BIO306 Exam 1 Review

Mendel and Beyond

• Mendel in context: Understanding the pre-Mendelian concepts of homunculus and blending inheritance. Peas were used as a model organism due to their clear traits and controlled mating.
• Monohybrid crosses: These crosses involve a single trait. Setting up the crosses includes using true-breeding parents (homozygous), resulting in an F1 generation that is heterozygous for the trait. Expected F2 genotypic and phenotypic ratios are crucial, demonstrating dominant and recessive relationships. It's important to remember that dominant does not equate to common.
• Dihybrid crosses: These crosses involve two traits. Initial setup involves true-breeding parents (homozygous), producing a heterozygous F1 generation. The expected F2 phenotypic ratio is key. Understanding dominant and recessive relationships for both traits is necessary.
• Laws of Segregation and Independent Assortment: These laws explain how alleles separate during meiosis. Segregation refers to the separation of alleles for a single gene into different gametes. Independent assortment refers to the independent segregation of alleles for different genes during gamete formation, assuming the genes are on different chromosomes or far apart on the same chromosome.
• Techniques for determining ratios: Punnett squares, branch diagrams (specifically for dihybrid crosses using forks), and probabilities (product rule and sum rule). Branch diagrams may simplify complex scenarios. The product rule states that the probability of two independent events occurring together is the product of their individual probabilities. The sum rule states that the probability of either of two mutually exclusive events occurring is the sum of their individual probabilities.
• Interpreting pedigrees: Use pedigree analysis to determine if a trait is dominant or recessive based on inheritance patterns.

The Life and Times of a Chromosome

• Chromosome theory of inheritance: Genes are located on chromosomes.
• Diploid, haploid, diploid number (2n), haploid number (n): Understanding the number of chromosomes in somatic cells (diploid) and gametes (haploid). The level of organism complexity is not determined by the number of chromosomes.
• Nonhomologous vs. homologous chromosomes: Nonhomologous chromosomes have different sizes, centromere positions, banding patterns, and genes. Homologous chromosomes, which pair during meiosis, share these characteristics except for the alleles they carry (different versions of the same gene).
• Cell cycle stages: G1 (growth), S (DNA synthesis/replication), G2 (preparation for division), M (mitosis), and G0 (quiescence or cell cycle arrest). Major events occur in each phase.
• Mitosis: Focus on chromosome behavior during each phase and the necessity of each event for the process to work. Note the differences between plant and animal cell division, particularly cytokinesis. Mitosis is for repair and growth, resulting in two cells identical to the original cell.
• Meiosis: Focus on chromosome actions and event rationale in each phase (Prophase I, Metaphase I, Anaphase I, Telophase I, Meiosis II). Cells become haploid after Meiosis I.
• Special features of meiosis: Pairing of homologs and crossing over (recombination) occur during Prophase I. Pairing ensures proper segregation, while crossing over increases genetic diversity.
• Gametogenesis: Examine gamete formation in humans and other mammals, noting similarities and differences compared to the general meiosis model.
• Relationship between Mendel's principles and meiosis: Segregation reflects the separation of homologous chromosomes during meiosis I, while independent assortment corresponds to the random alignment of chromosome pairs during metaphase I.
• Sex determination: Homologous vs. heterologous chromosome pairs determine sex.
• Sex-linked traits: Understand sex-linked inheritance patterns and use appropriate symbols when diagramming crosses.

It's Not as Simple as Mendel Thought

• Incomplete dominance: The heterozygote phenotype is intermediate between the two homozygous phenotypes (e.g., flower color). It can be distinguished from Mendelian inheritance.
• Codominance: Both alleles are expressed in the heterozygote (e.g., blood type AB). It can be distinguished from Mendelian inheritance.
• Heterozygous advantage (overdominance): The heterozygote has a higher fitness than either homozygote.
• Multiple alleles: Most genes have more than two allele options.
• Sex-limited and sex-influenced traits: Sex-limited traits are expressed in only one sex, while sex-influenced traits are expressed differently in each sex. These differ from X-linked traits.
• Lethal alleles: Certain genotypes result in death, altering expected inheritance ratios.
• Epistasis: One gene masks the expression of another gene, leading to modified dihybrid ratios other than 9:3:3:1. For example,