Ch 4: Beyond Mendelian Genetics

Alleles and Their Interactions

This topic explores the complexity of genetic inheritance beyond simple Mendelian patterns, focusing on allele types, their molecular basis, and how they interact to produce diverse phenotypes.

• Wild Type vs. Mutant Alleles: The wild type allele is the most common form found in a population, while mutant alleles arise from changes in DNA sequence.

• Molecular Basis of Dominance: Dominant alleles often produce functional proteins, while recessive alleles may result from loss-of-function mutations.

• Genotype-Phenotype Relationships: The same genotype can yield different phenotypes due to environmental effects, genetic background, or epigenetic factors.

• Allele Interactions:

  • Complete Dominance: One allele masks the effect of another.

  • Incomplete Dominance: Heterozygotes show an intermediate phenotype.

  • Codominance: Both alleles are fully expressed (e.g., AB blood type).

• Blood Type Inheritance: The ABO blood group system is an example of codominance and multiple alleles.

• Sex-Linked, Sex-Influenced, and Sex-Limited Traits:

  • Sex-linked: Traits determined by genes on sex chromosomes (e.g., X-linked color blindness).

  • Sex-influenced: Expression differs between sexes (e.g., pattern baldness).

  • Sex-limited: Trait appears only in one sex (e.g., milk production in cows).

• Recessive Lethal Alleles: Certain alleles cause death when homozygous, altering expected progeny ratios.

• Pleiotropy: One gene affects multiple traits (e.g., sickle cell anemia).

• Single vs. Multiple Gene Control: Cross results can indicate whether a trait is monogenic or polygenic.

• Gene Interactions:

  • Complementation: Two genes restore a phenotype when combined.

  • Epistasis: One gene masks the effect of another.

  • Redundancy: Multiple genes perform the same function.

• Extranuclear Inheritance: Traits inherited via organelles (mitochondria, chloroplasts) rather than the nucleus.

Example: The inheritance of coat color in mice demonstrates recessive lethal alleles, where homozygous yellow is lethal.

Ch 7: Linkage and Chromosome Mapping

Gene Linkage and Recombination

This section covers how genes located close together on chromosomes are inherited together, the process of recombination, and how genetic maps are constructed.

• Linkage: Genes located near each other on the same chromosome tend to be inherited together.

• Parental vs. Recombinant Gametes: Parental gametes retain original allele combinations; recombinant gametes result from crossing over.

• Detecting Linkage: Deviations from Mendelian ratios in crosses suggest linkage.

• Testcrosses: Used to determine linkage by analyzing offspring ratios.

• Physical Basis of Recombination: Crossing over occurs during meiosis, exchanging genetic material between homologous chromosomes.

• Recombination Frequency: Proportional to the distance between loci; measured as percentage of recombinant offspring.

• Slash Notation: Used to indicate linked alleles (e.g., A/B).

• Maximum Recombination Frequency: Limited to 50%, indicating independent assortment.

• Calculating Map Distance:

  • Map distance (in centimorgans, cM) =

• Genetic Maps: Constructed by ordering genes based on recombination frequencies.

• Linkage Groups: Each chromosome represents a linkage group.

• Mitotic Recombination: Rare, but can result in genetic mosaicism.

Example: In Drosophila, genes for body color and wing shape are linked, producing more parental-type offspring than recombinants.

Ch 10: DNA Replication

Mechanisms and Features of DNA Replication

This topic explains the structure of DNA, the process of its replication, and the differences between prokaryotic and eukaryotic systems.

• Chemical Components of DNA: DNA is composed of nucleotides, each containing a phosphate group, deoxyribose sugar, and a nitrogenous base (A, T, G, C).

• DNA Structure: Double helix with antiparallel strands.

• Semiconservative Replication: Each new DNA molecule contains one old and one new strand.

• Meselson-Stahl Experiment: Used heavy nitrogen () to demonstrate semiconservative replication.

• DNA Polymerase Requirements: Needs a template, primer, and nucleotides.

• Steps in DNA Replication:

  1. Initiation: Origin recognition and unwinding.

  2. Elongation: DNA polymerase adds nucleotides.

  3. Termination: Replication ends at specific sites.

• DNA Polymerase Features: Proofreading, high fidelity, and processivity.

• Prokaryotic vs. Eukaryotic Replication: Prokaryotes have a single origin; eukaryotes have multiple origins and more complex machinery.

• Replication of Chromosome Ends: Telomerase extends telomeres in eukaryotes.

Example: DNA polymerase III in E. coli synthesizes new DNA during replication.

Ch 12: Transcription

Transcription and Its Regulation

This section covers the process by which DNA is transcribed into RNA, the enzymes involved, and differences between prokaryotic and eukaryotic transcription.

• Role in Central Dogma: Transcription is the first step in gene expression, converting DNA to RNA.

• Prokaryotic Transcription: RNA polymerase core enzyme synthesizes RNA; sigma factor initiates transcription; rho factor terminates it.

• Stages of Transcription:

  1. Initiation: RNA polymerase binds to promoter.

  2. Elongation: RNA strand is synthesized.

  3. Termination: RNA polymerase releases the completed RNA.

• Promoters: Prokaryotic promoters contain -10 and -35 elements; eukaryotic promoters have TATA box and other regulatory elements.

• Cis vs. Trans Acting Factors: Cis factors are DNA sequences; trans factors are proteins or RNAs that bind to DNA.

• Transcription Initiation Differences: Eukaryotes require multiple transcription factors; prokaryotes use sigma factor.

• RNA Polymerase Function: Catalyzes RNA synthesis, requires template and nucleotides.

• Predicting mRNA Sequence: mRNA is complementary to the DNA template strand.

• Polycistronic mRNA: In prokaryotes, one mRNA can encode multiple proteins.

• Pre-mRNA Processing in Eukaryotes:

  • 5' capping

  • 3' polyadenylation

  • Splicing

  Importance: Stability, export, and translation efficiency.

• Splicing: Spliceosome removes introns; alternative splicing generates multiple mRNAs from one gene.

Example: The human beta-globin gene undergoes alternative splicing to produce different mRNA isoforms.