Allele frequency | Khan Academy

Understanding Allele Frequency

  • Allele: A variant of a gene inherited from both parents (one from mother, one from father).

Model for Allele Frequency

  • Simplification: Using eye color as an illustration for allele frequency.

  • Eye Color Alleles:

    • Capital B: Represents brown eye color (dominant allele).

    • Lowercase b: Represents blue eye color (recessive allele).

  • Dominance: If an individual has at least one capital B allele, they will display brown eyes.

    • Example: Genotype combinations that show brown eyes include (BB or Bb).

    • Genotype showing blue eyes is (bb).

Small Population Example

  • Artificial Population: Consider a population with 2 individuals: Person 1 and Person 2.

    • Genotypes:

      • Person 1: Genotype contains capital B (brown) and lowercase b (blue).

      • Person 2: Genotype consists of two lowercase b alleles (blue).

Calculating Allele Frequencies

  • Total Alleles: Each individual has 2 alleles (haploid alleles in diploid population).

    • Population total alleles: 4 (2 from each person).

  • Brown Allele Frequency Calculation:

    • Number of brown alleles (B): 1 (from Person 1 only).

    • Frequency of brown allele: 1 out of 4 = 0.25 or 25%.

  • Blue Allele Frequency Calculation:

    • Number of blue alleles (b): 3 (2 from Person 2 and 1 from Person 1).

    • Frequency of blue allele: 3 out of 4 = 0.75 or 75%.

Distinction from Phenotype Frequency

  • Phenotype Frequency: Refers to observable traits.

    • Brown-eyed individuals: 1 out of 2 = 50%.

    • Blue-eyed individuals: 1 out of 2 = 50%.

  • Allele Frequency vs. Phenotype Frequency:

    • Allele frequency gives insight into the genetic makeup; phenotype frequency indicates observed traits.

Hardy-Weinberg Principle

  • Internalizing Frequencies: Understanding allele frequencies p (dominant) and q (recessive) is key for exploring the Hardy-Weinberg principle.

  • Notation:

    • p = frequency of the dominant allele (capital B).

    • q = frequency of the recessive allele (lowercase b).

  • Relationship: The sum of p and q reflects the total frequencies:

    • p + q = 1 or 100%.

    • Example from previous calculations: 0.25 (B) + 0.75 (b) = 1 or 100%.

Conclusion and Next Steps

  • Preparation for Next Concept: Following this foundational understanding, the next video will delve deeper into the Hardy-Weinberg equation and its applications.