Part 1: The Basics of Heredity (Classical Genetics)
To understand genetics, you must first learn the "language" of inheritance.
1. Key Terminology
โ Gene: The fundamental unit of inheritance responsible for a specific trait.
โ Allele: Alternate forms of a single gene (e.g., a gene for height might have "tall" and "short" alleles).
โ Genotype: The actual genetic makeup (the letters, like Tt ).
โ Phenotype: The physical expression of those genes (the appearance, like "Tall").
โ Homozygous: Having two of the same alleles (e.g., TT or tt ).
โ Heterozygous: Having two different alleles (e.g., Tt ).
โ Dominant: An allele that is expressed even if only one copy is present (represented by capital letters).
โ Recessive: An allele that is only expressed when the dominant allele is absent (represented by lowercase letters).
2. Mendelโs Laws
Gregor Mendel, the "Father of Genetics," established three core principles through his work with pea plants:
1. Law of Segregation: During gamete formation, the two alleles for a trait separate so that each gamete receives only one.
2. Law of Independent Assortment: Genes for different traits are sorted into gametes independently of one another (this only
applies if they are on different chromosomes).
3. Law of Dominance: One allele can mask the expression of another.
3. Solving Genetic Crosses (Punnett Squares)
Punnett squares are diagrams used to predict the probability of an offspring's genotype.
โ Monohybrid Cross: A cross involving one trait (e.g., Aa x Aa ).
โ Phenotype Ratio: Usually 3:1.
โ Genotype Ratio: Usually 1:2:1.
โ Dihybrid Cross: A cross involving two traits (e.g., AaBb x AaBb ).
โ Phenotype Ratio: 9:3:3:1.
Part 2: Molecular Genetics
This section focuses on how DNA actually works at a chemical level.
1. The Central Dogma
The flow of genetic information in a cell follows this path: DNA โ RNA โ Protein.
โ Replication: DNA makes a copy of itself.
โ Transcription: DNA is used as a template to create Messenger RNA (mRNA) in the nucleus.
โ Translation: mRNA is "read" by ribosomes in the cytoplasm to build a protein chain using Transfer RNA (tRNA) to bring
amino acids.
2. DNA Structure
โ Components: DNA is a double helix made of nucleotides.
โ Nucleotide: Consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base.
โ Base Pairing: Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C).
โ Directionality: DNA strands are antiparallel, running in 5' to 3' and 3' to 5' directions.
3. DNA Replication Enzymes
โ Helicase: Unwinds and "unzips" the DNA double helix.
โ DNA Polymerase III: Assembles new DNA strands in the 5' to 3' direction.
โ Ligase: Joins DNA fragments (Okazaki fragments) on the lagging strand.
Part 3: Gene Regulation and Mutations
Cells do not express all genes at once; they use "switches" to control them.
1. Prokaryotic Regulation (Operons)
โ Lac Operon: A system in E. coli that controls the breakdown of lactose.
โ Promoter: Where RNA polymerase binds to start transcription.
โ Operator: The "on/off switch" where a repressor protein can bind.
โ Polycistronic mRNA: In prokaryotes, one mRNA molecule can code for multiple different proteins.
2. Eukaryotic Differences
โ RNA Processing: Unlike prokaryotes, eukaryotic mRNA is modified before leaving the nucleus. This includes adding a 5โ cap,
a poly-A tail, and splicing (removing non-coding introns).
โ Monocistronic: Eukaryotic mRNA typically codes for only one protein.
