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

Incomplete Dominance: Blended traits (e.g., pink flowers from red and white parents).
Codominance: Both alleles expressed (e.g., AB blood type).
Multiple Alleles: More than two alleles control a trait (e.g., blood type A, B, O).
Polygenic Inheritance: Traits controlled by multiple genes (e.g., skin color).
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.