DNA & Chromosomes

Complementary Base Pairing Rules

DNA structure is stabilized by specific pairing between nitrogenous bases, following Chargaff's rules:

• Adenine (A) pairs with Thymine (T) via 2 hydrogen bonds.

• Guanine (G) pairs with Cytosine (C) via 3 hydrogen bonds.

• In RNA, Uracil (U) replaces Thymine.

• Example: For a DNA strand 5'-ATGCCG-3', the complementary strand is 3'-TACGGC-5'.

Diploid and Haploid Chromosome Numbers

• Diploid (2n): Full chromosome set, found in somatic cells (e.g., humans 2n = 46).

• Haploid (n): Half the chromosome set, found in game\

• es (n = 23 in humans).

• Each chromosome pair consists of one chromosome from each parent.

Model Organisms in Genetics

• Drosophila melanogaster (fruit fly), Mus musculus (mouse), Caenorhabditis elegans (roundworm), Arabidopsis thaliana (plant) are common model organisms.

• Large mammals and humans are less commonly used due to ethical and practical reasons.

Cell Division: Mitosis & Meiosis

Cell Cycle & Chromosome Doubling

• G1: Cell growth and protein synthesis.

• S Phase: DNA replication (chromosome doubling occurs here).

• G2: Preparation for cell division.

• M Phase: Mitosis or meiosis occurs.

Chromosome doubling occurs during S phase, not during mitosis itself.

Mitosis

• Interphase (S Phase): Chromosomes replicate, each consisting of two sister chromatids.

• Prophase: Chromatin condenses, spindle fibers form.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate to opposite poles.

• Telophase: Nuclear envelope reforms.

• Cytokinesis: Cell splits into two diploid daughter cells.

Meiosis

• Reduces chromosome number from diploid (2n) to haploid (n).

• Consists of two divisions: Meiosis I and Meiosis II.

• Results in four genetically unique haploid gametes.

Mendelian Genetics

Mendel's Laws

• Independent assortment applies to genes on different (non-homologous) chromosomes.

Genotypes & Phenotypes

• Homozygous dominant: AA

• Homozygous recessive: aa

• Heterozygous (Carrier): Aa

• Wild type: Most common phenotype in a population.

• Proband: First affected individual identified in a pedigree.

Test Cross

• Cross an unknown dominant phenotype with a homozygous recessive (aa) to determine genotype.

Dihybrid Cross Example

• Cross: AaBb × Aabb

• Gametes for AaBb: AB, Ab, aB, ab

• Gametes for Aabb: Ab, Ab, ab, ab

• Phenotype Ratio: A_B_ : 3, A_bb : 3, aaB_ : 1, aabb : 1 (3:3:1:1)

Number of Gamete Types

• Formula: where n = number of heterozygous gene pairs

• For AaBbCcDd: n = 4, so gamete types

Forked-Line Method (Probability)

• Used to calculate probability of offspring genotypes across multiple genes.

• Example: Probability of aa offspring from Aa × Aa for each gene:

Inheritance Patterns

• Gain of Function Mutation: Creates new/enhanced protein function, usually autosomal dominant.

Pedigree Analysis

• X-linked Dominant: Affected fathers pass trait to all daughters, not sons; affected mothers (heterozygous) pass to 50% of sons and daughters.

Complementation Test (Fruit Flies)

The complementation test distinguishes whether mutations causing the same phenotype are in the same or different genes. If two mutants are crossed and the offspring are wild type, the mutations are in different genes (complementation occurs). If the offspring are mutant, the mutations are in the same gene (no complementation).

Sex Determination & X-Inactivation

Sex Chromosome Abnormalities & Nondisjunction

• Nondisjunction: Failure of chromosomes to separate properly during meiosis, leading to abnormal chromosome numbers.

Lyon Hypothesis / X-Inactivation / Barr Bodies

• In mammals, one X chromosome is randomly inactivated in each somatic cell, forming a Barr body.

• Formula:

• This process is called dosage compensation, ensuring equal expression of X-linked genes in both sexes.

• Calico cats (XX) display orange/black patches due to random X-inactivation.

Pseudoautosomal Region (PAR)

• PAR is a region on the Y chromosome that pairs with the X chromosome during meiosis.

• It behaves like an autosomal region and is located at the tips of X and Y chromosomes.

Metabolic & Chromosomal Disorders

Inborn Errors of Metabolism

• Albinism: Autosomal recessive, absence of melanin pigment.

Lethal Alleles

• Some alleles are lethal when homozygous dominant (AA).

• Example cross (Aa × Aa, AA lethal): Expected 1 AA : 2 Aa : 1 aa, but AA dies, so surviving ratio is 2 Aa : 1 aa.

Karyotype Analysis

• A karyotype shows chromosome number, size, and structure.

• It cannot reveal specific gene mutations, gene expression levels, epigenetic modifications, or behavioral traits.

Study Tip: Focus on understanding the reasoning behind each concept, not just memorization. Practice drawing mitosis, meiosis, and Punnett squares, and apply the product law for probability calculations.