Equilíbrio de Hardy-Weinberg - Aula 19 - Módulo II: Genética | Prof. Gui

Understanding Hardy-Weinberg Equilibrium

Overcoming Fear of Complex Concepts

• The speaker addresses the common fear students have regarding Hardy-Weinberg equilibrium, labeling it as a limiting belief.

• Emphasizes that if the same concepts were presented in a math class, they would seem simpler to students.

• Assures students that with practice, understanding will improve significantly over time.

Introduction to Hardy-Weinberg

• The instructor introduces himself as Guilherme and states the focus on Hardy-Weinberg equilibrium.

• Provides background on the two key figures: Hardy (a mathematician) and Weinberg (a gynecologist), who independently developed this concept.

• Clarifies that their independent work led to similar conclusions about allele frequency stability under certain conditions.

Key Postulates of Hardy-Weinberg

• States that populations tend to maintain constant allele frequencies under specific conditions.

• Addresses a common student question regarding polydactyly being dominant yet not prevalent in populations, linking it back to Hardy-Weinberg principles.

Conditions for Equilibrium

• Lists necessary conditions for maintaining allele frequency constancy:

• No mutations

• No migrations

• No strong natural selection pressures

• Minimal genetic drift

Population Dynamics

• Explains how significant changes like mass migration or natural disasters can disrupt allele frequencies.

• Introduces the concept of panmictic populations where random mating occurs without selective pressures.

Practical Application of Concepts

• Mentions creating an example population for practical calculations related to Hardy-Weinberg equilibrium.

Understanding Allele Frequencies and Hardy-Weinberg Equilibrium

Introduction to Allele Frequencies

• The dominant allele frequency is denoted as p and the recessive allele frequency as q. These are standard notations in genetics, where any letter can represent an allele.

Counting Alleles in a Population

• In a given population of 50 individuals, there are 20 dominant alleles (azão), leading to a frequency calculation of 20/50 = 0.4 or 40%.

• The remaining alleles must be recessive (azinho), calculated as 30/50 = 0.6 or 60%. The total frequencies must sum to 100%, confirming that p + q = 1 .

Hardy-Weinberg Principle

• Instead of using percentages, the Hardy-Weinberg equilibrium is expressed as p + q = 1 . This simplifies calculations by allowing the use of decimal values instead of percentages.

Distinguishing Between Allele and Genotype Frequencies

• It’s crucial to differentiate between allele frequencies (e.g., azão vs. azinho) and genotype frequencies (e.g., homozygous dominant vs. homozygous recessive).

Calculating Genotype Frequencies

• The frequency of homozygous dominant individuals (azão azão) is calculated as p^2 . Given p = 0.4 , this results in p^2 = 0.16 , or 16%.

• For homozygous recessive individuals (azinho azinho), the calculation follows as q^2 = (0.6)^2 = 0.36, equating to 36%.

Heterozygous Frequency Calculation

• The heterozygous genotype frequency (azão azinho or azinho azão) is represented by 2pq . With values for p and q, it calculates to:

• pq = (0.4)(0.6)=0.24

• Thus, multiplying by two gives a total of 48% for heterozygotes.

Application in Population Genetics Problems

• In practical problems, you may not always have direct access to p or q; sometimes you will need to derive them from given information about genotypes.

Example Problem: Population Analysis

• An example problem states that if 16% of a population consists of homozygous dominants, then:

• Calculate how many individuals this represents in a sample size of 25, resulting in four individuals being identified.

Final Count Verification

• If there are four homozygous dominants and nine homozygous recessives identified, the remainder must be heterozygotes:

• Totaling up leads us back to confirm that all counts align with expected proportions based on Hardy-Weinberg principles.

Understanding Allelic Frequencies and Heterozygosity

Introduction to Allelic Frequencies

• The discussion begins with the concept of allelic frequency, specifically focusing on a locus with an allele frequency of 0.36.

• The professor explains that if given either p or q (allele frequencies), one can derive the other using the equation p + q = 1 .

Calculating Allele Frequencies

• The calculation for q is established as q = 0.6 , leading to p = 1 - q , which results in p = 0.4 .

• The formula for calculating heterozygote frequency is introduced: 2pq . This accounts for both possible combinations of alleles.

Determining Heterozygous Individuals

• Using the previously calculated values, the heterozygote frequency is determined to be approximately 48%.

• To find the number of heterozygous females in a population of 10,000 individuals, it’s noted that only half are female, necessitating multiplication by 0.5.

Final Calculation and Conclusion

• After performing calculations, it’s concluded that there are approximately 2400 heterozygous females in this population.