Simulate a monohybrid cross using beads to understand Mendelian inheritance. Record results.

To simulate a monohybrid cross, you will represent alleles using beads.

• Assign red beads to represent the dominant allele ($\text{A}$).
• Assign white beads to represent the recessive allele ($\text{a}$).
• For a standard monohybrid cross (e.g., $\text{Aa} \times \text{Aa}$), you will need two small containers, each representing one parent.
• Place one red bead ($\text{A}$) and one white bead ($\text{a}$) into each of the two containers. Each container now represents a heterozygous parent ($\text{Aa}$).

This step simulates the random combination of alleles during fertilization.

• Without looking, randomly select one bead from the first parental container.
• Similarly, randomly select one bead from the second parental container.
• Pair these two selected beads together. This pair represents the genotype of one offspring.
• Record this genotype (e.g., $\text{AA}$, $\text{Aa}$, or $\text{aa}$) on your observation sheet.
• Return the beads to their respective containers.
• Repeat this process many times (e.g., 50-100 times) to generate a sufficient number of offspring genotypes for accurate analysis.

As you simulate each cross, record the following:

• Note the offspring genotype for each pair of beads (e.g., $\text{AA}$, $\text{Aa}$, or $\text{aa}$). Keep a tally for each genotype.
• Identify the corresponding phenotype:
  • If the genotype is $\text{AA}$ or $\text{Aa}$, the phenotype is the dominant trait (since red is dominant).
  • If the genotype is $\text{aa}$, the phenotype is the recessive trait (since white is recessive).
  • Keep a separate tally for the number of dominant phenotypes and recessive phenotypes.

After completing all your simulated crosses, calculate and compare the ratios:

• Calculate the observed genotypic ratio. For example, if you had 25 $\text{AA}$, 50 $\text{Aa}$, and 25 $\text{aa}$ offspring, the ratio would be $25:50:25$, which simplifies to $1:2:1$.
• Calculate the observed phenotypic ratio. Using the example above, 25 $\text{AA}$ + 50 $\text{Aa}$ = 75 dominant phenotypes, and 25 $\text{aa}$ = 25 recessive phenotypes. The ratio would be $75:25$, which simplifies to $3:1$.
• Compare your observed ratios with Mendelian predictions for a monohybrid cross ($\text{Aa} \times \text{Aa}$):
  • Predicted genotypic ratio: $\boxed{1 \text{ AA} : 2 \text{ Aa} : 1 \text{ aa}}$
  • Predicted phenotypic ratio: $\boxed{3 \text{ dominant} : 1 \text{ recessive}}$
• Discuss any differences between your observed ratios and the predicted Mendelian ratios. These differences are typically due to random chance, especially if the number of trials is small.