. Collaborative groups are also eligible to go to the front board to work through the practice problems if they would like the social support. The visual reinforcement will allow students the opportunity to check their work and gain confidence by seeing and hearing the correct procedure.
Homework Practice: Students will be challenged to write their own dihybrid cross practice problem as homework. To ensure that students are practicing the procedure, they will create an answer to their sample problem on a separate sheet of paper. Students need to include the following in their answer keys:
- Assign the parental genotypes for each parent.
- FOIL the parental genotypes to determine the 4 allele pairs for each parent
- Set up the 16-square grid and place one parent's allele pairs on top and the other on the side
- Cross the parental allele pairs to fill in each box of the gird (just like single trait crosses)
- Count up the number of the number of offspring with each of the PHENOTYPES
- Dominant, Dominant
- Dominant, Recessive
- Recessive, Dominant
- Recessive, Recessive
- Create a ratio to express the variations of traits expressed in the population.
Here is an example of a dihybrid cross from the IB Guides website:
The students should create a dihybrid cross similar to this as homework. The ratio for the outcome would be 9:3:3:1.
NGSS-based Writing Prompt: Students also need to write a paragraph explaining how Punnett Squares provide evidence to explain the variation and distribution of the expressed trait.