BIO 3303 Genetics: Exam 1 Study Guide – Core Concepts and Applications

Module 1: Introduction to Genetics

Branches of Genetics

Genetics is the scientific study of heredity and variation in living organisms. The field is divided into several branches, each focusing on different aspects of genetic information and inheritance.

• Molecular Genetics: Studies the structure and function of genes at a molecular level.

• Transmission Genetics: Examines how genetic traits are passed from parents to offspring.

• Population Genetics: Investigates genetic variation within populations and how it changes over time.

• Quantitative Genetics: Focuses on traits controlled by multiple genes and their statistical analysis.

• Genomics: Involves the comprehensive study of whole genomes.

Cell Theory

Cell theory is a foundational concept in biology, stating that all living organisms are composed of cells, and all cells arise from pre-existing cells. This principle underpins the study of genetics, as genetic material is contained within cells.

Genotype and Phenotype

• Genotype: The genetic makeup of an organism; the specific alleles present at a locus.

• Phenotype: The observable physical or biochemical characteristics of an organism, determined by both genotype and environment.

• Example: In pea plants, the genotype TT or Tt results in tall plants (phenotype), while tt results in short plants.

Methods of Science and Scientific Process

• Observation: Gathering data about natural phenomena.

• Hypothesis Formation: Proposing explanations based on observations.

• Experimentation: Testing hypotheses through controlled experiments.

• Analysis: Interpreting data to draw conclusions.

• Peer Review and Publication: Sharing results for validation by the scientific community.

Experimental Variables

• Independent Variable: The factor manipulated by the experimenter.

• Dependent Variable: The factor measured in response to changes in the independent variable.

• Controlled Variables: Factors kept constant to ensure a fair test.

Model Organisms

Model organisms are species widely used in genetic research due to their ease of maintenance, short generation times, and well-characterized genetics.

• Escherichia coli (bacterium)

• Saccharomyces cerevisiae (yeast)

• Drosophila melanogaster (fruit fly)

• Mus musculus (mouse)

• Arabidopsis thaliana (plant)

Module 2: Cell Division

Eukaryotic vs. Prokaryotic Cells and Genomes

• Eukaryotic Cells: Contain a nucleus and membrane-bound organelles; genomes are linear and organized into chromosomes.

• Prokaryotic Cells: Lack a nucleus; genomes are typically circular and found in the cytoplasm.

Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide.

• Interphase: Includes G1 (growth), S (DNA synthesis), and G2 (preparation for division).

• M Phase: Mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Ploidy Level, Homologous Chromosomes, and Sister Chromatids

• Ploidy: Number of sets of chromosomes in a cell (e.g., diploid = 2n, haploid = n).

• Homologous Chromosomes: Chromosome pairs with the same genes but possibly different alleles.

• Sister Chromatids: Identical copies of a chromosome, connected at the centromere, formed during DNA replication.

Dominant vs. Recessive, Homozygous vs. Heterozygous

• Dominant Allele: Expressed in the phenotype even if only one copy is present.

• Recessive Allele: Expressed only when two copies are present.

• Homozygous: Two identical alleles at a locus (e.g., AA or aa).

• Heterozygous: Two different alleles at a locus (e.g., Aa).

Bacterial Fission

Bacterial (binary) fission is the process by which prokaryotic cells divide, producing two genetically identical daughter cells.

Mitosis – Stages and Outcomes

• Prophase: Chromosomes condense, spindle forms.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate to opposite poles.

• Telophase: Nuclear envelopes reform, chromosomes decondense.

• Outcome: Two genetically identical diploid daughter cells.

Meiosis – Stages and Outcomes

• Meiosis I: Homologous chromosomes separate, reducing chromosome number by half.

• Meiosis II: Sister chromatids separate.

• Outcome: Four genetically unique haploid gametes.

Module 3: Transmission Genetics

Mendel’s Two Laws of Inheritance

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

• Law of Independent Assortment: Genes for different traits assort independently of one another during gamete formation.

• Connection to Meiosis: Segregation occurs during anaphase I; independent assortment results from random alignment of homologous chromosomes.

Determining Genotypes and Phenotypes

• Use of Punnett squares to predict offspring genotypes and phenotypes from parental crosses.

• Backcrosses and testcrosses help determine unknown genotypes.

Punnett Squares

Punnett squares are diagrams used to predict the outcome of genetic crosses.

Probability – Product and Sum Rules

• Product Rule: Probability of independent events occurring together is the product of their individual probabilities.

• Sum Rule: Probability of either of two mutually exclusive events is the sum of their individual probabilities.

• Example: Probability of getting heads twice in a row: $P = 0.5 \times 0.5 = 0.25$

Binomial Expansions

Used to calculate probabilities of specific combinations in multiple trials.

• General formula: $P = \frac{n!}{x! (n-x)!} p^x q^{n-x}$

• Where n = total trials, x = number of successes, p = probability of success, q = probability of failure.

Chi-Squared ($\chi^2$) Analysis

Statistical test to compare observed and expected results.

• Formula: $\chi^2 = \sum \frac{(O - E)^2}{E}$

• Where O = observed value, E = expected value.

• Used to test hypotheses about genetic ratios.

Pedigrees

Pedigree charts are diagrams that show inheritance patterns of traits across generations in families.

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

• Used to infer modes of inheritance (autosomal dominant, autosomal recessive, X-linked, etc.).

LOD Analysis and Genetic Linkage

• LOD (Logarithm of the Odds) Score: Statistical estimate of whether two loci are likely to be linked.

• LOD score > 3 indicates significant evidence for linkage.

• Genetic linkage refers to the tendency of genes located close together on a chromosome to be inherited together.

Module 4: Extensions of Transmission Genetics

Sex Determination and Inheritance/Expression

• Sex can be determined by chromosomes (e.g., XX/XY in mammals, ZZ/ZW in birds), environmental factors, or gene dosage.

• Sex-linked inheritance involves genes located on sex chromosomes, often resulting in different expression patterns in males and females.

Epistasis and Pleiotropy

• Epistasis: Interaction between genes where one gene masks or modifies the effect of another.

• Pleiotropy: A single gene influences multiple phenotypic traits.

• Example: The gene for sickle cell anemia affects both red blood cell shape and resistance to malaria.

Codominance, Incomplete Dominance, and Overdominance

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

• Incomplete Dominance: Heterozygote phenotype is intermediate between the two homozygotes (e.g., pink flowers from red and white parents).

• Overdominance: Heterozygote has a phenotype that is more extreme or advantageous than either homozygote.

Incomplete Penetrance

• Not all individuals with a particular genotype express the expected phenotype.

• Penetrance is the proportion of individuals with a genotype who display the phenotype.

Multiple Alleles

• More than two alleles exist for a gene in a population (e.g., ABO blood group system).

• Individuals still inherit only two alleles (one from each parent).

Module 5: Organellar Inheritance

Mitochondrial and Chloroplast Genome Structure and Size

• Mitochondria and chloroplasts contain their own circular DNA, distinct from nuclear DNA.

• Genome size is much smaller than the nuclear genome.

• Genes encode proteins essential for organelle function.

Organellar-Nuclear Genetic Transfer

• Some organellar genes have been transferred to the nuclear genome over evolutionary time.

• Many proteins required for organelle function are encoded by nuclear genes and imported into the organelle.

Organellar Inheritance

• Inheritance of mitochondrial and chloroplast DNA is typically uniparental (usually maternal).

• Leads to non-Mendelian inheritance patterns.

Organellar Evolutionary History

• Mitochondria and chloroplasts are believed to have originated from endosymbiotic bacteria.

• Evidence includes similarities in DNA structure, ribosomes, and reproduction by binary fission.