Chapter 2: Mendelian Genetics

Mendel's Experiments and Laws

Gregor Mendel, through his experiments with pea plants, established the foundational principles of inheritance. His work disproved the blended inheritance theory and introduced the concept of discrete hereditary units (genes).

• Pure Lines: Offspring produced from self-fertilization of pure lines are identical for the trait in question.

• Parental (P) Generation: The initial set of parents in a genetic cross.

• First Filial (F1) Generation: Offspring from the parental cross.

• Second Filial (F2) Generation: Offspring from selfing or crossing F1 individuals.

Phenotype and Genotype

• Phenotype: Observable traits of an organism.

• Genotype: Genetic constitution underlying the phenotype.

Mendel's Laws

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

• Law of Independent Assortment: Alleles of different genes assort independently during gamete formation (applies to genes on different chromosomes).

Genetic Crosses and Terminology

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

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

• Monohybrid Cross: Cross between individuals heterozygous for a single gene (e.g., Bb x Bb).

• Dihybrid Cross: Cross between individuals heterozygous for two genes (e.g., BbSs x BbSs).

• Punnett Squares: Diagrammatic method to predict genotypic and phenotypic ratios.

• Chi-square Test: Statistical test to compare observed and expected genetic ratios.

• Pedigrees: Family trees used to study inheritance patterns.

Example: In a monohybrid cross of two heterozygotes (Bb x Bb), the expected genotypic ratio is 1:2:1 (BB:Bb:bb), and the phenotypic ratio is 3:1 if B is dominant over b.

Chapter 3: Cell Division

The Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It consists of interphase (G1, S, G2) and the M phase (mitosis or meiosis).

• G1 Phase: Cell growth and preparation for DNA synthesis.

• S Phase: DNA replication and chromosome duplication.

• G2 Phase: Preparation for cell division.

• M Phase: Mitosis (somatic cells) or meiosis (germ-line cells).

Mitosis

Mitosis is the process by which a cell divides to produce two genetically identical diploid daughter cells. It consists of several stages:

• Prophase: Chromatin condenses into visible chromosomes; nuclear envelope breaks down; spindle fibers form.

• Prometaphase: Nuclear envelope disappears; spindle fibers attach to kinetochores.

• Metaphase: Chromosomes align at the metaphase plate; each chromosome is attached to spindle fibers from both poles.

• Anaphase: Sister chromatids separate at the centromere and move to opposite poles.

• Telophase: Nuclear envelopes reform; chromosomes de-condense; cytokinesis divides the cytoplasm.

Diagram: The provided image illustrates DNA replication, mitosis, and the formation of two diploid cells.

Centrosomes and Spindle Apparatus

• Centrosome: Organelle made of two centrioles; main microtubule-organizing center (MTOC).

• Spindle Fibers: Microtubules that attach to chromosomes and pull them apart during cell division.

Chromosome Numbers

• Humans have 46 chromosomes (23 pairs) and 2 centrosomes during mitosis.

• During S phase, chromosomes are duplicated, resulting in 92 chromatids.

Meiosis

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

• Meiosis I: Homologous chromosomes pair and segregate, reducing chromosome number to haploid.

• Meiosis II: Sister chromatids separate, similar to mitosis.

Meiosis I Stages

• Prophase I: Chromosomes condense, homologous chromosomes pair (synapsis), crossing over occurs (genetic recombination).

• Metaphase I: Homologous pairs align at the metaphase plate.

• Anaphase I: Homologs separate to opposite poles; sister chromatids remain joined.

• Telophase I: Nuclear membranes reform; cytokinesis produces two haploid cells.

Meiosis II Stages

• Prophase II: Nuclear envelope breaks down; chromosomes condense.

• Metaphase II: Chromosomes align at the metaphase plate.

• Anaphase II: Sister chromatids separate.

• Telophase II: Nuclear membranes reform; cytokinesis produces four haploid gametes.

Key Terms in Meiosis

• Synapsis: Pairing of homologous chromosomes during prophase I.

• Chiasmata: Sites where crossing over occurs between homologous chromosomes.

• Cohesion: Protein complex holding sister chromatids together; separase enzyme breaks cohesion during anaphase.

Genetic Linkage and Inheritance Patterns

• Sutton and Boveri: Proposed chromosome behavior in meiosis mirrors hereditary transmission of genes.

• Morgan's Experiments: Used fruit flies to test Mendel's rules; discovered sex-linked inheritance.

• Sex-linked Inheritance: Transmission of genes on sex chromosomes (e.g., X-linked inheritance).

• Wild Type: Most common phenotype in a population.

Summary Table: Mitosis vs. Meiosis

Key Equations

• Chi-square Test:

Where = observed value, = expected value.

• Probability of Independent Events:

Additional info: Some context and terminology were expanded for clarity and completeness, including definitions and examples of genetic crosses, and a summary table comparing mitosis and meiosis.