#AP Biology: Hardy-Weinberg Equilibrium - Your Last-Minute Guide 🚀

Hey there, future AP Bio rockstar! Let's break down Hardy-Weinberg, a key concept that might seem tricky but is totally manageable. We'll make sure you're feeling confident and ready to tackle any question on this topic. Let's get started! 💪

#Introduction to Hardy-Weinberg Equilibrium

The Hardy-Weinberg equilibrium is a model that describes a theoretical state where allele frequencies in a population remain constant from generation to generation. Think of it as a baseline to compare real-world populations against. If a population deviates from this equilibrium, it suggests that evolution is occurring! 💡

#Key Assumptions

This model operates under five crucial assumptions. Remember, these are rarely (if ever) met in nature, but they're essential for the model to work:

1. No Mutation: 🧬 No new alleles are popping up. The existing ones are simply passed down.
2. No Selection: 🎯 No allele has an advantage. Natural or artificial selection isn't playing favorites.
3. No Gene Flow: 🚶‍♀️ No migration or movement of individuals in or out of the population. No new alleles entering or leaving.
4. Infinite Population Size: ♾️ The population is huge, so random fluctuations in allele frequencies (genetic drift) are negligible.
5. Random Mating: 💘 Mating is random, with no preference for certain genotypes.

#Understanding the Equations

Okay, let's talk equations. Don't panic! They're not as scary as they look. We have two main equations:

1. Allele Frequencies: $p + q = 1$

   • p = frequency of the dominant allele
   • q = frequency of the recessive allele

2. Genotype Frequencies: $p^2 + 2pq + q^2 = 1$

   • $p^2$ = frequency of homozygous dominant genotype
   • $2pq$ = frequency of heterozygous genotype
   • $q^2$ = frequency of homozygous recessive genotype

#How to Use Them

1. Find q²: Start with the homozygous recessive phenotype. This is the only genotype directly observable from the phenotype.
2. Calculate q: Take the square root of q².
3. Calculate p: Use p = 1 - q.
4. Plug and Play: Use p and q in the second equation to find genotype frequencies.

#Visual Aid

#Example Walkthrough

Let's make it concrete with an example:

Imagine a bird population where 84 out of 100 birds have white feathers (dominant), and 16 have grey feathers (recessive). Here's how we solve it:

1. q²: 16/100 = 0.16 (frequency of grey birds)
2. q: √0.16 = 0.4 (frequency of the recessive allele)
3. p: 1 - 0.4 = 0.6 (frequency of the dominant allele)
4. p²: 0.6² = 0.36 (frequency of homozygous dominant birds)
5. 2pq: 2 * 0.6 * 0.4 = 0.48 (frequency of heterozygous birds)

#Example Problem and Explanation

Let's tackle a more complex problem:

Problem: A rare genetic disorder is caused by a recessive allele, "a", with a frequency of 0.02. Calculate the frequencies of the dominant allele "A", homozygous recessive (aa), and heterozygous (Aa) individuals. Also, list the assumptions made in this scenario.

Solution:

1. Assign variables: q = frequency of "a" = 0.02, p = frequency of "A"
2. Calculate p: p = 1 - q = 1 - 0.02 = 0.98
3. Calculate q²: q² = 0.02² = 0.0004 (0.04% homozygous recessive)
4. Calculate 2pq: 2pq = 2 * 0.98 * 0.02 = 0.0392 (3.92% heterozygous)

Assumptions:

• No mutation
• No selection
• No gene flow
• Infinite population size
• Random mating

#Final Exam Focus

Alright, let's focus on what's most important for the exam:

• Understand the conditions: Know why each condition is essential for Hardy-Weinberg equilibrium.
• Master the equations: Practice using the equations to solve for allele and genotype frequencies. Focus on starting with q².
• Apply the concept: Understand that Hardy-Weinberg is a null hypothesis. Deviations from the equilibrium indicate evolution.
• Connect to other units: Relate Hardy-Weinberg to natural selection, genetic drift, and other evolutionary mechanisms.

#Last-Minute Tips

• Time Management: Start with the easier parts of the problem, like finding q². Then, work your way to p and 2pq.
• Common Pitfalls: Don't mix up allele and genotype frequencies. Always double-check your calculations.
• FRQ Strategy: Clearly state your assumptions and show all your work. Even if you make a mistake, you might get partial credit for the correct process.

#Practice Questions

Okay, let's test your knowledge with some practice questions:

Practice Question

Multiple Choice Questions:

1. In a population of butterflies, the allele for black wings (B) is dominant over the allele for white wings (b). If 16% of the butterflies in a population are white, what is the frequency of the black wing allele? (A) 0.16 (B) 0.36 (C) 0.4 (D) 0.6

2. Which of the following conditions is NOT an assumption of the Hardy-Weinberg equilibrium? (A) No mutation (B) Random mating (C) Small population size (D) No gene flow

Free Response Question:

In a population of 500 pea plants, 320 have purple flowers (dominant), and 180 have white flowers (recessive). Assume that the population is in Hardy-Weinberg equilibrium.

(a) Calculate the frequencies of the dominant and recessive alleles. (b) Calculate the number of plants that are homozygous dominant, heterozygous, and homozygous recessive. (c) Explain how a change in the environment that favors white flowers would affect the allele frequencies in the next generation.

Scoring Breakdown:

(a) 3 points * 1 point for calculating q² correctly (180/500 = 0.36) * 1 point for calculating q correctly (√0.36 = 0.6) * 1 point for calculating p correctly (1 - 0.6 = 0.4)

(b) 3 points * 1 point for calculating p² correctly (0.4² = 0.16), and multiplying by 500 (80 plants) * 1 point for calculating 2pq correctly (2 * 0.4 * 0.6 = 0.48), and multiplying by 500 (240 plants) * 1 point for stating the number of homozygous recessive plants (180 plants)

(c) 3 points * 1 point for stating that the frequency of the recessive allele would increase * 1 point for stating that the frequency of the dominant allele would decrease * 1 point for explaining that natural selection is favoring the recessive allele

You've got this! Go ace that exam! 🎉