Population Genetics

Key Concepts and Vocabulary

Population genetics explores how allele frequencies change within populations due to various evolutionary forces. Understanding these changes is fundamental to interpreting genetic variation and evolution.

• Allele: A version of a gene (e.g., A or a).

• Genotype: The combination of alleles an individual possesses (e.g., AA, Aa, or aa).

• Allele frequency: The proportion of a specific allele in the population (e.g., p for A, q for a).

• Genotype frequency: The proportion of a specific genotype in the population.

• Fixation: When an allele reaches a frequency of 1.0 (100% of individuals carry it).

Allele frequency equation:

Hardy–Weinberg Equilibrium (HWE)

HWE is a null model describing allele and genotype frequencies in a population where no evolution is occurring. It predicts genotype frequencies based on allele frequencies:

HWE holds only if all five conditions are met:

• No mutation

• No gene flow (migration)

• No natural selection

• No genetic drift (infinitely large population)

• Random mating

If any condition is violated, allele frequencies change and evolution occurs.

The Four Evolutionary Forces

• Genetic Drift: Random changes in allele frequency, strongest in small populations. Can lead to fixation or loss of alleles by chance.

• Natural Selection: Differential survival and reproduction based on genotype fitness. Directional selection increases one allele's frequency; heterozygote advantage maintains both alleles.

• Mutation: The only source of new genetic variation. Mutation rates are low, but can counteract selection.

• Gene Flow (Migration): Movement of alleles between populations, making allele frequencies more similar across populations.

Reading Simulator Graphs

• x-axis: Generations

• y-axis: Frequency of allele A (p), from 0 to 1

• Colored lines: Replicate simulation runs

• If , allele A is fixed; if , allele A is lost

Example: In a small population (N = 20), genetic drift causes allele frequencies to fluctuate and may lead to fixation or loss. In a large population (N = 2000), frequencies remain more stable.

Diffusion and Osmosis

Membranes and Permeability

Cell membranes are selectively permeable, allowing water to pass freely while regulating solute movement. Transport can be active (requires energy) or passive (no energy required).

• Active transport: Movement requiring energy.

• Passive transport: Movement requiring no energy (diffusion, osmosis).

Diffusion

Diffusion is the net movement of molecules from high to low concentration, driven by the concentration gradient, until equilibrium is reached.

• Example: Dye dropped in water spreads until uniform.

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane. Water moves from low solute concentration (high water concentration) to high solute concentration (low water concentration).

• Key point: Osmosis refers only to water movement, not solute movement.

Tonicity

Tonicity describes the relative solute concentration of a solution compared to the cell's interior.

• Isotonic: Solute concentration outside equals inside; no net water movement; cells maintain normal shape.

• Hypotonic: Solute concentration outside is less than inside; water moves into the cell; animal cells swell and may burst (lysis); plant cells become turgid due to water entering the vacuole.

• Hypertonic: Solute concentration outside is greater than inside; water moves out of the cell; animal cells shrink (crenation); plant cells undergo plasmolysis (membrane pulls away from cell wall).

Plant Cells: Special Considerations

• Plant cells have a cell wall and central vacuole.

• Hypotonic solution: water enters vacuole, turgor pressure increases, cell becomes turgid (ideal state).

• Hypertonic solution: water leaves, plasmolysis occurs.

• Turgor pressure is essential for plant rigidity.

Dialysis Tubing Experiment

Dialysis tubing acts as a selectively permeable membrane. Glucose and starch were placed inside; tubing was immersed in I₂KI solution.

• Benedict's test: Detects glucose (positive = orange/red/brick color).

• IKI test: Detects starch (positive = blue-black color).

• Glucose and IKI are small and can cross the membrane; starch is large and cannot.

• Predict which compartment tests positive for each molecule after the experiment.

Osmotic Activity in Cells

• Red blood cells and plant cells (Elodea/red onion) were placed in solutions of different tonicities.

• Identify solution tonicity based on cell appearance under the microscope.

Example: A red blood cell in a hypotonic solution swells and may burst (lysis); a plant cell in a hypertonic solution undergoes plasmolysis.

Mitosis

Key Vocabulary

• Chromosome: Condensed DNA and histone proteins.

• Sister chromatids: Identical copies of a chromosome joined at the centromere.

• Centromere: Region connecting sister chromatids.

• Kinetochore: Protein complex at the centromere where spindle fibers attach.

• Spindle fibers: Microtubules that pull chromosomes apart.

• Centrioles: Organize the spindle in animal cells.

• Diploid (2n): Two copies of each chromosome (somatic cells).

• Haploid (n): One copy of each chromosome (gametes).

• Homologous chromosomes: Chromosome pairs with the same genes (one from each parent).

• Cytokinesis: Division of the cytoplasm following mitosis.

Purpose of Mitosis

• Growth and tissue repair in multicellular organisms.

• Asexual reproduction in single-celled eukaryotes.

• Produces two daughter cells genetically identical to the parent cell.

• Maintains diploid chromosome number ().

The Cell Cycle

• Interphase: G1 (cell growth), S (DNA replication), G2 (preparation for division).

• M Phase: Mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Stages of Mitosis (PMAT)

• Prophase: Chromosomes condense; spindle forms; nuclear envelope breaks down; centrioles move to poles.

• Prometaphase: Nuclear envelope fully breaks down; spindle fibers attach to kinetochores; chromosomes move toward the middle.

• Metaphase: Chromosomes align at the metaphase plate; spindle fibers from both poles attach.

• Anaphase: Sister chromatids are pulled apart to opposite poles; cell elongates.

• Telophase: Chromosomes arrive at poles; decondense; nuclear envelope re-forms; spindle breaks down; two nuclei present.

Cytokinesis

• Animal cells: Cleavage furrow pinches cell in two.

• Plant cells: Cell plate forms between daughter cells.

• Result: Two genetically identical daughter cells.

Modeling and Observing Mitosis

• Identify mitotic stages from diagrams or models.

• Know chromosome number and location at each stage.

• Onion root tip slides are ideal for observing mitosis due to actively dividing cells.

Human Chromosomes in Leukocytes

• Cytogeneticists treat white blood cells with chemicals to stop spindle formation, allowing chromosomes to condense but not separate.

• This enables chromosome counting and karyotyping.

• Recognize karyotype images and understand their purpose.

Example: At the end of mitosis and cytokinesis, a human somatic cell (2n = 46) produces two daughter cells, each with 46 chromosomes, genetically identical to the parent.

Practice Questions: Key Concepts

Population Genetics

• Genetic drift is strongest in small populations and can lead to allele fixation.

• Natural selection increases the frequency of advantageous alleles.

• Gene flow makes populations more similar genetically.

• Heterozygote advantage maintains both alleles at intermediate frequencies.

Diffusion and Osmosis

• Red blood cells in hypotonic solutions swell and may burst (lysis).

• Plant cells in hypertonic solutions undergo plasmolysis.

• Dialysis tubing experiment demonstrates selective permeability: small molecules (glucose, IKI) cross; large molecules (starch) do not.

• Osmosis is water movement from low solute to high solute concentration.

Mitosis

• Metaphase: Chromosomes align at the equator.

• Anaphase: Sister chromatids are pulled apart.

• S phase: DNA replication occurs.

• Telophase and cytokinesis: Chromosomes decondense, cell plate forms in plant cells.

• Kinetochore: Spindle fibers attach to chromosomes.

• After mitosis, daughter cells are genetically identical to the parent.

Summary Table: Tonicity Effects on Cells

Summary Table: Stages of Mitosis

How to Study

• Focus on interpreting lab results and understanding concepts.

• Connect evolutionary forces to patterns in allele frequency graphs.

• Identify cell tonicity and mitotic stages from images.

• Understand the difference between allele and genotype frequencies.

Additional info: Academic context was expanded for clarity and completeness, including definitions, examples, and summary tables.