Monohybrid cross ratio = 3:1

“A pea plant’s yellow-green seed colors split like a traffic light: three yellows to one green.”

In a monohybrid cross with complete dominance, the F₂ phenotypic ratio is 3 dominant : 1 recessive. The genotypic ratio is 1 : 2 : 1, not 3 : 1. This ratio fails when there is incomplete dominance, codominance, or lethal alleles. Trap: applying 3:1 blindly without checking dominance type.

Monohybrid cross ratio = 3:1

Memory trick: 3 dominant :1 recessive → complete dominance only.

Dihybrid cross ratio = 9:3:3:1

“Crossing two heterozygous pea plants, 9 out of 16 seedlings sprout round yellow seeds, the classic 9:3:3:1 garden pattern.”

When two genes assort independently and both parents are heterozygous (AaBb × AaBb), the F2 phenotypic ratio is 9:3:3:1. The ratio fails if the genes are linked or if any parent is homozygous for a locus. Remember it describes phenotypes, not genotypes, so hidden recessive genotypes can be missed.

Dihybrid cross ratio = 9:3:3:1

Memory trick: 9-3-3-1 = 3² : 3 : 3 : 1

Test cross ratio = 1:1

“When you cross a pea plant with purple flowers (unknown genotype) with a white-flowered plant, the seedlings split like a traffic light: half purple, half white.”

A test cross mates the unknown individual with a homozygous recessive (rr). If the gene is autosomal and viable, heterozygotes give a 1:1 phenotypic ratio, confirming the unknown is heterozygous. The trap: ratios deviate when the gene is linked, lethal, or shows incomplete dominance.

Test cross ratio = 1:1

Memory trick: Test = T × rr → expect 1 T : 1 r

Recombination frequency = 1% ~ 1 map unit

“If two eye-color genes on a fruit fly chromosome swap in 1 out of 100 gametes, they are said to be 1 map unit apart, like a centimeter on a ruler.”

Recombination frequency is the percent of recombinant offspring; 1 % ≈ 1 cM (centiMorgan). This linear relation holds only up to about 20-30 cM; beyond 50 % the observed frequency caps because double crossovers restore parental types. The exam trap is to treat any % as map distance without correcting for >50 %.

Recombination frequency = 1% ~ 1 map unit

Memory trick: 1 % = 1 cM, but max 50 %

incomplete dominance vs codominance

“Mixing red and white paint yields pink, while sprinkling red and white sugar crystals shows both colors side by side.”

In incomplete dominance each allele contributes partially, so the heterozygote shows an intermediate phenotype and a 1:2:1 phenotypic ratio in a monohybrid cross. In codominance both alleles are fully expressed, giving a heterozygote that displays both parental phenotypes (e.g., AB blood). The trap is to label any mixed appearance as incomplete dominance; equal expression of both alleles means codominance.

Memory trick: Incomplete = 'in-between'; Codominant = 'both show'

genotype vs phenotype

“Identical twins may share the same eye-color genes, yet one raised in sunny hills may develop darker skin than the other raised indoors.”

Genotype is the exact set of alleles an organism carries; phenotype is the observable trait, resulting from genotype plus environmental influences. For traits affected by environment, identical genotypes can yield different phenotypes. The trap is to infer genotype directly from phenotype without considering environmental modulation.

Memory trick: Genotype = genes; Phenotype = genes + environment (G+E)

linked genes vs unlinked genes

“Two freckles on chromosome 2 inches apart behave like distant cousins, shuffling independently during meiosis, unlike two nearby freckles that travel together.”

Linked genes are on the same chromosome and tend to be inherited together; the nearer they are, the lower the recombination frequency. Genes separated by >~50 cM recombine often enough to appear independent, giving a 9:3:3:1 ratio. The trap: assuming any same-chromosome pair is linked without checking distance.

Memory trick: Close = linked; far >50 cM = independent

Memory trick: Linked = train cars; crossover = track switch.

DNA structure

“Two opposite-direction ladders (5′→3′ and 3′→5′) interlock their rungs A-T and G-C to build the double helix.”

DNA is an antiparallel double helix; A pairs with T (2 H-bonds) and G with C (3 H-bonds). Major/minor grooves arise from strand orientation, guiding protein binding. Trap: forgetting opposite polarity when designing primers.

Memory trick: 5′-to-3′ opposite, AT2 GC3.

DNA structure — common mistake

“A lock (A) fits only its key (T); trying to fit a wrong key (G) won’t turn the DNA lock.”

Correct base pairing is strict: A-T (2 H-bonds) and G-C (3 H-bonds). Mismatches destabilize the helix and are corrected by polymerase proofreading. Trap: assuming A can pair with G or C with T.

Memory trick: A-T two, G-C three, no cross-pair.

Semiconservative replication

“Copying a zipper: one side is pulled straight (leading strand), the other is stitched in patches (lagging strand).”

Semiconservative replication yields each daughter DNA with one parental and one new strand, as shown by Meselson-Stahl. The leading strand is synthesized continuously; the lagging strand forms Okazaki fragments later joined. Trap: forgetting discontinuous synthesis on the lagging strand.

Memory trick: One old, one new; lagging in fragments.

Semiconservative replication — common mistake

“Like repairing a torn notebook, each new page keeps one original half and one fresh half.”

Semiconservative replication means each daughter DNA contains one parental and one newly synthesized strand. The parental strands separate and serve as templates, so after one round there are exactly two old strands. Mistaking it for conservative replication leads to wrong counts of old vs new DNA.

Memory trick: Half old, half new = semiconservative

Central dogma

“DNA is the blueprint, RNA the recipe, and protein the cooked dish in a kitchen.”

The central dogma states that genetic information flows DNA → RNA (transcription) → protein (translation). This one-way flow is due to the chemical nature of nucleic acids and ribosomes. Forgetting the exception of reverse-transcribing viruses gives a wrong answer about DNA synthesis.

Memory trick: DNA → RNA → Protein, no back arrows (except retro)

Central dogma — common mistake

“A 3-D printer (protein) can only print objects from a digital file (RNA), it never creates the file itself from scratch.”

The central dogma states DNA → RNA → protein; nucleic acids, not proteins, serve as templates. Reverse transcription (RNA → DNA) occurs only via specific reverse-transcriptase enzymes in retroviruses, retrotransposons or telomerase, never by a protein acting as a template. Forgetting the enzyme requirement leads to the common error of thinking proteins can synthesize DNA.

Memory trick: DNA→RNA→Protein; only RT enzymes reverse the flow.

Transcription

“RNA polymerase is a typewriter that writes a draft, which eukaryotes later add a cover (cap) and a back page (poly-A).”

During transcription RNA polymerase reads the DNA template strand and synthesizes a pre-mRNA. In eukaryotes this transcript receives a 5′ cap, a 3′ poly-A tail and introns are spliced out before export. Prokaryotes lack these steps, so marking them for bacteria is a trap.

Memory trick: Cap-Tail-Splice = eukaryote only

Transcription — common mistake

“Bacterial mRNA is like a sticky note—no cover page, no index.”

Prokaryotic transcription produces a mature mRNA directly; there is no 5′ capping, poly-A tail, or intron splicing. Their mRNAs are often polycistronic. Adding eukaryotic processing steps to a bacterial gene is a frequent error.

Memory trick: Prokaryote = plain note

Genetic code

“The genetic code is a LEGO set where many brick colors (codons) build the same model (amino acid).”

The code reads three nucleotides (a codon) to specify an amino acid; it is degenerate because several codons code for the same amino acid, and it is nearly universal across species. Assuming each codon is unique or that stop codons code for amino acids is a classic pitfall.

Memory trick: Degenerate = many codons, one amino

Genetic code — common mistake

“Just like a telephone keypad where several letters share one number, many codons code the same amino acid.”

The genetic code is degenerate: 64 codons encode 20 amino acids plus stop signals. Most amino acids are specified by 2–6 codons, allowing silent mutations. Trap: treating each amino acid as having a unique codon leads to wrong predictions about mutation effects.

Memory trick: Degenerate = many codons per AA (2-6).

Translation

“A ribosome is like an assembly line, reading mRNA beads and adding matching beads from tRNA trucks.”

During translation the ribosome moves 5'→3' on mRNA, positioning codons in the A, P and E sites. tRNA anticodons pair with each codon and the growing peptide is transferred from the P-site tRNA to the A-site tRNA. Trap: forgetting the reading frame set by the start codon causes frame-shift errors.

Memory trick: A-P-E: Accept, Peptidyl, Exit sites.

Translation — common mistake

“Think of a lock (codon) and its key (anticodon) – they are opposite, not the same.”

A codon is the three-base sequence on mRNA read 5'→3'; the anticodon on tRNA is complementary and reads 3'→5'. Correct pairing is antiparallel. Trap: mixing up the directions gives mismatched bases in answer keys.

Memory trick: Codon 5'→3', anticodon 3'→5' – opposite.

Lac operon

“The lac operon is like a smart thermostat that turns the heater (genes) on only when the house (cell) detects lactose.”

The lac operon is induced when allolactose (lactose derivative) binds the repressor, freeing the promoter. Low glucose raises cAMP, allowing CAP to bind and enhance transcription. Trap: assuming lactose alone activates the operon ignores catabolite repression by glucose.

Memory trick: Lactose on, glucose off → CAP-cAMP on → genes on.

Lac operon — common mistake

“Imagine a gate that stays closed unless lactose hands the key; glucose can lock it again.”

When glucose is abundant, cAMP levels fall, CAP cannot bind, and even with lactose the lac operon stays off (catabolite repression). Only when glucose is low and lactose present does transcription proceed. Trap: overlooking glucose’s repressive effect leads to incorrect expression predictions.

Memory trick: Glucose high → cAMP low → CAP off → operon off.

Splicing

“Splicing is like editing a film: cut out the boring scenes (introns) and stitch the exciting clips (exons) together.”

Splicing removes introns from pre-mRNA via the spliceosome and ligates exons to form mature mRNA. Alternative splicing can join different exon combinations, producing protein variants. Trap: assuming a single, fixed mRNA ignores alternative splicing possibilities.

Memory trick: Introns out, exons join – spliceosome’s edit.

Splicing — common mistake

“Like a chef sometimes keeps a chili seed for extra spice, cells can retain an intron segment, producing a variant protein.”

Splicing removes introns, usually at GU-AG boundaries; the branch-point A lies ~18–40 nt upstream of the 3' splice site. However, regulated intron retention can export mRNA that is translated, yielding altered proteins. A rare U12-type intron (AU-AC) also exists. The trap: assuming every intron is removed and only GU-AG occurs.

Memory trick: GU-AG default; AU-AC for rare U12 introns; intron retention = keep seed.