Amoeba Sisters Monohybrid Crosses Answer Key

Amoeba Sisters Monohybrid Crosses: A Deep Dive with Answer Key

The Amoeba Sisters have become a beloved resource for biology students worldwide, simplifying complex concepts with engaging videos and clear explanations. Their monohybrid cross tutorials are particularly helpful, providing a solid foundation in Mendelian genetics. This article will delve deep into the principles of monohybrid crosses, offering a detailed explanation alongside example problems and, importantly, an answer key to help you master this fundamental aspect of genetics.

Understanding Monohybrid Crosses: The Basics

A monohybrid cross focuses on the inheritance of a single gene. This contrasts with dihybrid crosses, which consider two genes simultaneously. These crosses are crucial for understanding fundamental genetic principles like dominance, recessiveness, alleles, genotypes, and phenotypes.

Key Terminology:

• Gene: A specific segment of DNA that codes for a particular trait.
• Allele: Different versions of a gene. For example, a gene for flower color might have an allele for purple and an allele for white.
• Genotype: The genetic makeup of an organism, represented by the combination of alleles. For example, PP, Pp, or pp.
• Phenotype: The observable characteristics of an organism, determined by its genotype. For example, purple flowers or white flowers.
• Homozygous: Having two identical alleles for a particular gene (e.g., PP or pp). These are also referred to as true-breeding.
• Heterozygous: Having two different alleles for a particular gene (e.g., Pp). Also known as a hybrid.
• Dominant Allele: An allele that masks the expression of another allele (represented by a capital letter).
• Recessive Allele: An allele whose expression is masked by a dominant allele (represented by a lowercase letter).

Punnett Squares: The Visual Tool for Monohybrid Crosses

Punnett squares are the cornerstone of visualizing monohybrid crosses. They are grids that predict the possible genotypes and phenotypes of offspring from a given cross.

Constructing a Punnett Square:

1. Determine the genotypes of the parents: Identify the alleles each parent carries for the trait in question.
2. Set up the square: Draw a square grid with the parental alleles along the top and side.
3. Combine alleles: Fill in the squares by combining the alleles from the parents. Each square represents a possible genotype of an offspring.
4. Determine genotypes and phenotypes: Count the number of each genotype and phenotype to determine the expected ratios.

Example Monohybrid Cross Problems and Solutions (Answer Key)

Let's work through some example problems, mirroring the type you might encounter in the Amoeba Sisters' videos or in a genetics course. We'll focus on a simple trait: flower color in pea plants. Let's assume purple (P) is dominant to white (p).

Problem 1: Homozygous Dominant x Homozygous Recessive

Cross a homozygous dominant purple-flowered plant (PP) with a homozygous recessive white-flowered plant (pp).

Solution:

• Genotypic Ratio: 100% Pp (Heterozygous)
• Phenotypic Ratio: 100% Purple flowers

Problem 2: Heterozygous x Heterozygous

Cross two heterozygous purple-flowered plants (Pp).

Solution:

• Genotypic Ratio: 1 PP : 2 Pp : 1 pp
• Phenotypic Ratio: 3 Purple flowers : 1 White flower

Problem 3: Homozygous Recessive x Heterozygous

Cross a homozygous recessive white-flowered plant (pp) with a heterozygous purple-flowered plant (Pp).

Solution:

• Genotypic Ratio: 1 Pp : 1 pp
• Phenotypic Ratio: 1 Purple flower : 1 White flower

Problem 4: Analyzing Offspring Ratios

You perform a monohybrid cross and observe the following offspring: 75 purple-flowered plants and 25 white-flowered plants. What are the likely genotypes of the parent plants?

Solution:

The 3:1 phenotypic ratio strongly suggests that both parents were heterozygous (Pp). This is because a heterozygous cross (Pp x Pp) produces this ratio (as shown in Problem 2).

Beyond the Basics: Exploring More Complex Scenarios

While the examples above cover the fundamentals, understanding monohybrid crosses can extend to more complex scenarios.

Incomplete Dominance:

In incomplete dominance, neither allele is completely dominant. The heterozygote shows a blended phenotype. For example, if red (R) and white (r) flowers exhibit incomplete dominance, a Rr plant would have pink flowers.

Example: Cross a red-flowered plant (RR) with a white-flowered plant (rr) under incomplete dominance.

Solution:

• Genotypic Ratio: 100% Rr
• Phenotypic Ratio: 100% Pink flowers

Codominance:

In codominance, both alleles are fully expressed in the heterozygote. For example, if red (R) and white (r) are codominant, a Rr plant would have red and white patches.

Example: Cross a red-flowered plant (RR) with a white-flowered plant (rr) under codominance.

Solution:

• Genotypic Ratio: 100% Rr
• Phenotypic Ratio: 100% Red and white patches

Applying Your Knowledge: Test Your Understanding

To solidify your understanding of monohybrid crosses, try solving these additional problems:

1. Problem 5: In pea plants, tall (T) is dominant to short (t). Cross two heterozygous tall plants. What are the genotypic and phenotypic ratios of the offspring?

2. Problem 6: In rabbits, black fur (B) is dominant to white fur (b). A black rabbit is crossed with a white rabbit, and all the offspring are black. What are the genotypes of the parent rabbits?

3. Problem 7: A certain flower exhibits incomplete dominance for petal color. Red (R) and white (r) alleles produce pink (Rr) flowers. Cross a pink flower with a white flower. What are the expected genotypic and phenotypic ratios?

Conclusion: Mastering Monohybrid Crosses

Understanding monohybrid crosses is a crucial stepping stone in genetics. By mastering the principles of dominance, recessiveness, Punnett squares, and interpreting results, you'll be well-equipped to tackle more complex genetic problems. Remember to practice regularly – the more problems you solve, the more confident and proficient you will become. The Amoeba Sisters' videos serve as an excellent resource to further enhance your comprehension and provide a fun learning experience. Use this article as a companion guide, referencing the examples and answer key to check your work and solidify your understanding of this fundamental area of biology.