Genetics Final Exam Study Guide: Gene Regulation, DNA Technology, and Quantitative Genetics

Gene Regulation in Prokaryotes

The lac Operon

The lac operon in Escherichia coli is a classic model for understanding gene regulation in prokaryotes. It controls the metabolism of lactose and is regulated by several genetic elements and proteins.

• Regulator Gene Product: The product of the regulator gene (I gene) is the repressor protein, which binds to the operator to inhibit transcription.

• Inducer: Lactose (specifically, allolactose) acts as the inducer by binding to the repressor and preventing it from binding to the operator, thus allowing transcription.

• Partial Diploid Analysis: In merozygotes (partial diploids) with two different alleles at the z locus, transcription depends on the presence of lactose and the functionality of the operator and repressor alleles.

• Mutation Types: Operator-constitutive mutations (oc) are cis-dominant and can also be described as trans-acting in some contexts.

• Catabolite Repression: This process involves cyclic-AMP (cAMP) levels and ensures that E. coli preferentially metabolizes glucose over lactose.

Example: If lactose is present, the repressor is inactivated, and the structural genes are transcribed. If a mutation makes the operator constitutive, transcription occurs regardless of lactose presence.

DNA Technology and Molecular Genetics

Restriction Enzymes and Cloning Vectors

Restriction enzymes and cloning vectors are essential tools in recombinant DNA technology.

• Type II Restriction Endonucleases: Recognize specific palindromic DNA sequences such as GAATTC (EcoRI) and GGATCC (BamHI).

• Chimeric Plasmids: Plasmids containing foreign DNA inserts are called chimeric plasmids or hybrid vectors.

• Selectable Markers: Common loci for selection include EcoRI sites, ampr (ampicillin resistance), and terr (tetracycline resistance).

• DNA Cloning Techniques: Blunt-ended DNA can be cloned using linkers, blunt-end ligation, or the poly-dA/poly-dT technique.

• Key Enzyme: T4 DNA ligase is crucial for ligating blunt-ended DNA fragments.

• cDNA: Complementary DNA (cDNA) is synthesized from mRNA and used in cloning eukaryotic genes.

• Blotting Techniques: Southern blotting is used to transfer and detect specific DNA fragments after electrophoresis.

• Expression Vectors: Plasmids designed for transcription and translation of cloned genes are called expression vectors.

• Chromosome Walking: A technique for studying long DNA sequences by using overlapping clones.

• Yeast Plasmids: For prokaryotic plasmids to function in yeast, they must contain a CEN (centromere) sequence.

• PCR: Polymerase Chain Reaction (PCR) is a technique for amplifying DNA between known sequences.

Example: Southern blotting is used to detect a specific gene in a DNA sample after restriction enzyme digestion and gel electrophoresis.

Quantitative Genetics and Polygenic Traits

Polygenic Inheritance

Quantitative traits are controlled by multiple genes (polygenes), each contributing a small effect to the phenotype.

• Additive Inheritance: When several loci each contribute an equivalent increment to a phenotype, the pattern is called additive inheritance.

• Quantitative Traits: Traits showing continuous variation (e.g., height, weight) are called quantitative, continuous, or metrical traits.

• Polygenes: These genes are typically located on all chromosomes and contribute to the trait in an additive manner.

• Genotype Categories: For a trait controlled by n loci, the number of genotypic categories in the F2 generation is $2n + 1$.

Example: In rye grass, seed color is a polygenic trait. Crossing true-breeding red and white varieties produces F1 seeds of intermediate color. Self-fertilization of F1 yields a range of colors in the F2, with 1 in 64 being white if six loci are involved.

Cost Calculation in Polygenic Crosses

Calculating the cost of producing a crop with all possible genotypes involves multiplying the number of genotypes by the cost per individual.

• Number of Genotypes: For six additive loci, the number of genotypes is $2^6 = 64$.

• Cost Example: If each hamster costs $2 to raise, producing all genotypes would cost $64 \times 2 = $128$.

• Additional info: The actual cost may be higher if the goal is to obtain a full bell-shaped curve distribution, requiring larger sample sizes.

Developmental Genetics

ABC Model of Flower Development

The ABC model explains the genetic control of flower organ identity in Arabidopsis thaliana.

• Class A Genes: Specify sepals and, with class B, petals.

• Class B Genes: Specify petals (with A) and stamens (with C).

• Class C Genes: Specify stamens (with B) and carpels.

• Mutation Effects: Loss of class B gene function results in missing petals and stamens.

Example: A mutation knocking out class B genes leads to flowers with only sepals and carpels.

Gene Interaction and Dominance

Dominant Negative Mutations

Some mutations produce proteins that interfere with the function of the wild-type protein, especially in multimeric complexes.

• Dominant Negative Effect: A mutant protein incorporated into a tetrameric complex can inactivate the entire complex.

• Probability Calculation: If wild-type and mutant proteins are produced equally, the probability that a tetramer contains at least one mutant subunit (and is thus inactive) is $1 - (1/2)^4 = 15/16$.

Example: In a heterozygote producing both wild-type and mutant X proteins, 15/16 of the tetrameric complexes will be inactive.

Gene Expression Regulation

Trans-acting Regulators

Gene expression is regulated by both cis-acting and trans-acting elements.

• Trans-acting Factors: Proteins such as silencers and histone deacetylases can regulate gene expression from a distance.

• Cis-acting Elements: DNA sequences like the TATA box and promoters are required for transcription initiation but are not trans-acting.

Example: Histone deacetylases modify chromatin structure, affecting gene accessibility and expression.

Summary Table: Key Terms and Concepts

Concept	Definition	Example/Application
lac operon	Gene system for lactose metabolism in E. coli	Regulated by repressor and inducer (allolactose)
cDNA	Complementary DNA synthesized from mRNA	Used for cloning eukaryotic genes
Southern blotting	DNA detection technique after electrophoresis	Identifies specific DNA fragments
Expression vector	Plasmid for gene transcription and translation	Produces recombinant proteins
PCR	Amplifies DNA between known sequences	Genotyping, diagnostics
Polygenic trait	Trait controlled by multiple genes	Height, weight, seed color
ABC model	Genetic model for flower organ identity	Arabidopsis flower development