Genetics, Evolution, and Heritability

Genetics, Evolution, & Heritability

  • Unit 4 Overview

Introduction

  • This unit covers the intersection of genetics, evolution, heritability, and behavior.

Outline

  • Genes, Proteins, and Behaviour

  • Evolution by Natural Selection

  • Evolutionary Psychology

  • Heritability and the Interplay between Nature and Nurture

  • Take Home Message

Chromosomes and DNA

  • Humans: 46 chromosomes arranged in 23 pairs found in the nucleus of cells (including neurons).

    • Sperm and Egg Cells: Contain 23 unpaired chromosomes.

    • Sex Chromosomes: 23rd pair (XX or XY) determines sex; the remaining 22 pairs are autosomes.

  • Structure: Chromosomes are made of long strands of Deoxyribonucleic Acid (DNA).

Deoxyribonucleic Acid (DNA) and Genes

  • Composition of DNA: Made of 4 bases: Guanine (G), Cytosine (C), Thymine (T), and Adenine (A).

  • Genes: Short segments of DNA with specific sequences of bases.

    • Vary in size from a few hundred bases to millions.

    • Number of genes per chromosome ranges from hundreds to thousands.

Genes and Proteins

  • Process of Gene Activation:

    • Transcription: DNA is copied to mRNA.

    • Translation: mRNA is used to create a chain of amino acids, forming proteins.

  • Function of Proteins: The specific order of amino acids determines the type of protein formed.

Proteins, Cells, and Behavior

  • Role of Proteins: Critical in the structure and function of all cells, including neurons.

    • Underlies organ function and physical traits (e.g., hair color).

  • Influence on Behavior and Traits: Genes influence traits through protein creation.

Genotype vs. Phenotype

  • Genotype: All genes an individual possesses.

  • Phenotype: Observable physical and behavioral traits.

    • Distinction:

      1. Dominant vs. Recessive Traits: in a gene pair, one is usually expressed.

      2. Polygenic Influence: most human traits are influenced by multiple gene pairs.

      3. Environmental Impact: Experience and environment affect gene expression (epigenetics).

Epigenetics

  • Nature vs. Nurture: Genes influence behavior while environment affects gene expression.

Evolution

  • Mechanisms: Changes through mutations in DNA, errors during copying, and environmental influences.

  • Definition: Gradual change in inherited traits of a population over generations.

Evolution by Natural Selection

  • Founders: Developed by Charles Darwin and Alfred Wallace in the 1800s.

  • Adaptive Traits: Traits that enhance survival and reproduction become more common in a population.

Principles of Natural Selection

  • Trait Adaptability: Adaptation depends on the environment's nature and experiences.

  • Importance of Variation: Variation within a species helps it adapt to environmental changes.

  • Population Changes: Over generations, common traits will shift based on survival benefits.

Concept of Sexual Selection

  • Intersexual Selection: One sex selects mates based on traits.

  • Intrasexual Selection: Competition among the same sex for mating opportunities.

Evolutionary Biologists and Psychologists

  • Focus Areas:

    • Biologists: Physical traits and evolutionary factors (e.g., beak shape in finches).

    • Psychologists: Behavioral traits and evolutionary explanations (e.g., taste preferences).

Not All Traits are Adaptive

  • Existence of Traits: Not all existing traits are adaptive; some may persist without adaptive benefits.

Heritability Defined

  • Concept: Proportion of variance in a trait attributable to genetic differences within a group.

    • Maximum value: 1.0 (100% genetic).

    • Variation in heritability for different traits.

Heritability Example

  • Environmental Influence: Heritability can be influenced by environmental factors; illustrated with the example of uniform nutrient solutions affecting groups.

Misconceptions about Heritability

  1. Not a Fixed Number: It varies.

  2. Environmental Modification: Even highly heritable traits can be influenced by the environment.

  3. Group Specific: Estimates apply only to variations within specific groups and contexts.

Studying Heritability

  • Approaches to Estimate:

    1. Family Studies: Assessing genetic relatedness.

    2. Twin Studies: Comparing identical vs. fraternal twins.

    3. Adoption Studies: Comparing traits of adopted children with biological and adoptive relatives.

Family Studies

  • Genetic Relatedness: Known levels of shared DNA among family members.

  • Shared Environment: Environmental factors complicate disentangling genetic influence from shared environments.

Twin Studies

  • Comparison Method: Estimate heritability by comparing identical twins (100% shared DNA) to fraternal twins (50% shared DNA).

  • Increased Similarity = Genetic Influence: More similarity in identical twins suggests genetics play a larger role.

Adoption Studies

  • Trait Comparison: Observing adopted children helps compare traits with biological vs. adoptive relatives.

  • Identical Twins Raised Apart: Similar traits are indicative of genetic influences due to differing environments.

Example: Heritability of Intelligence

  • IQ Scores: Correlate performance on IQ tests against peers of similar age.

    • Heritability Estimates: Higher for intelligence, with scores for identical twins showing significant correlation regardless of upbringing.

Environmental Factors Affecting IQ Scores

  • Hindering Experiences: Poor prenatal care, malnutrition, toxins, stress.

  • Helping Experiences: Good health care, nutrition, and mental enrichment.

Take Home Message

  • Nature and Nurture Interaction: Both genetic factors and environmental influences shape who we are as individuals and affect species evolution.