Genetics, Gene Expression, and Viruses: Study Guide for General Biology

Genetics

Basic Principles of Genetics

Genetics is the study of heredity and variation in organisms. It explains how traits are passed from parents to offspring through genes.

• Hybridization: The process of crossing two genetically different individuals to produce offspring with traits from both parents.

• True Breeding: Organisms that, when self-fertilized, produce offspring identical to themselves for a given trait.

• Dominant and Recessive Alleles: Dominant alleles mask the effect of recessive alleles in heterozygotes.

• Monohybrid Cross: A genetic cross involving a single trait.

• Homozygous: Having two identical alleles for a trait (e.g., BB or bb).

• Heterozygous: Having two different alleles for a trait (e.g., Bb).

Example: Mendel's pea plant experiments demonstrated the inheritance of flower color, seed shape, and other traits.

Punnett Squares and Genetic Crosses

Punnett squares are used to predict the possible genotypes and phenotypes of offspring from a genetic cross.

• Punnett Square: A diagram that shows the possible combinations of alleles from parental gametes.

• Genotype: The genetic makeup of an organism (e.g., BB, Bb, bb).

• Phenotype: The observable traits of an organism (e.g., blue or white flowers).

	B	b
B	BB	Bb
b	Bb	bb

• Homozygous Dominant: BB

• Homozygous Recessive: bb

• Heterozygous: Bb

Example: If B = blue flowers (dominant) and b = white flowers (recessive), BB and Bb will have blue flowers, bb will have white flowers.

Blood Type Inheritance

Blood type inheritance involves multiple alleles (A, B, O) and codominance.

• Type O: Genotype ii

• Type A: Genotype IAIA or IAi

• Type B: Genotype IBIB or IBi

• Type AB: Genotype IAIB

Example: A mother with type O (ii) and a daughter with type B (IBi) must have a father with at least one IB allele.

Sex-Linked Inheritance

Sex-linked traits are associated with genes located on sex chromosomes, often the X chromosome.

• X-linked Recessive: Traits like colorblindness are more common in males (XY) than females (XX).

• Punnett Square for X-linked Cross: Used to predict the probability of offspring inheriting sex-linked traits.

	XC	Xc
XC	XCXC	XCXc
Y	XCY	XcY

Example: If a woman with normal vision (XCXc) marries a man with normal vision (XCY), their sons have a 50% chance of being colorblind if the mother is a carrier.

Meiosis and Nondisjunction

Meiosis is the process by which gametes are formed. Nondisjunction is the failure of chromosomes to separate properly, leading to genetic disorders.

• Nondisjunction: Can result in gametes with abnormal chromosome numbers (e.g., trisomy 21).

• Genetic Disorders: Caused by changes in chromosome number (e.g., Down syndrome) or structure (e.g., deletions, duplications).

Example: If nondisjunction occurs in meiosis I, half the gametes will have an extra chromosome, and half will be missing one.

Gene Expression

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information from DNA to RNA to protein.

• DNA: A polymer made from monomers called nucleotides.

• Transcription: The process by which RNA is synthesized from DNA, performed by the enzyme RNA polymerase.

• RNA: Comes in three kinds: mRNA, tRNA, rRNA.

• Translation: The process by which proteins are synthesized from mRNA, performed by ribosomes.

• Protein: A polymer made from monomers called amino acids.

Process	Polymer	Monomer	Enzyme/Organelle
Transcription	RNA	Nucleotides	RNA Polymerase
Translation	Protein	Amino Acids	Ribosome

Equation:

$\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

Transcription and Translation Components

Transcription requires DNA template, RNA polymerase, and nucleotides. Translation requires mRNA, ribosomes, tRNA, and amino acids.

• Transcription Location: Nucleus (in eukaryotes)

• Translation Location: Cytoplasm (on ribosomes)

Example: The DNA sequence TACGATTCCGA is transcribed to mRNA and then translated to a specific amino acid sequence using the genetic code.

Point Mutations

Point mutations are changes in a single nucleotide pair of a gene. They can be classified by their effect and cause.

Mutation Type	Caused by (base-pair substitution, insertion and/or deletion)	Brief Description
Silent	Substitution	No change in amino acid sequence
Missense	Substitution	Changes one amino acid in the protein
Nonsense	Substitution	Creates a stop codon, truncating the protein
Frameshift	Insertion or Deletion	Alters the reading frame, changing all downstream amino acids

Example: A substitution mutation changing GAA to GAG is silent if both code for glutamic acid.

Viruses

Structure and Properties of Viruses

Viruses are infectious agents composed of genetic material (DNA or RNA) surrounded by a protein coat (capsid). Some have an additional lipid envelope.

• Capsid: Protein shell that encases viral genetic material.

• Envelope: Lipid membrane derived from host cell (in some viruses).

• Genetic Material: DNA or RNA, single or double stranded.

Example: Influenza virus has an RNA genome and a lipid envelope.

Why Viruses Are Not Considered Alive

Viruses lack cellular structure and cannot carry out metabolism or reproduce independently. They require a host cell for replication.

• No Metabolism: Viruses do not perform metabolic processes.

• No Independent Reproduction: Replication occurs only inside host cells.

Viral Replication

Viruses replicate by infecting host cells and using the host's machinery to produce new viral particles.

• Lytic Cycle: Virus injects genetic material, replicates, and lyses the host cell.

• Lysogenic Cycle: Viral DNA integrates into host genome and replicates with it.

Equation:

$\text{Virus} + \text{Host Cell} \rightarrow \text{Viral Replication}$

Vaccines and Viral Protection

Vaccines stimulate the immune system to recognize and fight viruses, providing immunity.

• Immunity: Memory cells recognize viral antigens and respond rapidly upon exposure.

• Types of Vaccines: Inactivated, attenuated, and subunit vaccines.

Example: The polio vaccine contains inactivated virus particles to induce immunity.

Additional info: Academic context and explanations have been expanded for completeness and clarity. Tables have been recreated and filled with logical entries based on standard biology knowledge.