Genetics, Evolution, and Heritability Notes

Chromosomes and DNA

  • Humans have 46 chromosomes (23 pairs) in the nucleus of cells, including neurons.

  • Sperm and egg cells contain 23 unpaired chromosomes.

  • The 23rd pair consists of sex chromosomes (XX or XY), while the other 22 pairs are autosomes (non-sex-specific).

  • Chromosomes comprise long strands of Deoxyribonucleic Acid (DNA).

Structure of DNA

  • DNA consists of 4 different bases arranged in pairs: Guanine (G), Cytosine (C), Thymine (T), and Adenine (A).

  • A gene is a “short” segment of DNA with a particular sequence of bases.

  • Human genes range from a few hundred to millions of bases in size; their number varies from hundreds to thousands per chromosome.

Genes and Protein Synthesis

  • Gene activation involves transcription: DNA is copied to mRNA.

  • The mRNA undergoes translation to form a chain of amino acids, which then folds into a protein.

  • The sequence of amino acids in a chain determines the protein's identity and function.

Role of Proteins

  • Proteins are essential within all body cells, including neurons, where they contribute to structure and functioning.

  • The functioning of cells, influenced by proteins, is critical for organ activity (like the brain) and physical characteristics (e.g., hair color).

  • Genes influence traits through protein production, affecting both physical and behavioral characteristics.

Genotype vs. Phenotype

  • Genotype: the complete set of genes in an individual.

  • Phenotype: the observable traits influenced by genotype and environment.

  • Key points:

    1. One gene in a pair may be expressed (dominant), while the other is not (recessive).

    2. Many traits are influenced by multiple gene pairs.

    3. The environment plays a role in gene expression, known as epigenetics.

Epigenetics Overview

  • Nature vs. Nurture:

    • Nature: genes influence behavior.

    • Nurture: behavior can influence gene expression.

Evolution and Natural Selection

  • Evolution involves changes due to:

    • Mutations during DNA copying.

    • Environmental factors and experiences.

  • It refers to the gradual change of inherited traits over generations.

Natural Selection Principle

  • Proposed by Charles Darwin and Alfred Wallace in the 1800s:

    • Traits that increase survival (finding food, avoiding predators) lead to greater reproductive success.

    • Adaptive traits become more common in the population over time.

      • tend to survive longer

      • reproduce in greater numbers

Factors Influencing Natural Selection

  • Trait adaptiveness depends on environmental conditions.

  • Variation within a species is crucial for adaptation and survival.

Sexual Selection

  • Additional concept by Darwin:

    • Intersexual selection: choice by the opposite sex for mating.

    • Intrasexual selection: competition among the same sex for mates.

Evolutionary Psychology

  • Evolutionary biologists study physical traits and their evolutionary causes (e.g., beak shape in finches).

  • Evolutionary psychologists examine behavioral traits and their evolutionary significance (e.g., taste preference).

  • Not all traits are adaptive; persistence does not imply current utility.

Understanding Heritability

  • Heritability: the proportion of variance in a trait that is attributable to genetic differences among individuals within a group

    • Maximum value is 1.0 (i.e., 100% genetic influence).

    • Some traits exhibit high heritability; others show low heritability.

Limitations of Heritability

  1. Heritability is not a fixed value; it can change.

  2. Highly heritable traits can still be influenced by environmental factors.

  3. Heritability estimates apply only to group variations, not individuals.

Approaches to Estimate Heritability

  1. Family Studies: Examines genetic relatedness within families, considering shared environments; environment is often shared too, making it difficult to disentangle genetic and environmental influences on traits.

  2. Twin Studies: Compares identical twins (100% shared DNA) and fraternal twins (50% shared DNA) to evaluate genetic influence.

    • Greater similarity in identical twins suggests a genetic basis for traits.

  3. Adoption Studies: Analyzes similarities between adopted children and biological versus adoptive relatives to estimate genetic influence.

    • Studies of separated identical twins offer insights into genetic versus environmental contributions.

Heritability of Intelligence

  • Intelligence Quotient (IQ) reflects performance on intelligence tests.

  • IQ heritability scores for adults can reach 0.80, indicating strong genetic influence.

  • Environmental Factors:

    • Hindrances: poor prenatal care, malnutrition, toxins, stress.

    • Enhancements: good health care, nutrition, mental stimulation.