Escience Lab 15 Population Genetics Answers

eScience Lab 15: Population Genetics - A Comprehensive Guide and Answer Key

Understanding population genetics is crucial for comprehending the mechanisms driving evolution. eScience Lab 15 provides a hands-on approach to exploring these concepts, but navigating the complexities can be challenging. This comprehensive guide will delve into the key concepts covered in the lab, provide detailed explanations, and offer answers to help solidify your understanding. We'll explore Hardy-Weinberg equilibrium, allele and genotype frequencies, and the factors that disrupt this equilibrium, all within the context of the eScience Lab 15 exercises.

1. Understanding Hardy-Weinberg Equilibrium

The cornerstone of population genetics is the Hardy-Weinberg principle. It states that the genetic variation in a population will remain constant from one generation to the next in the absence of disturbing factors. This equilibrium is maintained under five specific conditions:

• No Mutation: The rate of mutation must be negligible. Mutations introduce new alleles, altering allele frequencies.
• Random Mating: Individuals must mate randomly, without any preference for certain genotypes. Non-random mating, such as assortative mating (mating with similar individuals), can alter genotype frequencies.
• No Gene Flow: There should be no migration of individuals into or out of the population. Gene flow introduces new alleles or alters existing allele frequencies.
• No Genetic Drift: The population must be large enough to avoid random fluctuations in allele frequencies. Genetic drift is particularly pronounced in small populations.
• No Natural Selection: All genotypes must have equal survival and reproductive rates. Natural selection favors certain genotypes, leading to changes in allele frequencies.

The Hardy-Weinberg equations:

The principle is mathematically expressed through two equations:

• p + q = 1: This equation represents the allele frequencies, where 'p' represents the frequency of the dominant allele and 'q' represents the frequency of the recessive allele. The sum of these frequencies always equals 1 (or 100%).

• p² + 2pq + q² = 1: This equation represents the genotype frequencies, where:

  • p² represents the frequency of the homozygous dominant genotype (e.g., AA).
  • 2pq represents the frequency of the heterozygous genotype (e.g., Aa).
  • q² represents the frequency of the homozygous recessive genotype (e.g., aa).

2. Analyzing Allele and Genotype Frequencies

eScience Lab 15 likely presents scenarios where you need to calculate allele and genotype frequencies from given data (e.g., the number of individuals with each genotype in a population). Here's how:

Example:

Let's say you have a population of 100 individuals with the following genotypes:

• AA: 36 individuals
• Aa: 48 individuals
• aa: 16 individuals

1. Calculate allele frequencies:

• Number of 'A' alleles: (36 individuals × 2 alleles/individual) + (48 individuals × 1 allele/individual) = 120 alleles
• Number of 'a' alleles: (48 individuals × 1 allele/individual) + (16 individuals × 2 alleles/individual) = 80 alleles
• Total number of alleles: 120 + 80 = 200 alleles
• Frequency of 'A' allele (p): 120/200 = 0.6
• Frequency of 'a' allele (q): 80/200 = 0.4

2. Calculate genotype frequencies:

• Frequency of AA genotype (p²): 36/100 = 0.36 (This should approximately equal p² = 0.6²)
• Frequency of Aa genotype (2pq): 48/100 = 0.48 (This should approximately equal 2pq = 2 × 0.6 × 0.4)
• Frequency of aa genotype (q²): 16/100 = 0.16 (This should approximately equal q² = 0.4²)

3. Factors that Disrupt Hardy-Weinberg Equilibrium

The eScience Lab 15 likely explores how deviations from the five conditions listed earlier affect allele and genotype frequencies. Let's examine each:

3.1 Mutation:

Mutations introduce new alleles into the population. A high mutation rate can significantly alter allele frequencies over time, moving the population away from Hardy-Weinberg equilibrium. The direction and magnitude of the shift depend on the type and rate of mutations.

3.2 Non-Random Mating:

Assortative mating (mating with similar phenotypes) increases the frequency of homozygous genotypes. Disassortative mating (mating with dissimilar phenotypes) increases the frequency of heterozygous genotypes. Both deviate from the random mating assumption of Hardy-Weinberg.

3.3 Gene Flow:

Migration of individuals into or out of a population introduces or removes alleles. The impact depends on the allele frequencies in the migrating population and the size of the receiving population. High gene flow can homogenize allele frequencies between populations.

3.4 Genetic Drift:

Random fluctuations in allele frequencies are more pronounced in small populations. Genetic drift can lead to the loss of alleles or the fixation of certain alleles, irrespective of their selective advantage. The bottleneck effect and founder effect are examples of genetic drift.

3.5 Natural Selection:

Natural selection favors genotypes that enhance survival and reproduction. This leads to a change in allele frequencies, as advantageous alleles become more common. The strength of selection determines the rate of change.

4. eScience Lab 15 Specific Exercises (Hypothetical Examples and Answers)

Since I don't have access to the specific exercises within eScience Lab 15, I'll provide hypothetical examples and illustrate how to approach them. These examples cover the core concepts explored in the lab.

Hypothetical Exercise 1: Calculating Allele and Genotype Frequencies

A population of wildflowers has 200 individuals. 100 are red (RR), 80 are pink (Rr), and 20 are white (rr). Calculate the allele frequencies (p and q) and genotype frequencies. Determine if the population is in Hardy-Weinberg equilibrium.

Answer:

1. Allele Frequencies:

   • Number of R alleles: (100 × 2) + 80 = 280
   • Number of r alleles: 80 + (20 × 2) = 120
   • Total alleles: 400
   • p (frequency of R): 280/400 = 0.7
   • q (frequency of r): 120/400 = 0.3

2. Genotype Frequencies:

   • Observed frequency of RR: 100/200 = 0.5
   • Observed frequency of Rr: 80/200 = 0.4
   • Observed frequency of rr: 20/200 = 0.1

3. Hardy-Weinberg Expectation:

   • Expected frequency of RR (p²): 0.7² = 0.49
   • Expected frequency of Rr (2pq): 2 × 0.7 × 0.3 = 0.42
   • Expected frequency of rr (q²): 0.3² = 0.09

4. Comparison: The observed and expected genotype frequencies are significantly different. Therefore, the population is not in Hardy-Weinberg equilibrium. This suggests that one or more of the Hardy-Weinberg assumptions are violated.

Hypothetical Exercise 2: Impact of Selection

Imagine a population of beetles where green coloration (G) is dominant to brown (g). Green beetles are better camouflaged and have a higher survival rate. How will this affect allele and genotype frequencies over time?

Answer:

Natural selection favors the green (G) allele. Over time, the frequency of the G allele (p) will increase, while the frequency of the g allele (q) will decrease. The genotype frequencies will also shift, with an increased proportion of GG and Gg genotypes and a decreased proportion of gg genotypes. The population will move away from Hardy-Weinberg equilibrium.

Hypothetical Exercise 3: Effect of Genetic Drift

Consider two small populations of butterflies, one with 10 individuals and another with 100 individuals. Both populations initially have equal frequencies of a rare allele. Which population is more likely to experience significant changes in allele frequencies due to genetic drift?

Answer:

The smaller population (10 individuals) is more susceptible to genetic drift. Random fluctuations in allele frequencies are much more pronounced in small populations compared to larger populations. The smaller population is more likely to experience a loss of the rare allele or its fixation through chance events.

5. Conclusion

eScience Lab 15 provides invaluable experience in applying population genetics principles. By understanding Hardy-Weinberg equilibrium, calculating allele and genotype frequencies, and recognizing the factors that disrupt equilibrium, you gain a strong foundation in evolutionary biology. This guide offers a framework for approaching the lab's exercises. Remember to carefully analyze the data provided and apply the concepts discussed to understand the dynamics of allele and genotype frequencies within populations. By mastering these concepts, you'll be well-prepared to tackle more advanced topics in population genetics and evolutionary biology. Remember to consult your lab manual and instructor for specific guidance and clarification on your assigned exercises.