Comprehensive Genetics Study Notes (SI Handouts Combined) Summary & Study Notes

🧬 Core Concepts and Definitions

Genetics: The study of heredity — how inherited variation is encoded, replicated, expressed, and evolves over time. Genome: The complete set of genetic instructions (DNA/RNA) for an organism. Transmission genetics, molecular genetics, and population genetics focus respectively on inheritance patterns, chemical nature and expression of genetic information, and genetic composition changes in populations.

🧪 Historical Ideas and Model Organisms

Early incorrect ideas: pangenesis, inheritance of acquired characteristics, preformationism, and blending inheritance. Correct foundations: cell theory and germplasm. Common model organisms: E. coli, Mus musculus, C. elegans, S. cerevisiae, D. melanogaster, A. thaliana — chosen for short generation times and large progeny.

🌍 Domains, Viruses, and Nucleic Acids

Two-domain model: Bacteria and Archaea with eukaryotes branching from archaea. Archaea share replication/transcription/translation features with eukaryotes. Viruses are not in the three domains because they lack independent metabolism and need a host. Nucleic acids (DNA/RNA) are polymers of nucleotides: sugar + phosphate + nitrogenous base.

🔬 Cell Types and Reproduction Overview

Prokaryotes reproduce by binary fission (simple asexual). Eukaryotes use mitosis (growth/maintenance) and meiosis (sexual reproduction, generating genetic diversity).

🔁 Cell Cycle, Chromosomes, and Cohesin

Cell cycle phases: G1 / S / G2 / M. G1/S/G2 prepare the cell and replicate DNA; mitosis segregates replicated chromosomes into two genetically identical daughter cells. A chromosome consists of DNA packaged with proteins; homologous chromosomes are matching pairs (one from each parent) carrying the same genes with possible different alleles. Cohesin is the protein complex that holds sister chromatids together to ensure accurate segregation.

⚖️ Mitosis vs Meiosis (Key Differences)

Mitosis: one division → two genetically identical diploid cells; no reduction in chromosome number; no crossing over. Meiosis: two divisions (Meiosis I & II) → four haploid, genetically distinct cells; reductional division in Meiosis I; crossing over and independent assortment in Meiosis I increase genetic variation.

🧬 Spermatogenesis & Oogenesis (Animals) and Plant Gametogenesis

Spermatogenesis: spermatogonia (2n) → primary spermatocyte → meiosis I → secondary spermatocytes (n) → meiosis II → four spermatids (n) → sperm. Oogenesis: oogonia → primary oocyte arrested in prophase I → meiosis I → secondary oocyte + polar body → meiosis II completes at fertilization producing egg + second polar body. Plants: sporophyte → meiosis → haploid spores → gametophyte → mitosis → gametes; double fertilization in angiosperms forms zygote and triploid endosperm.

❓Selected Quick Questions

Why viruses need hosts: lack ribosomes and independent metabolic machinery. Example cell-counting: four chromosomes in G1 → after S (beginning of G2) still 4 chromosomes, 8 DNA molecules (sister chromatids), and 16 telomeres. Crossing over occurs in prophase I of meiosis, not in mitosis. Polar body: small byproduct of unequal cytokinesis in oogenesis.

🧬 Mendelian Terms and Principles

Dominant allele: expressed in both homozygote (AA) and heterozygote (Aa). Recessive allele: expressed only in homozygote (aa). Phenotype = observable trait; genotype = allele combination. Homozygous (AA or aa) vs heterozygous (Aa). Gene vs allele: gene = locus; alleles = variant forms.

⚖️ Mendel’s Laws and Probability Rules

Law of Segregation: two alleles at a locus separate during gamete formation (occurs in anaphase I). Law of Independent Assortment: alleles of different genes assort independently (applies to genes on different chromosomes or far apart on same chromosome). Probability rules: addition rule for mutually exclusive events, multiplication rule for independent events, and binomial probability for counts of successes across trials.

🔬 Crosses: Testcrosses, Backcrosses, Mono-/Dihybrid

Testcross: unknown genotype × homozygous recessive to reveal genotype. Backcross: F1 × parental genotype (used to introgress traits). Monohybrid cross (1 gene) classic 3:1 ratio; dihybrid cross (2 genes) classic 9:3:3:1 ratio in F2 for independently assorting loci.

📉 Chi-square Goodness-of-Fit (Example)

Compute whether observed counts deviate from expected ratios using $\chi^2 = \sum \frac{(observed - expected)^2}{expected}$. Example: observed black 95, blue 65 (total 190); expected 1:1 → expected each 95? (given handout uses 80 expected from 1:1 with different parental genotypes). Handout example produced $\chi^2 = 5.625$ with df = 1, p between $0.025$ and $0.01$, leading to rejection of the null hypothesis that deviation is due to chance.

🧾 Inheritance Modes & Pedigree Basics

Autosomal dominant/recessive, X-linked dominant/recessive, Y-linked. Pedigree: graphical family history; proband = individual that starts pedigree. Consanguinity increases chance of recessive trait expression. Use patterns (sex bias, transmission) and reciprocal crosses to distinguish modes (e.g., X-linked vs autosomal).

🎭 Allelic Interactions and Phenotypic Complexity

Incomplete dominance: heterozygote shows intermediate phenotype (heterozygote ratio often yields $1:2:1$ phenotypic ratio). Codominance: heterozygote expresses both homozygote phenotypes simultaneously (also often $1:2:1$). Lethal alleles can alter expected Mendelian ratios (e.g., heterozygote crosses may yield $2:1$ viable offspring ratio).

🔁 Penetrance, Modifier Genes, and Environment

Penetrance: percent of individuals with a genotype who express the expected phenotype (e.g., 80% penetrance = 80/100 show trait). Phenotype = genotype + modifier genes + environment + developmental and stochastic effects — genotype does not guarantee phenotype.

♀️ Dosage Compensation and X-Inactivation

Dosage compensation balances X-linked gene expression between sexes. Lyon hypothesis: random X-inactivation in females produces a Barr body; once inactivated it is stably maintained in daughter cells. Pseudo-autosomal regions (PARs) on X/Y remain homologous and recombine. Example: calico cats show patchy coat color due to X-inactivation mosaicism.

⚧ Sex Determination Systems

Common systems: XX-XY, ZZ-ZW, XX-XO, genic, and environmental determination. Despite different triggers, all systems funnel into gonad development → hormone production → adult anatomy. In Drosophila, sex is determined by the X:A ratio (e.g., $X:AA = 1.0$ → female; $0.5$ → male).

🧬 Human Sex Chromosome Disorders and Barr Bodies

Examples: Turner syndrome (XO), Klinefelter (XXY), Triple-X, Poly-X. Number of Barr bodies correlates with number of extra X chromosomes (e.g., XX → 1 Barr body), and XY → none.

🧪 Sex-linked Examples: Fruit Fly & Hemophilia

X-linked recessive traits (e.g., white eyes in Drosophila, hemophilia in humans) disproportionately affect males. In a classic X-linked recessive cross (carrier female × normal male), 50% of sons may be affected and 100% of daughters normal but 100% carriers in certain crosses depending on parental genotypes. Use reciprocal crosses to distinguish X-linked vs autosomal inheritance.

🔗 Linked Genes and Recombination Basics

Linked genes lie on the same chromosome and do not assort completely independently. Nonrecombinant (parental) gametes are the original allele combinations; recombinant gametes arise from crossing over. Map unit / centimorgan (cM) corresponds to recombination frequency (e.g., $1\text{ cM} \approx 1$ recombinant frequency).

🔁 Crossing Over, Double Crossovers, and Configurations

Crossing over produces recombinants; with linkage you typically observe more nonrecombinants than recombinants unless genes are far apart. Complete linkage yields 0% recombinants. A double crossover (DCO) involves two exchange events between chromatids and is rare but key for ordering three genes. Coupling (cis): wild-type alleles on one chromosome, mutants on the other. Repulsion (trans): wild-type and mutant alleles on the same chromosome.

🧭 Genetic vs Physical Maps and Three-Point Mapping

A genetic map is based on recombination frequencies; a physical map is based on DNA distance. A three-point testcross reveals gene order, single crossover (SCO) and double crossover frequencies, and allows calculation of map distances using: • Recombinant frequency: $RF = (SCO + DCO)/Total$ • Coefficient of coincidence: $COC = Observed\ DCO/Expected\ DCO$ • Interference: $I = 1 - COC$

📐 Interpreting Interference and Negative Values

High interference (e.g., $I=0.59$) means fewer DCOs observed than expected (59% of expected DCOs are blocked). A negative interference (e.g., $I=-0.23$) indicates more double crossovers occurred than expected based on single-crossover frequencies.

🧮 Practical Approach to Three-Point Data

1. Identify the two most frequent phenotypes → parental genotypes. 2. Identify the least frequent → DCOs. 3. Compare parents vs DCOs to find the middle gene (the gene whose allele switches in DCOs). 4. Compute SCO and DCO counts to get map distances and interference.

These concepts are essential for constructing linkage maps and interpreting recombination data in classical genetics.