Chargaff's rules — the foundation of DNA structure
In DNA: Adenine always pairs with Thymine (2 hydrogen bonds). Guanine always pairs with Cytosine (3 hydrogen bonds). "AT the GC" — a way to remember both pairs.
A
Adenine pairs with Thymine — 2 hydrogen bonds
T
Thymine pairs with Adenine
G
Guanine pairs with Cytosine — 3 hydrogen bonds
C
Cytosine pairs with Guanine
🧬 Genetics
Dominant masks Recessive
Dominance Rule
Dominant vs recessive — one simple rule
Dominant alleles (capital letter) always express when present. Recessive alleles (lowercase) only express when homozygous (two copies). Dd looks like DD.
🧬 Genetics
DNA → RNA → Protein
Central Dogma of Molecular Biology
The central dogma — information only flows one direction
DNA is transcribed into mRNA, which is translated into protein. Transcription happens in the nucleus. Translation happens at the ribosome.
🧬 Genetics
"Genotype is Genes, Phenotype is Physical"
Genotype vs Phenotype
Genotype and phenotype — easy G vs P distinction
Genotype = the actual alleles an organism carries (e.g., Bb). Phenotype = what you can observe (e.g., brown hair). Same phenotype can have different genotypes.
🧬 Genetics
X-linked: sons get it from mom
X-Linked Inheritance Pattern
X-linked recessive — why sons are affected more than daughters
Males have only one X (XY). A single recessive allele on that X expresses. Females need two copies (XX) to show the trait. Carrier moms pass it to 50% of sons.
Two classic Punnett square crosses every genetics student must know
Monohybrid cross (Tt × Tt): 1 TT : 2 Tt : 1 tt genotype, 3 dominant : 1 recessive phenotype. Test cross: cross unknown dominant with homozygous recessive (tt) — if any recessive offspring, unknown was heterozygous.
Incomplete vs Codominance
Incomplete dominance: RR + WW → RW (pink). Codominance: both show (AB blood type).
Incomplete vs Codominance
Two exceptions to simple dominant-recessive inheritance
Incomplete dominance: neither allele fully dominant → blend. Red + White = Pink snapdragon. Codominance: both alleles fully expressed simultaneously. Blood type AB: both A and B antigens present on red blood cells. Neither masks the other.
Meiosis vs Mitosis
Meiosis makes gametes (sperm/egg) with half the chromosomes (haploid, n)
Meiosis vs Mitosis
The critical difference between cell division for body cells vs sex cells
Mitosis: 1 cell → 2 identical diploid cells (2n). For growth and repair. Meiosis: 1 cell → 4 haploid gametes (n). For reproduction. Meiosis has two rounds of division (Meiosis I and II). Crossing over in Meiosis I creates genetic diversity.
Sex-Linked Inheritance
Sex-linked traits: females are XX, males are XY. Sons get X from mom, Y from dad.
Sex-Linked Inheritance
Why X-linked conditions affect males more than females
Males (XY) only have one X chromosome — if it carries a recessive allele, the trait is expressed (no second X to mask it). Females (XX) need two copies of the recessive allele to be affected. Color blindness and hemophilia are classic X-linked recessive examples.
Types of Mutations
Mutations: point mutation changes one base. Frameshift (insertion/deletion) shifts everything after it.
Types of Mutations
Two categories of DNA mutations — one is usually more damaging
Point mutation: one nucleotide substituted. May be silent (same amino acid), missense (different amino acid), or nonsense (creates stop codon). Frameshift: insertion or deletion shifts the reading frame → completely different amino acid sequence from that point on → usually nonfunctional protein.
Hardy-Weinberg Equilibrium
Hardy-Weinberg: p² + 2pq + q² = 1. No evolution if population is in equilibrium.
Hardy-Weinberg Equilibrium
The baseline — a non-evolving population's allele frequencies stay constant
p = frequency of dominant allele, q = frequency of recessive allele. p + q = 1. Genotype frequencies: p² (homozygous dominant) + 2pq (heterozygous) + q² (homozygous recessive) = 1. Equilibrium requires: large population, random mating, no mutation, no migration, no selection.
Polygenic Inheritance
Polygenic traits: controlled by many genes → bell curve distribution (height, skin color)
Polygenic Inheritance
When multiple genes together control one trait — produces a range, not categories
Unlike simple Mendelian traits (tall/short), polygenic traits show continuous variation. Height, skin color, intelligence, weight — all influenced by many gene loci plus environment. When you graph the population, you get a normal distribution (bell curve). Environment also plays a significant role.