Biology Exam 1 Study Guide: Genetics, Inheritance, and Molecular Biology

Scientific Inquiry and Experimental Design

Scientific Method

The scientific method is a systematic approach used to investigate natural phenomena. It involves several key steps:

• Observation: Gathering information about a phenomenon.

• Question: Formulating a question based on observations.

• Hypothesis: Proposing a testable explanation or prediction.

• Experiment: Designing and conducting tests to evaluate the hypothesis.

• Data Collection: Gathering and recording results.

• Analysis: Interpreting data to draw conclusions.

• Conclusion: Accepting, rejecting, or modifying the hypothesis based on evidence.

Independent variable: The factor that is changed or manipulated in an experiment. Dependent variable: The factor that is measured or observed in response to changes in the independent variable.

Theory vs. Hypothesis:

• Hypothesis: A specific, testable prediction about the outcome of an experiment.

• Theory: A broad, well-supported explanation for a wide range of observations, supported by extensive evidence.

Genetics: Genes, Chromosomes, and Inheritance

Genes and Chromosomes

• Genes: Units of heredity made up of DNA segments that code for proteins.

• Chromosomes: Structures within cells that contain genetic material. Humans have 23 pairs (46 total).

• Sex chromosomes: Chromosomes that determine biological sex (X and Y in humans).

• Autosomal chromosomes: All other chromosomes not involved in sex determination (22 pairs in humans).

Meiosis and Genetic Variation

Meiosis is the process by which gametes (sperm and egg cells) are produced, reducing the chromosome number by half and introducing genetic diversity.

• Meiosis I: Homologous chromosomes separate, resulting in two haploid cells (each with one chromosome from each pair).

• Meiosis II: Sister chromatids separate, producing four haploid gametes.

• Haploid (n): Cells with one set of chromosomes (gametes).

• Diploid (2n): Cells with two sets of chromosomes (somatic cells).

Independent assortment: During meiosis I, homologous chromosome pairs align randomly, leading to different combinations of maternal and paternal chromosomes in gametes.

Crossing over: Exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I of meiosis, increasing genetic diversity.

Random fertilization: The combination of any sperm with any egg further increases genetic variability in offspring.

Mendelian Genetics

Gregor Mendel conducted experiments with pea plants to uncover the basic principles of inheritance.

• Monohybrid cross: A cross between individuals differing in one trait.

• Dihybrid cross: A cross between individuals differing in two traits.

Mendel's Laws:

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation so that each gamete carries only one allele.

• Law of Independent Assortment: Genes for different traits can segregate independently during the formation of gametes.

Complex Patterns of Inheritance

Some traits do not follow simple Mendelian inheritance. Five factors that lead to more complex patterns include:

• Incomplete dominance: Heterozygotes have an intermediate phenotype (e.g., pink flowers from red and white parents).

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

• Multiple alleles: More than two alleles exist for a gene (e.g., ABO blood group).

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

• Polygenic inheritance: Multiple genes influence a single trait (e.g., skin color).

Sex-Linked Inheritance and X Inactivation

• Sex-linked genes: Genes located on sex chromosomes, often the X chromosome. These exhibit unique inheritance patterns, such as higher frequency of X-linked recessive disorders in males.

• Barr bodies: In female mammals, one X chromosome in each cell is inactivated and condensed into a Barr body, ensuring dosage compensation.

Linkage and Crossing Over

• Linked genes: Genes located close together on the same chromosome tend to be inherited together.

• Crossing over: Can separate linked genes, resulting in recombinant gametes.

• 50% parental type and 50% recombinant type gametes: When genes are not linked, crossing over produces equal numbers of parental and recombinant gametes.

• Linkage disequilibrium: Genes in close proximity have lower recombination frequencies due to reduced likelihood of crossing over between them.

Morgan's Drosophila test cross: Provided evidence for crossing over by showing that some gene pairs did not assort independently, but could be separated by recombination.

Molecular Biology: DNA, Gene Expression, and Mutations

DNA Structure and Replication

• DNA: Double helix composed of nucleotides (adenine, thymine, cytosine, guanine).

• Base pairing: Adenine (A) pairs with Thymine (T); Cytosine (C) pairs with Guanine (G).

DNA proofreading and repair: After replication, DNA polymerases check for errors and correct them. Two main repair mechanisms:

• Mismatch repair: Enzymes correct errors missed during replication.

• Excision repair: Damaged DNA segments are removed and replaced.

Central Dogma and Gene Expression

• Central dogma: Information flows from DNA to RNA to protein.

• Transcription: Synthesis of RNA from a DNA template.

• Translation: Synthesis of proteins using mRNA as a template.

Genetic code: Triplet base sequences (codons) in mRNA specify amino acids during protein synthesis.

Protein Function and Phenotype

• Proteins play three primary roles in physiology:

  • Transporters: Move substances across membranes (e.g., hemoglobin).

  • Receptors: Receive and transmit signals (e.g., insulin receptor).

  • Enzymes: Catalyze biochemical reactions (e.g., DNA polymerase).

• Protein function depends on shape and amino acid sequence.

Mutations

• Substitution mutations: One base is replaced by another.

  • Silent mutation: No change in amino acid sequence.

  • Missense mutation: Changes one amino acid in the protein.

  • Nonsense mutation: Creates a premature stop codon.

• Insertions and deletions: Addition or loss of nucleotides can cause frameshift mutations, altering the reading frame and potentially disrupting protein function.

Summary Table: Types of Mutations

Type	Description	Effect	Example
Silent	Base change does not alter amino acid	No effect on protein	GAA to GAG (both code for Glu)
Missense	Base change alters one amino acid	May affect protein function	Sickle cell anemia (Glu to Val)
Nonsense	Base change creates stop codon	Truncated, nonfunctional protein	Duchenne muscular dystrophy
Frameshift	Insertion/deletion shifts reading frame	Usually nonfunctional protein	Cystic fibrosis (ΔF508 mutation)

Example: A missense mutation in the hemoglobin gene causes sickle cell anemia, demonstrating how a single base change can alter protein function and phenotype.

Additional info: This study guide covers foundational concepts in genetics and molecular biology, including experimental design, Mendelian and non-Mendelian inheritance, chromosome behavior in meiosis, and the molecular basis of gene expression and mutation.