Fundamentals
The core facts every aspirant should own — each a titled nugget with a real-world story, the concept in plain words, and a memory trick. Works even when the internet doesn't.
300 fundamentals
“Mendel cracked the code of heredity by patiently counting pea plants in a monastery garden.”
A frequent error is thinking both alleles blend rather than segregate. In reality, the laws of dominance, segregation, and independent assortment govern how traits pass to offspring.
Memory trick: alleles separate cleanly into gametes — they don't blend.
“Mendel cracked the code of heredity by patiently counting pea plants in a monastery garden.”
A frequent error is confusing the 3:1 phenotype ratio with the 1:2:1 genotype ratio. In reality, a cross of one trait gives a 3:1 phenotype ratio in the F2 generation.
Memory trick: 3:1 is what you SEE; 1:2:1 is the underlying genotype.
“Mendel cracked the code of heredity by patiently counting pea plants in a monastery garden.”
A frequent error is confusing incomplete dominance with codominance. In reality, heterozygotes show a blended, intermediate phenotype (red × white → pink).
Memory trick: incomplete = blend (pink); codominance = both show (roan, AB blood).
“Mendel cracked the code of heredity by patiently counting pea plants in a monastery garden.”
A frequent error is thinking sex-linked traits affect both sexes equally. In reality, genes on the X chromosome (like those for haemophilia and colour blindness) affect males more often.
Memory trick: males have one X, so a single recessive X allele shows up in them.
“Mendel cracked the code of heredity by patiently counting pea plants in a monastery garden.”
Homozygous means two identical alleles (TT or tt); heterozygous means two different alleles (Tt).
“Mendel cracked the code of heredity by patiently counting pea plants in a monastery garden.”
In incomplete dominance the heterozygote is a blend (pink); in codominance both alleles show fully and separately (AB blood group).
“Mendel cracked the code of heredity by patiently counting pea plants in a monastery garden.”
3:1 is the PHENOTYPE ratio of a monohybrid cross; the genotype ratio is 1:2:1.
“The DNA in one of your cells, stretched out, is about 2 metres long — folded into a microscopic nucleus.”
A frequent error is pairing adenine with guanine or cytosine. In reality, a double helix held by complementary base pairing: adenine with thymine, guanine with cytosine.
Memory trick: A–T (2 hydrogen bonds), G–C (3 hydrogen bonds) — always.
“The DNA in one of your cells, stretched out, is about 2 metres long — folded into a microscopic nucleus.”
A frequent error is thinking both strands of the daughter DNA are brand new. In reality, each new DNA molecule keeps one original strand and one newly made strand.
Memory trick: 'semi' — half old, half new in each copy (Meselson–Stahl).
“The DNA in one of your cells, stretched out, is about 2 metres long — folded into a microscopic nucleus.”
Copying a gene's DNA into messenger RNA in the nucleus.
Memory trick: transcription writes the message; translation reads it.
“The DNA in one of your cells, stretched out, is about 2 metres long — folded into a microscopic nucleus.”
A frequent error is confusing transcription (DNA→RNA) with translation (RNA→protein). In reality, copying a gene's DNA into messenger RNA in the nucleus.
Memory trick: transcription writes the message; translation reads it.
“The DNA in one of your cells, stretched out, is about 2 metres long — folded into a microscopic nucleus.”
Reading mRNA codons at the ribosome to build a protein.
Memory trick: a codon is THREE bases → one amino acid.
“The DNA in one of your cells, stretched out, is about 2 metres long — folded into a microscopic nucleus.”
A frequent error is thinking each base codes for one amino acid. In reality, reading mRNA codons at the ribosome to build a protein.
Memory trick: a codon is THREE bases → one amino acid.
“The DNA in one of your cells, stretched out, is about 2 metres long — folded into a microscopic nucleus.”
A codon is a triplet of bases on mRNA that codes for an amino acid; an anticodon is the complementary triplet on tRNA that reads that codon.
“The DNA in one of your cells, stretched out, is about 2 metres long — folded into a microscopic nucleus.”
Transcription makes mRNA from DNA in the nucleus; translation makes protein from mRNA at the ribosome.
“The DNA in one of your cells, stretched out, is about 2 metres long — folded into a microscopic nucleus.”
Purines (adenine, guanine) are double-ring bases; pyrimidines (cytosine, thymine, uracil) are single-ring bases.
“The DNA in one of your cells, stretched out, is about 2 metres long — folded into a microscopic nucleus.”
Replication is semi-conservative — each daughter molecule retains one parental strand.
“Whales still have tiny hip bones — fossils of legs from land-walking ancestors.”
A frequent error is reading it as 'survival of the strongest'. In reality, individuals best adapted to their environment survive and reproduce more, so their traits spread.
Memory trick: it's survival of the best-ADAPTED, not the biggest or strongest.
“Whales still have tiny hip bones — fossils of legs from land-walking ancestors.”
Inherited differences among individuals provide the raw material selection acts on.
Memory trick: variation arises randomly; selection then favours the useful ones.
“Whales still have tiny hip bones — fossils of legs from land-walking ancestors.”
A frequent error is thinking organisms change themselves on purpose to adapt. In reality, inherited differences among individuals provide the raw material selection acts on.
Memory trick: variation arises randomly; selection then favours the useful ones.
“Whales still have tiny hip bones — fossils of legs from land-walking ancestors.”
A frequent error is thinking allele frequencies always drift over time. In reality, allele frequencies in a large, undisturbed population stay constant — no evolution.
Memory trick: deviations from Hardy–Weinberg equilibrium signal that evolution is happening.
“Whales still have tiny hip bones — fossils of legs from land-walking ancestors.”
Homologous organs share structure and ancestry but differ in function (human arm vs whale flipper); analogous organs share function but not ancestry (bird wing vs insect wing).
“Whales still have tiny hip bones — fossils of legs from land-walking ancestors.”
Natural selection acts on inherited variation over generations; Lamarckism (disproved) claimed traits acquired in life are passed on.
“Whales still have tiny hip bones — fossils of legs from land-walking ancestors.”
Divergent evolution makes related species differ (homologous organs); convergent evolution makes unrelated species resemble each other (analogous organs).