General Biology: Genetics, DNA, and Inheritance – Study Guide and Practice Questions

Genetics and Inheritance

Errors in Meiosis

Meiosis is a specialized type of cell division that produces gametes (sperm and egg cells) with half the number of chromosomes of the parent cell. Errors during meiosis can lead to abnormal chromosome numbers in offspring.

• Non-disjunction: Failure of chromosomes to separate properly during meiosis.

  • Trisomy: Presence of an extra chromosome (e.g., Down syndrome is trisomy 21).

  • Monosomy: Absence of a chromosome.

• Meiosis I non-disjunction:

  • 2 gametes with an extra chromosome

  • 2 gametes with a missing chromosome

• Meiosis II non-disjunction:

  • 1 gamete with an extra chromosome

  • 1 gamete with a missing chromosome

  • 2 normal gametes

Mendelian Genetics

Mendelian genetics describes how traits are inherited through discrete units called genes.

• Alleles: Variants of a gene.

  • Homozygous: Two copies of the same allele.

  • Heterozygous: Two different alleles present.

• Genotype vs. Phenotype:

  • Genotype: Genetic makeup.

  • Phenotype: Physical appearance.

• Sources of Genetic Variation:

  • Law of Segregation: Only one allele per gene is passed to each gamete.

  • Law of Independent Assortment: Alleles for different genes are distributed independently.

  • Crossing Over: Exchange of genetic material between homologous chromosomes during meiosis.

Pedigrees

Pedigrees are diagrams that show the inheritance of traits through generations.

• Symbols: Squares (males), circles (females), shaded (affected), unshaded (unaffected).

• Calling alleles and genotypes: Assigning genotypes based on inheritance patterns.

• Calculating probabilities:

  1. Assign genotypes for affected and carriers.

  2. Assign genotypes based on what was passed on.

  3. Calculate probabilities.

Diseases

• Sickle cell anemia: Inherited as an autosomal recessive trait.

• Achondroplasia: Example of a dominant trait.

Non-Mendelian Inheritance

• Incomplete dominance: Heterozygotes show a mixed phenotype.

• Co-dominance: Both alleles are expressed (e.g., AB blood type).

• Multiple alleles: More than two alleles possible for a trait.

• Epistasis: One gene affects the expression of another.

• Polygenic traits: Multiple genes affect a trait, often showing continuous variation.

Environmental Effects

• Environment can influence the expression of genetic traits.

• Some traits are determined by both genetic and environmental factors.

DNA and Gene Expression

DNA Structure and Replication

DNA is the molecule that carries genetic information in cells.

• Double helix: Two strands twisted into a spiral.

• Chargaff's rules: Base pairing (A-T, C-G).

• Antiparallel nature: Strands run in opposite directions (5' to 3' and 3' to 5').

• Function: DNA acts as a genetic carrier.

Requirements for DNA Replication

• Origin of replication

• Unwinding with helicase and topoisomerase

• DNA synthesis with primase and DNA polymerase

• Leading and lagging strands

• Okazaki fragments and ligase

Steps of DNA Replication

• Initiation: Recognition of origin and unwinding

• Elongation: DNA polymerase adds nucleotides

• Termination: Completion and joining of fragments

Transcription

Transcription is the process by which RNA is synthesized from a DNA template.

• RNA: Functions as a genetic messenger.

• Differences to DNA: RNA contains ribose sugar and uracil instead of thymine.

• Requirements: Promoter, RNA polymerase, transcription bubble.

• Steps: Initiation, elongation, termination.

Transcriptional Regulation

• RNA processing:

  • 5' cap addition

  • 3' poly-A tail

  • Splicing: Removal of introns

• Reasons for RNA processing:

  • Increased stability

  • Increased protein diversity

• Promoter recognition: Promoters are DNA sequences where RNA polymerase binds to initiate transcription.

Gene Regulation

• Lac operon: Model for gene regulation in bacteria.

• Lactose sensing: Involves lac repressor and CAP protein.

• Positive vs. negative control: Positive control increases transcription; negative control decreases it.

• General vs. specific transcription factors: General factors are required for all transcription; specific factors regulate particular genes.

• Enhancers: DNA elements that increase transcription.

Translation

Translation is the process by which mRNA is decoded to build proteins.

• Codon table: Used to convert mRNA sequences into amino acids.

• Start codon: AUG (methionine).

• Ribosome assembly: Initiation, elongation, termination.

• tRNA: Brings amino acids to the ribosome.

• Cycle of amino acid addition: Peptide bond formation, translocation.

• Stop codons: Signal the end of translation.

Mutations

Mutations are changes in the DNA sequence that can affect gene function.

• Spontaneous vs. non-spontaneous mutations: Occur naturally or due to external factors.

• Types:

  • Silent: No change in amino acid sequence.

  • Missense: Change in one amino acid.

  • Nonsense: Creates a stop codon.

  • Frameshift: Addition or deletion shifts the reading frame.

Practice Problems and Pedigree Analysis

Sample Multiple Choice Questions

• Calculating chromosome numbers in trisomy (e.g., Drosophila melanogaster).

• Understanding non-disjunction and its consequences.

• Identifying haploid, diploid, triploid, and aneuploid cells.

• Applying Mendel's laws to genetic crosses.

• Determining genotypes from phenotypes and pedigrees.

• Recognizing inheritance patterns (autosomal dominant, autosomal recessive, X-linked).

• Interpreting DNA structure and replication mechanisms.

• Translating RNA sequences using the codon table.

• Classifying mutations and their effects on proteins.

Pedigree Table Example

Pedigrees are used to track inheritance patterns. Below is a sample table describing pedigree symbols and their meanings:

Symbol	Meaning
Square	Male
Circle	Female
Shaded	Affected individual
Unshaded	Unaffected individual

Sample Genetic Cross Table

Below is a table showing possible genotypes and phenotypes for a monohybrid cross:

Genotype	Phenotype
AA	Dominant trait
Aa	Dominant trait
aa	Recessive trait

Key Equations and Concepts

• Probability of inheritance: $P = \frac{\text{Number of desired outcomes}}{\text{Total number of possible outcomes}}$

• Law of Segregation: Each gamete receives one allele for each gene.

• Law of Independent Assortment: Alleles of different genes assort independently during gamete formation.

• DNA base pairing: $A = T$, $C = G$

Additional info:

• Some context and explanations have been expanded for clarity and completeness.

• Tables have been recreated to illustrate pedigree and genetic cross concepts.