Amoeba Sisters Video Recap Monohybrid Crosses Answer Key

Amoeba Sisters Video Recap: Monohybrid Crosses – A Comprehensive Guide

The Amoeba Sisters have created a fantastic resource for understanding genetics, particularly the concept of monohybrid crosses. This guide serves as a comprehensive recap of their video, providing a detailed explanation, incorporating key terms, and offering practice problems to solidify your understanding. We'll go beyond a simple answer key, delving into the underlying principles and extending your knowledge beyond the basics.

Understanding Basic Genetics Terminology

Before diving into monohybrid crosses, let's review some fundamental genetic terms. This will provide a solid foundation for understanding the concepts covered in the Amoeba Sisters' video.

Genes and Alleles:

• Genes: These are the basic units of heredity, carrying the instructions for specific traits. Think of them as the recipe for a specific characteristic.
• Alleles: These are different versions of the same gene. For example, a gene for flower color might have an allele for red flowers and an allele for white flowers.

Genotype and Phenotype:

• Genotype: This refers to the genetic makeup of an organism, representing the combination of alleles it possesses. It's the actual genetic code.
• Phenotype: This describes the observable physical characteristics of an organism, determined by its genotype and environmental factors. It's what you see.

Homozygous and Heterozygous:

• Homozygous: An organism is homozygous for a particular gene if it carries two identical alleles. For instance, RR (homozygous dominant) or rr (homozygous recessive).
• Heterozygous: An organism is heterozygous if it carries two different alleles for a particular gene. For example, Rr.

Dominant and Recessive Alleles:

• Dominant Alleles: These alleles mask the expression of recessive alleles when present. They are represented by uppercase letters (e.g., R).
• Recessive Alleles: These alleles are only expressed when two copies are present (homozygous recessive). They are represented by lowercase letters (e.g., r).

Monohybrid Crosses: A Deep Dive

A monohybrid cross involves tracking the inheritance of a single trait. The Amoeba Sisters' video expertly illustrates the process using Punnett squares, a valuable tool for predicting the probabilities of offspring genotypes and phenotypes.

Punnett Squares: The Visual Tool

Punnett squares are a graphical representation of the possible combinations of alleles from two parents. The video emphasizes their use in determining the probability of different genotypes and phenotypes in the offspring.

Example: Let's consider a monohybrid cross involving flower color. Let's say "R" represents the dominant allele for red flowers and "r" represents the recessive allele for white flowers.

A homozygous dominant parent (RR) is crossed with a homozygous recessive parent (rr).

All offspring (100%) will have the genotype Rr and the phenotype of red flowers because R is dominant.

Another Monohybrid Cross Example (Heterozygous x Heterozygous):

Now, let's consider a cross between two heterozygous parents (Rr x Rr):

In this case, we observe:

• Genotypic Ratio: 1 RR : 2 Rr : 1 rr (1:2:1)
• Phenotypic Ratio: 3 Red Flowers : 1 White Flower (3:1)

This demonstrates that even with heterozygous parents, there's a chance of the recessive trait (white flowers) appearing in the offspring.

Beyond the Basics: Extending Your Knowledge

The Amoeba Sisters’ video lays a strong foundation. However, to truly master monohybrid crosses, we need to explore some further concepts:

Probability in Genetics:

The Punnett square visually represents the probability of each genotype and phenotype. Understanding probability is crucial for interpreting these results. For instance, in the Rr x Rr cross, the probability of an offspring having the RR genotype is 1/4 or 25%.

Test Crosses:

A test cross is used to determine the genotype of an organism with a dominant phenotype. If the organism is homozygous dominant (RR), all offspring from a cross with a homozygous recessive (rr) individual will have the dominant phenotype. However, if the organism is heterozygous (Rr), approximately half of the offspring will exhibit the recessive phenotype.

Incomplete Dominance and Codominance:

The Amoeba Sisters' video mainly focuses on complete dominance. However, other inheritance patterns exist:

• Incomplete Dominance: In this pattern, neither allele is completely dominant. The heterozygote exhibits a blend of the two parental phenotypes. For example, a red flower (RR) crossed with a white flower (WW) might produce pink flowers (RW).
• Codominance: Both alleles are fully expressed in the heterozygote. For example, a red flower (RR) crossed with a white flower (WW) might produce flowers with both red and white patches (RW).

Practice Problems: Test Your Understanding

Let's solidify your understanding with some practice problems:

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

Problem 2: A black cat (BB) is crossed with a white cat (bb). Assuming incomplete dominance, where gray is the heterozygous phenotype, what will the offspring look like?

Problem 3: In humans, free earlobes (E) are dominant to attached earlobes (e). A person with attached earlobes marries a person with heterozygous free earlobes. What are the chances that their child will have attached earlobes?

Problem 4: A homozygous dominant red-flowered plant (RR) is crossed with a white-flowered plant (rr). If codominance occurs, describing the phenotype of the F1 generation.

Solutions to Practice Problems

Problem 1:

• Punnett Square: | | T | t | | :---- | :- | :- | | T | TT | Tt | | t | Tt | tt |

• Genotypic Ratio: 1 TT : 2 Tt : 1 tt (1:2:1)

• Phenotypic Ratio: 3 Tall : 1 Short (3:1)

Problem 2: All offspring will be gray (Bb).

Problem 3:

• Punnett Square: | | E | e | | :---- | :- | :- | | e | Ee | ee | | e | Ee | ee |

• There's a 50% chance the child will have attached earlobes (ee).

Problem 4: The F1 generation will have flowers with both red and white patches (RW), exhibiting codominance.

Conclusion

The Amoeba Sisters' video provides an excellent introduction to monohybrid crosses. This comprehensive guide expands on their work, clarifying key concepts and providing practice problems to ensure a thorough understanding. Remember, mastering genetics requires practice and a solid understanding of underlying principles. By consistently applying the concepts discussed here and working through additional problems, you can build a strong foundation in genetics and confidently tackle more complex inheritance patterns.