Chapter 2 - Mitosis and Meiosis

Haploid vs. Diploid Chromosome Number

Understanding chromosome number is fundamental to genetics. Diploid (2n) cells contain two homologous copies of each chromosome, while haploid (n) cells contain one chromosome from each homologous pair.

• Diploid (2n): Two sets of chromosomes (e.g., human somatic cells: 2n = 46).

• Haploid (n): One set of chromosomes (e.g., human gametes: n = 23).

• Homologous chromosomes: Carry the same genes at the same loci but may have different alleles.

• Chromosome counting: Chromosome number is determined by centromeres, not chromatids.

Example: In a species with 2n = 12, gametes have n = 6 chromosomes; somatic cells have 6 homologous pairs.

DNA Content During Interphase

DNA replication occurs during S phase, doubling the DNA content but not the chromosome number.

• G1 phase: 2n chromosomes, DNA content = 2C.

• After S/G2: 2n chromosomes, DNA content = 4C.

• After meiosis I: n chromosomes, DNA content = 2C.

• After meiosis II: n chromosomes, DNA content = 1C.

Example: A diploid cell with 10 chromosomes after S phase has 10 chromosomes and 20 chromatids.

Cell Cycle Checkpoints

Checkpoints ensure proper cell cycle progression and prevent errors.

• G1/S checkpoint: Checks conditions and DNA suitability for replication.

• G2/M checkpoint: Ensures replication is complete and DNA is intact.

• Spindle/metaphase checkpoint: Prevents anaphase until chromosomes are properly attached to spindle fibers.

• Cyclins and CDKs: Regulate cell cycle progression.

Example: The spindle checkpoint stops the cell if a chromosome is not attached to spindle fibers at metaphase.

Chromosome Separation During Mitosis

Mitosis ensures equal distribution of chromosomes to daughter cells.

• Prophase: Chromosomes condense.

• Prometaphase: Spindle attaches to kinetochores.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate.

• Telophase/Cytokinesis: Nuclei reform and cell divides.

Key distinction: Mitosis separates sister chromatids; meiosis I separates homologous chromosomes.

Meiosis: Gametes, Reductional Division, DNA Complements

Meiosis produces four haploid gametes from one diploid cell, increasing genetic diversity.

• Meiosis I: Reductional division; homologs separate.

• Meiosis II: Equational division; sister chromatids separate.

• Prophase I: Homologs synapse, forming tetrads; crossing over occurs.

• Genetic variation: Independent assortment and crossing over.

Example: A cell with 2n = 8 after meiosis I has n = 4 chromosomes (still duplicated); after meiosis II, gametes have 4 unduplicated chromosomes.

Chapter 3 - Mendelian Genetics

Law of Segregation

Each diploid individual has two alleles for each gene; these alleles segregate during gamete formation.

• Segregation: Each gamete receives only one allele.

• Example: Aa produces 1/2 A and 1/2 a gametes.

Monohybrid Cross Ratio

Crosses between heterozygotes (Aa × Aa) yield predictable genotype and phenotype ratios.

• Genotype ratio: 1 AA : 2 Aa : 1 aa.

• Phenotype ratio: 3 dominant : 1 recessive (for complete dominance).

Example: Probability of recessive phenotype (aa) is 1/4.

Testcross Logic

A testcross reveals the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive.

• Result: Presence of recessive offspring indicates heterozygosity.

Example: Pp × pp yields a 1:1 ratio of purple:white, indicating Pp.

Independent Assortment

Alleles of different genes assort independently if the genes are unlinked.

• Four gamete types: RY, Ry, rY, ry (for RrYy).

• Law: Independent assortment explains equal gamete frequencies.

Dihybrid Cross Ratio

Dihybrid crosses (RrYy × RrYy) produce a classic 9:3:3:1 phenotype ratio under independent assortment and complete dominance.

• Phenotype ratio: 9 R_Y_, 3 R_yy, 3 rrY_, 1 rryy.

Example: Probability of rrY_ = (1/4)(3/4) = .

Product Rule

The product rule calculates the probability of independent events occurring together.

• Formula:

• Application: Useful for multi-gene crosses.

Example: Probability of aabb in AaBb × AaBb is .

Number of Gametes

The number of distinct gamete types depends on the number of heterozygous loci.

• Formula: where n = number of heterozygous gene pairs.

Example: AaBBCcDd produces gamete types.

Degrees of Freedom in Chi-Square

Chi-square tests compare observed and expected genetic ratios.

• Formula:

• Degrees of freedom: Number of categories minus one.

Example: Four phenotype categories: df = 4 - 1 = 3.

Chapter 4 - Extensions of Mendelian Genetics

Mutation Types and Neutral Mutations

Mutations can alter gene function in various ways.

• Loss-of-function: Reduces or eliminates gene product activity.

• Gain-of-function: Increases or creates new gene activity.

• Neutral mutation: No detectable effect on phenotype or fitness.

• Wild-type allele: Most common in nature, not necessarily dominant.

Dominance Relationships: Complete, Incomplete, Codominance

Dominance patterns describe heterozygote phenotypes.

• Complete dominance: Heterozygote resembles one homozygote.

• Incomplete dominance: Heterozygote has intermediate phenotype; F2 ratio often 1:2:1.

• Codominance: Both alleles are distinctly expressed (e.g., ABO blood type).

Example: Pink snapdragons from red × white show incomplete dominance; pink × pink yields 1 red : 2 pink : 1 white.

Epistasis / Gene Interaction

Epistasis occurs when one gene masks or modifies the effect of another gene.

• Interaction: Between loci, not alleles at the same locus.

• Effect: Modifies expected dihybrid ratios.

Example: Dihybrid cross with epistasis may not show 9:3:3:1 phenotypes.

Heterogeneous Traits

Genetically heterogeneous traits can result from mutations in different genes.

• Example: Hereditary deafness can arise from mutations in multiple genes.

X-Linked Inheritance

X-linked genes are located on the X chromosome; inheritance patterns differ between sexes.

• Hemizygosity: Individuals with one X chromosome express recessive alleles directly.

• Transmission: Fathers transmit X to daughters, Y to sons; no father-to-son X-linked transmission.

• Clues: More affected males; carrier mothers can have affected sons.

Example: Carrier mother (XNXn) and unaffected father (XNY): 1/2 of sons expected to be affected.

Non-Mendelian Inheritance Patterns

Phenotypes may not follow simple dominant/recessive expectations due to penetrance, expressivity, genetic background, and environmental effects.

• Penetrance: Proportion of individuals expressing a genotype.

• Expressivity: Degree/intensity of phenotype expression.

• Genetic background: Other genes modify phenotype.

• Environment: Can alter genotype expression.

Example: Incomplete penetrance: 8/10 individuals with genotype show phenotype.

Chapter 6 - Bacterial and Bacteriophage Genetics

Spontaneous Mutation

Spontaneous mutations arise without specific selective agents; selection acts on pre-existing variants.

• Example: Antibiotic resistance can pre-exist before exposure; selection increases frequency.

Horizontal Gene Transfer: Conjugation, Transformation, Transduction

Bacteria acquire DNA through three main mechanisms.

• Conjugation: Cell-to-cell contact; F+ or Hfr donor transfers DNA.

• Transformation: Uptake of naked extracellular DNA; recombination required for stable change.

• Transduction: Bacteriophage-mediated DNA transfer.

Hfr transfer: F factor integrated; gene order determined by interrupted mating.

Example: Hfr experiment: gene A at 5 min, B at 11 min, C at 18 min; order A-B-C, spacing A-B = 6 min, B-C = 7 min.

Lysogenic vs. Lytic Cycles

Bacteriophages can follow two life cycles.

• Lytic cycle: Phage replicates, cell lyses, new phages released.

• Lysogenic cycle: Phage DNA integrates as prophage, replicated with host.

• Temperate phage: Can switch between cycles; virulent phage only lytic.

Example: Integrated phage DNA copied with host, no new virions: lysogeny.

Bacteriophage Genetics

Phage genetics uses plaques and recombination to map mutations.

• Plaques: Clear areas from phage lysis.

• Recombinant frequency: Estimates distance between mutations.

• Transduction: Generalized transduction transfers bacterial DNA; cotransduction indicates close linkage.

Complementation and Cistrons

Complementation tests determine if mutations are in the same gene (cistron).

• Complementation: Two mutants restore wild-type function together: different genes.

• Failure to complement: Mutations in the same gene.

• Cistron: Functional genetic unit defined by complementation.

Example: Two phage mutants complement: mutations in different cistrons.

Chapter 8 - Chromosomal Mutations: Number and Arrangement

Aneuploidy

Aneuploidy is the gain or loss of individual chromosomes, not whole sets.

• Monosomy: 2n - 1 chromosomes.

• Trisomy: 2n + 1 chromosomes.

Example: Trisomic individual in 2n = 20 species has 21 chromosomes.

Nondisjunction

Nondisjunction is the failure of chromosomes to separate properly during meiosis.

• Meiosis I: Homologs fail to separate; all gametes abnormal.

• Meiosis II: Sister chromatids fail to separate; 50% normal gametes.

Example: Two normal, one n+1, one n-1 gamete: nondisjunction in meiosis II.

Down, Turner, and Klinefelter Syndromes

Common human aneuploidies involve autosomes and sex chromosomes.

Example: 47 chromosomes with three copies of chromosome 21: Down syndrome.

Polyploidy

Polyploidy involves more than two complete sets of chromosomes.

• Triploid: 3n; Tetraploid: 4n.

• Autopolyploidy: Multiple sets from same species.

• Allopolyploidy: Sets from different species.

• Odd ploidy: Reduced fertility due to pairing problems.

Example: Four complete sets from same species: autotetraploid (4n).

Chromosomal Rearrangements and Unequal Crossing Over

Chromosome rearrangements alter gene order and DNA content.

• Deletion: Segment lost.

• Duplication: Segment repeated.

• Inversion: Segment reversed.

• Translocation: Segment moved to another chromosome.

• Unequal crossing over: Misaligned homologs produce duplication and deletion.

Example: ABCBCDEF and ADEF from crossover: unequal crossing over.

Translocations

Reciprocal translocations exchange segments between nonhomologous chromosomes.

• Balanced carrier: Phenotypically normal, but meiosis can produce unbalanced gametes.

• Robertsonian translocation: Fusion of long arms of acrocentric chromosomes; can cause familial Down syndrome.

Example: Balanced carrier may have reproductive problems due to unbalanced gametes.

Evolutionary Importance of Inversions

Inversions can suppress recombination, preserving adaptive allele combinations.

• Paracentric: Excludes centromere.

• Pericentric: Includes centromere.

• Inversion loop: Forms during pairing in heterozygotes.

• Suppression of recombination: Maintains linked alleles.

Example: Inversion reduces recombination, preserving adaptive allele combinations.

Rapid Review - High-Yield Distinctions

• DNA doubles but chromosome number does not: S phase / sister chromatid formation.

• Homologs separate: Meiosis I.

• Sister chromatids separate: Mitosis or meiosis II.

• 3:1 phenotype ratio: Monohybrid complete dominance.

• 9:3:3:1: Dihybrid, independent assortment, complete dominance.

• 1:2:1 phenotype ratio: Often incomplete dominance or codominance.

• Cross to homozygous recessive: Testcross.

• AND probability: Product rule.

• Same phenotype from different genes: Genetic heterogeneity.

• One gene masks another: Epistasis.

• No father-to-son transmission: Possible X-linked inheritance.

• Cell-to-cell DNA transfer: Conjugation.

• Naked extracellular DNA: Transformation.

• Phage-mediated bacterial DNA: Transduction.

• Integrated phage DNA: Prophage / lysogeny.

• Mutants restore wild type together: Complementation; different cistrons.

• 2n + 1: Trisomy / aneuploidy.

• All gametes abnormal after nondisjunction: Meiosis I nondisjunction.

• Two normal + two abnormal gametes: Meiosis II nondisjunction.

• Whole extra chromosome sets: Polyploidy.

• Misalignment + crossover: Unequal crossing over.

• Reversed segment: Inversion.

• Exchange between nonhomologous chromosomes: Reciprocal translocation.

Final Checklist for Exam Preparation

• Track chromosome number vs. chromatid/DNA amount through S phase, mitosis, meiosis I, and meiosis II.

• Solve monohybrid/dihybrid probabilities using product rule.

• Distinguish segregation from independent assortment.

• Recognize dominance patterns, epistasis, penetrance, expressivity, and genetic heterogeneity.

• Identify X-linked inheritance clues.

• Distinguish conjugation, transformation, and transduction.

• Explain Hfr interrupted mating and gene order determination.

• Describe lytic vs. lysogenic cycles and prophage state.

• Use complementation to determine same vs. different genes.

• Distinguish aneuploidy from polyploidy and meiosis I vs. II nondisjunction.

• Identify chromosomal rearrangements from gene-order diagrams.