Behavioral Genetics Lecture Review

Behavioral Genetics Overview

What is Behavioral Genetics?

  • Behavioral genetics is a field that explores the interplay between genetic and environmental influences on behavioral traits.

Calculating Allele Frequencies

Key Concepts

  • In many human autosomal recessive traits, the heterozygote cannot be distinguished from the normal homozygote.

  • When indistinguishability occurs, the Hardy-Weinberg equilibrium is assumed to apply.

Hardy-Weinberg Principle

General Description

  • The Hardy-Weinberg principle explains a population that is not evolving.

    • It is expressed as

    • Gamete + Gamete = New genotype in progeny

    • It maintains constancy in allele frequencies and genotype frequencies across generations.

Conditions for Hardy-Weinberg Equilibrium

  • The following conditions must be met:

    1. No mutations.

    2. Random mating.

    3. No natural selection.

    4. Extremely large population size.

    5. No gene flow.

  • These conditions are rarely fully met in nature but the principle provides a good approximation of allelic frequencies when the fourth condition is met.

Equation of Allelic Frequencies

  • The Hardy-Weinberg equation is:

    • p^2 + 2pq + q^2 = 1

    • Where:

    • p = frequency of one allele (e.g., dominant)

    • q = frequency of the other allele (e.g., recessive)

Gene Pools and Allele Frequencies

Definitions

  • A population is a localized group of interbreeding individuals capable of producing fertile offspring.

  • A gene pool consists of all the alleles for all loci in a population.

Calculating Allele Frequencies

  • For diploid organisms:

    • Total number of alleles at a locus = Total number of individuals x 2.

  • Convention used:

    • If there are 2 alleles at a locus, p and q represent their respective frequencies.

    • p + q = 1.

Example of Allele Frequencies

  • Consider the following example of allele frequencies:

    • If we have 80% chance of one allele and 20% chance of another:

    • Frequency of CR allele, p = 0.8

    • Frequency of CW allele, q = 0.2

Hardy-Weinberg Equilibrium in Practice

Relationship to Population Genetics

  • The Hardy-Weinberg principle applies in populations where:

    • Gametes contribute to the next generation randomly.

    • Allele frequencies remain constant over generations.

  • Mendelian inheritance preserves genetic variation in populations.

Phenotype Frequency Calculation

  • Corresponding frequencies for genotypes:

    • p^2 : Homozygous Dominant

    • 2pq : Heterozygous

    • q^2 : Homozygous Recessive

    • Total formula for phenotype frequencies:

    • 1 = p^2 + 2pq + q^2

Case Study: Carrier Frequency for PKU

PKU Carrier Detection

  • Understanding how to determine the carrier frequency for PKU (Phenylketonuria):

    • Individuals can be identified using a Punnett square to explore genotype combinations:

    • Individual genotype possibilities: pp (affected), qq (normal), pq (carriers).

Population-Level Calculation of Carrier Frequency

  • If the frequency of affected individuals for an autosomal recessive disorder is 1/10000:

    1. Assume total population is 10000 individuals; thus, the affected frequency (q²):

    • q^2 = 0.0001

    1. Calculate q:

    • q=\sqrt{\left(0.0001\right)}=0.01

    1. Calculate p:

    • p = 1 - q = 1 - 0.01 = 0.99

    1. Calculate carrier frequency (2pq):

    • 2pq=2\cdot0.99\cdot0.01=0.0198=0.02

Hardy-Weinberg Theorem Summary

Theoretical Implications

  • The Hardy-Weinberg theorem describes a hypothetical population model.

  • In real populations, allele and genotype frequencies change over time, but the theorem provides a good approximation for large populations.

Behavioral Genetics and Evolution

Evolution of Behavior

  • Behavioral genetics explores how genetic variation leads to behavioral variation that can be subject to selection.

    • Example: Social behaviors can have both short-term and long-term effects. Defining Behavior

  • Behavior is described as a response to a stimulus that can be internal or external.

  • Responses are the coordinated actions or inactions of organisms to stimuli, excluding developmental changes.

Conceptualization of Behavior

  • The definition of behavior remains debated among behavioral biologists:

    • “Behavior is the internally coordinated responses (actions or inactions) of whole living organisms (individuals or groups) to internal and/or external stimuli.” (Levitis, D.A., Lidicker, W.Z. & Freund, G. 2009)

  • Changes in life processes like growth or death are not classified as behavior.

Is Behavior a Phenotype?

Genetic and Environmental Influences on Behavior

  • A behavior that is inherited can be classified as a phenotype.

  • A phenotype is the observable trait resulting from a genotype, with behaviors being inherently polygenic—influenced by many genes.

  • However, behaviors can also be impacted by environmental factors, making them multifactorial phenotypes.

  • Not every behavior is affected by the environment consistently:

    • Examples: The white gene or dunce gene in Drosophila and the Huntingtin gene in Huntington's disease in humans.

    • Not every type of behavior equates to one gene leading to one behavior outcome.

Key Points to Remember

  • Familiarize yourself with the Hardy-Weinberg equilibrium and its limitations, specifically regarding non-evolving populations.

  • Learn how to calculate phenotype frequency given allele frequency, as well as allele frequency when the phenotype frequency is known.

  • Be prepared to define behavior succinctly.