# Monohybrid & Dihybrid Cross Activities

Instructor: Joanne Abramson

Joanne has taught middle school and high school science for more than ten years and has a master's degree in education.

Are you beginning your unit on genetics? These interactive, hands-on activities will illuminate monohybrid crosses and dihybrid crosses for all of your students.

## Monohybrid and Dihybrid Crosses

Monohybrid and dihybrid crosses are an essential part of any biology curriculum. The standards-based activities below are arranged in order from simplest to most complex. They are all hands-on learning tasks that promise to engage students, deepen their understanding of the process, and challenge their critical thinking skills. The set up for some of these activities might seem tedious, especially if you have a large class. Offer extra credit to trustworthy students and have them assist in creating the materials.

## Monohybrid Cross Activities

A proper study of genetics begins with the monohybrid cross. The following two activities introduce the concept and create a foundation for future learning.

### Building Block Punnett Squares

This is a great kinesthetic activity to both introduce students to test crosses and to solidify their understanding of dominant and recessive alleles. For this activity, you will need colored building blocks that come in various sizes. Choose two different colors. Pick all the rectangles of one color (let's say red) and all the squares of the other color (blue). In this case, red would represent the dominant allele (since it is larger) and blue would represent the recessive. (Alternatively, you could simply cut two different colors of cardstock into rectangles and squares.) Also, make handouts of Punnett squares that are large enough for students to fit two blocks into each section.

Have students work in pairs and distribute a handout and eight blocks of each color to the partners. Demonstrate how to cross two parents by placing the blocks representing one parent across the top of the Punnett square and the blocks of the other parent down the left. Next, show students how to distribute the alleles across and down by placing an appropriate block into the box. Finally, show students how to calculate the genotypic and phenotypic ratios of the offspring. As students catch on, they can move away from the blocks and start writing the monohybrid crosses by hand.

### Plastic Egg Punnett Squares

This colorful activity can be completed any time of year. Students will practice completing monohybrid crosses and learn about incomplete dominance. The materials needed are plastic Easter eggs of various colors, matching colors of construction paper, scissors, and colored pencils. The idea behind this activity is that the top and the bottom of the eggs represent the parents, and colored pieces of paper inside the egg represent the offspring. An example of possible genotypes and phenotypes for this activity are:

• BB = blue
• bb = yellow
• Bb = green
• PP = purple
• pp = pink
• Pp = orange

Thus, an egg with a blue top and a blue bottom would represent BB x BB. An egg with a blue top and a green bottom would represent BB x Bb.

To set up, create eggs for each possible combination of parents (bb x bb, bb x Bb, etc.) If you have a large class, you may want to create more than one set. Next, cut small squares of blue, yellow, green, purple, pink and orange construction paper. Place a square of construction for each possible offspring phenotype into the eggs. For example, a pink and orange egg (pp x Pp) would have 2 orange square and 2 pink squares.

Students may work independently or with a partner. Have students choose an egg and create a monohybrid cross for the two parent genotypes. Using the colored pencils, they can color-code the offspring on the cross and then calculate the phenotypic ratios. Students can check if they completed the cross correctly by checking the colored squares inside the egg.

Alternatively, you can use these eggs as a reward or treat by replacing the colored squares of paper with colored candy.

## Dihybrid Cross Activities

Dihybrid crosses are a little more difficult than monohybrid crosses, but the concept is essentially the same. The following two activities will break down the steps and allow students to deepen their understanding.

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