Monohybrid Cross Worksheet

Howard Cosell

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28-11-2024

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Understanding monohybrid crosses is fundamental to grasping the principles of Mendelian genetics. This worksheet provides a structured approach to solving monohybrid cross problems, along with explanations to reinforce your understanding.

What is a Monohybrid Cross?

A monohybrid cross examines the inheritance of a single trait controlled by a single gene with two alleles (alternative forms of a gene). This contrasts with dihybrid crosses, which consider two traits simultaneously. We typically represent alleles with letters: a capital letter for the dominant allele (e.g., 'R' for red flowers) and a lowercase letter for the recessive allele (e.g., 'r' for white flowers).

Key Terms to Remember

Before we proceed, let's define some essential terms:

• Genotype: The genetic makeup of an organism (e.g., RR, Rr, rr).
• Phenotype: The observable characteristics of an organism (e.g., red flowers, white flowers).
• Homozygous: Having two identical alleles for a trait (e.g., RR, rr).
• Heterozygous: Having two different alleles for a trait (e.g., Rr).
• Dominant Allele: The allele that expresses its phenotype even when paired with a recessive allele.
• Recessive Allele: The allele that is only expressed when paired with another recessive allele.

Working Through a Monohybrid Cross

Let's tackle a typical monohybrid cross problem: We'll cross two heterozygous plants for flower color, where red (R) is dominant over white (r).

1. Parental Genotypes: Both parents are heterozygous, so their genotypes are Rr.

2. Gamete Formation: Each parent produces two types of gametes (sex cells) during meiosis: R and r.

3. Punnett Square: We use a Punnett square to visualize the possible combinations of alleles in the offspring.

4. Genotypic Ratio: From the Punnett square, we determine the genotypic ratio of the offspring: 1 RR : 2 Rr : 1 rr.

5. Phenotypic Ratio: Based on the genotypes, we determine the phenotypic ratio: 3 red flowers (RR and Rr) : 1 white flower (rr).

Practice Problems

Now it's your turn! Try these problems to solidify your understanding:

1. Cross two homozygous recessive plants (rr) for flower color. What are the genotypic and phenotypic ratios of the offspring?

2. Cross a homozygous dominant plant (RR) with a heterozygous plant (Rr). Determine the genotypic and phenotypic ratios.

3. A tall pea plant (TT) is crossed with a short pea plant (tt). Tallness is dominant. Predict the genotypic and phenotypic ratios of the F1 generation. What would the F2 generation look like if the F1 generation self-pollinated?

Remember to follow the steps outlined above for each problem. By practicing these, you will develop confidence in solving monohybrid cross problems and further your understanding of Mendelian inheritance.