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NDA Biology · Genetics and Evolution

Heredity, DNA and Genes

Heredity is the passing of traits from parents to offspring through genes — segments of DNA, the double-helix molecule whose A–T and G–C base pairing carries all genetic information.

All Genetics and Evolution notesNDA Biology strategy

Why this matters

This is the foundation of the whole chapter — the NDA tests it as straight recall (base pairing, the meaning of 'allele', who discovered DNA's structure). Three facts carry almost every PYQ: A pairs with T and G pairs with C, an allele is a variant form of a gene, and Watson, Crick and Wilkins shared the 1962 Nobel Prize for the double helix. All EASY or MODERATE — learn the facts, win the marks.

Concept 1 of 5

Heredity — genes, chromosomes and DNA

Intuition

Children resemble their parents because they inherit instructions written in a chemical code. That code is DNA; a meaningful stretch of it is a gene; genes are packaged into thread-like structures called chromosomes inside the nucleus of every cell. Heredity is simply the transfer of these instructions from one generation to the next.

Definition

The key terms of inheritance, smallest to largest:

  • DNA (deoxyribonucleic acid) — the molecule that stores genetic information as a sequence of four bases.
  • Gene — a segment of DNA that codes for one trait or one protein; the basic unit of heredity.
  • Chromosome — a long, coiled thread of DNA wrapped on protein; humans have 23 pairs (46 in total).
  • Genome — the complete set of genes in an organism.

Heredity is the transmission of these genes from parents to offspring; the study of heredity is called genetics (a term coined by William Bateson; Gregor Mendel is the 'Father of Genetics').

Worked example

Arrange from smallest to largest: chromosome, gene, DNA base, genome.
  1. A single DNA base (A, T, G or C) is the smallest unit of the code.
  2. A gene is a sequence of many bases that codes for one trait.
  3. A chromosome is one long DNA thread carrying many genes.
  4. The genome is the entire set of chromosomes/genes in the organism.
Answer:DNA base → gene → chromosome → genome.

Concept 2 of 5

DNA structure and base pairing

Intuition

DNA is shaped like a twisted ladder — the double helix. The two side-rails are made of sugar and phosphate; the rungs are pairs of nitrogen bases. The bases pair in a fixed way, and that fixed pairing is what lets DNA copy itself faithfully every time a cell divides.

Definition

DNA is a double helix of two strands held together by base pairs:

  • The four bases are Adenine (A), Thymine (T), Guanine (G), Cytosine (C).
  • A always pairs with T (2 hydrogen bonds); G always pairs with C (3 hydrogen bonds). This is complementary base pairing.
  • Chargaff's rule follows from it: in any DNA, amount of A = amount of T, and G = C.
  • A and G are purines (double-ring); T and C are pyrimidines (single-ring). A purine always pairs with a pyrimidine, keeping the helix an even width.

In RNA, thymine (T) is replaced by uracil (U), so A pairs with U.

The base-pairing rule

A=TG≡CA = T \qquad G \equiv C
  • A = TAdenine pairs with Thymine via 2 hydrogen bonds
  • G ≡ CGuanine pairs with Cytosine via 3 hydrogen bonds
sugar–phosphatesugar–phosphateAT2 H-bondsTA2 H-bondsGC3 H-bondsCG3 H-bondsAT2 H-bondsGC3 H-bonds

A–T = 2 hydrogen bonds · G–C = 3 hydrogen bonds (so G–C-rich DNA is more stable)

Worked example

One strand of a DNA molecule reads A–G–C–T. What is the sequence of the complementary strand?
  1. Replace each base with its partner: A pairs with T, G pairs with C, C pairs with G, T pairs with A.
  2. A → T, G → C, C → G, T → A.
Answer:The complementary strand reads T–C–G–A.

From the bank · past-year question

Example 2Genetics and EvolutionEASY
Which is the correct base pairing found in a normal DNA molecule?

[Q110 · Apr · 2025]

A pairs with T, not with G or C

The distractors offer Adenine–Guanine, Adenine–Cytosine, Thymine–Guanine. None are valid. A purine pairs only with its specific pyrimidine: A–T and G–C. Mnemonic: 'Apple in the Tree, Car in the Garage' (A–T, C–G).

G–C has 3 hydrogen bonds, A–T has 2

DNA rich in G–C pairs is more stable (harder to separate) because each G–C rung has three hydrogen bonds versus two for A–T.

Concept 3 of 5

Genes, alleles and genotype

Intuition

A gene controls a trait — say, flower colour. But a gene can come in different versions: one version makes red flowers, another makes white. These alternative versions of the same gene are called alleles, and the particular combination an organism carries is its genotype.

Definition

The vocabulary of variation:

  • Allele — one of the different forms (variants) of the same gene; alleles sit at the same locus (position) on homologous chromosomes.
  • Genotype — the actual alleles an organism carries for a trait (e.g. Tt).
  • Phenotype — the visible, expressed trait (e.g. tall).
  • Homozygous — two identical alleles (TT or tt); Heterozygous — two different alleles (Tt).
  • Dominant allele — expressed even when one copy is present (written capital, T); Recessive — expressed only when both copies are recessive (tt).

Worked example

A pea plant has the genotype Tt for height, where T (tall) is dominant over t (short). Is the plant tall or short, and is it homozygous or heterozygous?
  1. The plant carries one tall allele (T) and one short allele (t) → two different alleles → heterozygous.
  2. T is dominant, so a single T is enough to show the tall trait.
Answer:The plant is tall (phenotype) and heterozygous (genotype Tt).

From the bank · past-year question

Example 3Genetics and EvolutionEASY
Different varieties of the same gene are called

[Q112 · Sep · 2021]

Allele vs genotype vs isomer

An allele is a variant of a gene. A genotype is the whole allele combination. An isomer is a chemistry term (same formula, different structure) and never describes genes — it is a classic distractor here.

Concept 4 of 5

Mendel's laws of inheritance

Intuition

Gregor Mendel crossed pea plants and worked out the rules of inheritance long before anyone knew DNA existed. His two laws explain why a trait can disappear in one generation and reappear in the next, and why different traits are inherited independently.

Definition

Mendel's three principles:

  • Law of Dominance — in a heterozygote, only the dominant allele is expressed; the recessive one is masked.
  • Law of Segregation — the two alleles of a gene separate during gamete formation, so each gamete carries only one allele.
  • Law of Independent Assortment — alleles of different genes are distributed to gametes independently of one another.

A monohybrid cross (Tt × Tt) gives a phenotype ratio of 3 : 1 (3 tall : 1 short) and a genotype ratio of 1 : 2 : 1 (TT : Tt : tt).

Worked example

Two heterozygous tall pea plants (Tt × Tt) are crossed. What fraction of the offspring are expected to be short (tt)?
  1. Set up the cross Tt × Tt. Each parent gives T or t with equal chance.
  2. The four equally likely combinations are TT, Tt, tT, tt.
  3. Only tt is short → 1 out of 4.
Answer:1/4 (25%) of the offspring are expected to be short.

3 : 1 is the phenotype ratio, 1 : 2 : 1 is the genotype ratio

A Tt × Tt cross gives 3 tall : 1 short by appearance (phenotype) but 1 TT : 2 Tt : 1 tt by genetic make-up (genotype). The bank can ask for either — read which one is wanted.

Concept 5 of 5

Who discovered the structure of DNA

Intuition

The double-helix model is one of the most famous discoveries in biology, and the NDA likes to test who did what. Watson and Crick built the model, but they relied on Wilkins' and Franklin's X-ray photographs and Chargaff's base-ratio rule. The 1962 Nobel Prize went to Watson, Crick and Wilkins — Franklin had died in 1958 and the prize is not awarded posthumously.

Definition

The scientists behind the DNA story and the contribution the bank tests. Note the trap: the 1962 Nobel Prize was shared by Watson, Crick and Wilkins — not Franklin or Chargaff.

ScientistContribution
James Watson & Francis CrickBuilt the double-helix model of DNA (1953)
Maurice WilkinsX-ray diffraction studies of DNA; shared the 1962 Nobel Prize
Wilkins is the name the 1962-Nobel question asks for — alongside Watson and Crick.
Rosalind FranklinX-ray photograph ('Photo 51') that revealed the helix; died 1958, so not in the 1962 prize
Erwin ChargaffChargaff's rules — in DNA, A = T and G = C
Watson + Crick + Wilkins shared the 1962 Nobel Prize in Physiology or Medicine for the molecular structure of nucleic acids.
Practice this conceptself-check · 4 quick reps

Try it yourself

Whose X-ray photograph was crucial to discovering the helix but who was not awarded the 1962 Nobel Prize, and why?

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Who built the double-helix model of DNA?
  2. 2.
    Which three scientists shared the 1962 Nobel Prize for DNA's structure?
  3. 3.
    Whose rule states that in DNA, A = T and G = C?
  4. 4.
    Who is called the 'Father of Genetics'?

From the bank · past-year question

Example 5Genetics and EvolutionMODERATE
Who among the following shared the Nobel Prize in 1962 along with Francis Crick and James Watson for their discoveries concerning the molecular structure of nucleic acids?

[Q106 · Sep · 2017]

Franklin and Chargaff are the distractors in the 1962-Nobel question

The 1962 Nobel was shared by Watson, Crick and Wilkins. Rosalind Franklin (who died in 1958) and Erwin Chargaff are offered as wrong options — both contributed, but neither shared that prize.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Reference tables (1)

Who discovered the structure of DNA4 rows
ScientistContribution
James Watson & Francis CrickBuilt the double-helix model of DNA (1953)
Maurice WilkinsX-ray diffraction studies of DNA; shared the 1962 Nobel Prize
Wilkins is the name the 1962-Nobel question asks for — alongside Watson and Crick.
Rosalind FranklinX-ray photograph ('Photo 51') that revealed the helix; died 1958, so not in the 1962 prize
Erwin ChargaffChargaff's rules — in DNA, A = T and G = C
Watson + Crick + Wilkins shared the 1962 Nobel Prize in Physiology or Medicine for the molecular structure of nucleic acids.

Watch out for (5)

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