Borgon Molec Bio lecture 1 & 2 objectives

Define and Understand Mendel’s Laws of Segregation and Independent Assortment

• Law of Segregation: Each organism has two alleles for each trait, which segregate during gamete formation, ensuring that each gamete carries only one allele.

• Law of Independent Assortment: Genes for different traits assort independently of one another in the formation of gametes, applicable when genes are on different chromosomes.

Mendel’s Experiments and Conclusions

• Mendel’s methodical approach involved crossbreeding pea plants and tracking inheritance patterns.

• Observed 3:1 phenotypic ratios in F2 generations, leading to conclusions about dominant and recessive traits.

• Model Choice: Pea plants were ideal due to distinct traits, controlled mating, and rapid growth cycles.

Alleles

Definitions and Differentiation

• Alleles: Variants of a gene found at a specific locus on a chromosome.

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

• Heterozygous: Two different alleles for a trait (e.g., Aa).

Punnett Square Calculations

Genotypic and Phenotypic Probability

• Single Trait: Use a 2x2 grid; multiple traits require expanded grids (4x4 for two traits).

• Example: F1 (Rr x Rr) leads to a 1:2:1 genotypic ratio and 3:1 phenotypic ratio.

• Dihybrid crosses show a 9:3:3:1 phenotypic ratio in F2 generations.

Inheritance Patterns

Inheritance Types and Examples

1. Incomplete Dominance: Blended traits (e.g., pink flowers from red and white parents).

2. Codominance: Both alleles expressed (e.g., AB blood type).

3. Multiple Alleles: More than two alleles control a trait (e.g., blood type A, B, O).

4. Polygenic Inheritance: Traits controlled by multiple genes (e.g., skin color).

5. Epistasis: One gene modifies the expression of another (e.g., coat color in dogs).

Mitosis and Meiosis Overview

Steps and Comparison

• Mitosis: Produces identical diploid cells; stages are prophase, metaphase, anaphase, and telophase.

• Meiosis: Produces haploid gametes; includes meiosis I (homologous chromosomes separate) and meiosis II (sister chromatids separate).

• Chromosome count halves in meiosis but remains constant in mitosis.

Gametes and Genotypes

• Gamete formation involves random segregation of alleles.

• Independent Assortment: Genes on separate chromosomes or far apart on the same chromosome assort independently.

• Linkage: Close genes on the same chromosome are inherited together unless separated by crossing over.

Independent Assortment and Linkage Analysis

• Independent assortment applies when genes are unlinked.

• Linked genes show recombination less frequently, calculable via recombination frequency.

Genotypic and Phenotypic Probabilities

• Multiply probabilities of individual trait outcomes for multi-trait crosses.

• Example: Probability of RRYy = (1/4) x (1/2) = 1/8.

Sex-Linked Traits

• Traits associated with sex chromosomes (e.g., X-linked color blindness).

• Example: Cross XwXw x XWY; use Punnett squares to calculate probabilities.

McClintock Experiment

• Demonstrated crossing over in corn using chromosomal markers.

• Observed recombination rates to establish linkage and crossover events.

Linkage and Crossing Over Overview

• Conditions for Linkage: Genes close on the same chromosome.

• Recombination Frequency: Proportional to the distance between genes.

Linkage and Crossing Over Calculations

• Probability of recombination = distance in cM.

• Double Crossovers: Adjust calculations using the formula AB = AC + BC - 2(AC)(BC).

Recombination Frequency

• Measure of how often crossing over occurs between two genes.

• Example: A 10% recombination rate equals 10 cM.

Gene Markers

• Used to map genes using two- and three-factor crosses.

• Consider double crossovers for accurate recombination frequency calculations.

Genetic Mapping

• Minimum markers required: (Chromosome length in Mb / 50 cM) - 1.

• Each cM corresponds to ~1 million bp in humans.

Gene Order and Double Crossovers

• Determine gene order using least frequent recombinants.

• Double crossovers restore parental phenotype but affect calculations.

Three-Factor Crosses

• Analyze phenotypes of offspring to determine gene order and crossover frequencies.

• Least frequent classes represent double crossovers.

Drosophila Genetic Map

• Early genetic maps linked gene loci with recombination frequencies.

• Example: X-linked traits analyzed in fruit flies.

Human Genetics

• Disorders like alkaptonuria and phenylketonuria linked to specific enzyme mutations.

• Observations advanced understanding of metabolic pathways and genetic inheritance.