Year 10 Science · Biological Sciences · 18m read

DNA, Genes and Inheritance

Ever wondered why you have your mum's eyes but your dad's curly hair? That's all down to DNA—the instruction manual inside every cell that tells your body how to build and run itself. You'll learn how DNA is structured, how genes switch on and off to create different traits, and why some characteristics skip generations or show up differently in boys and girls. Understanding inheritance isn't just fascinating—it's the foundation for everything from predicting genetic diseases to improving crops, and it's key to your VCE Biology success.

Learning objective: Explain DNA structure, gene expression, and patterns of inheritance including sex-linkage.

By the end of this lesson you can:

I describe DNA replication and the central dogma.
I apply Punnett squares including sex-linked traits.
I explain how mutations affect proteins.

DNA is an instruction manual. Every cell carries the full manual. Genes are chapters for building proteins, and mutations are typos that may change the product.

DNA and inheritance

DNA STRUCTURE:
• DNA = deoxyribonucleic acid.
• Shape: DOUBLE HELIX (two strands twisted together).
• Building blocks: NUCLEOTIDES (sugar + phosphate + base).
• Four bases: A (adenine), T (thymine), C (cytosine), G (guanine).
• Base pairing: A–T, C–G.

LEVELS OF ORGANISATION:
• DNA → GENES → CHROMOSOMES.
• A GENE is a section of DNA that codes for a trait.
• Humans have 46 chromosomes (23 pairs).

INHERITANCE:
• ALLELE — a variant of a gene (e.g., brown vs blue eyes).
• HOMOZYGOUS — two same alleles (BB or bb).
• HETEROZYGOUS — two different alleles (Bb).
• DOMINANT (B) masks RECESSIVE (b).

GENOTYPE vs PHENOTYPE:
• GENOTYPE: actual alleles (Bb).
• PHENOTYPE: observable trait (brown eyes).

PUNNETT SQUARES:
• Predict offspring from two parents' alleles.
• Bb × Bb → 25% BB : 50% Bb : 25% bb.
• Phenotypic ratio: 3 brown : 1 blue.

SEX DETERMINATION:
• Humans: XX = female, XY = male.
• Sex chromosomes passed from parents.

GENETIC DISORDERS:
• Cystic fibrosis (recessive).
• Huntington's disease (dominant).
• Sex-linked (colour blindness on X).

Genetics shorthand

Dominant: Capital letter.
Recessive: Lowercase.
Ratios: Genotype 1:2:1, phenotype 3:1 for Bb × Bb.

Quick reference

| Term | Meaning |
|---|---|
| DNA | Double helix; bases A–T, C–G |
| Gene | DNA section coding a protein |
| Allele | Version of a gene |
| Genotype/phenotype | Alleles / observed trait |
| Sex-linked | On X chromosome (e.g. colour blindness) |
| Mutation | Change in DNA sequence |

Worked example (Medium)

Two heterozygous brown-eyed parents (Bb × Bb). What % offspring have BLUE eyes?

Punnett. Bb × Bb → BB, Bb, Bb, bb.
Genotypes. 1 BB : 2 Bb : 1 bb.
Blue phenotype. Blue = homozygous recessive bb = 1/4 = 25%.

Answer: 25% of offspring will have blue eyes.

Molecular Biology Formulas

Central dogma: \text{DNA} \xrightarrow{\text{transcription}} \text{mRNA} \xrightarrow{\text{translation}} \text{Protein}
DNA base pairs: A \leftrightarrow T, \quad C \leftrightarrow G
RNA base pairs: A \leftrightarrow U, \quad C \leftrightarrow G

Worked example (Hard)

A colour-blind father ($X^n Y$) and carrier mother ($X^N X^n$) have children. What proportion of daughters will be colour-blind?

Cross. $X^N X^n$ (mother) $\times$ $X^n Y$ (father).
Daughters' Punnett. Daughters receive $X^n$ from father AND $X^N$ or $X^n$ from mother: 50% $X^N X^n$ (carrier), 50% $X^n X^n$ (colour-blind).
Result. 50% of daughters are colour-blind ($X^n X^n$).

Answer: 50% of daughters are colour-blind.

Common mistakes

Saying transcription produces a protein directly Transcription produces mRNA. Protein is made during TRANSLATION at the ribosome. Fix: Central dogma: DNA → mRNA (transcription, nucleus) → protein (translation, ribosome). Two separate steps.
Writing colour-blind males as $X^n X^n$ instead of $X^n Y$ Males have XY chromosomes. A colour-blind male is $X^n Y$ (hemizygous — only one X). Fix: Males = XY. Colour-blind male = $X^n Y$. $X^n X^n$ is only possible for females.
Treating all mutations as harmful Most mutations are silent or in non-coding regions and have no observable effect. Fix: Silent = same amino acid. Most mutations are neutral. Only some missense/nonsense mutations cause disease.

Practice questions

DNA base pairing rule:
1. A-C, G-T.
2. A-T, C-G.
3. A-G, C-T.
4. Any can pair.
(1 marks)
Explain the difference between genotype and phenotype.
(2 marks)
Haemophilia A is X-linked recessive ($X^H$ = normal, $X^h$ = haemophilia). A carrier mother and normal father have children. What proportion of ALL children have haemophilia?
(3 marks)
A deletion mutation removes one base from a coding sequence. Why is this typically more severe than a substitution mutation?
(2 marks)
Why is DNA replication described as semi-conservative?
(3 marks)

Going Deeper — CRISPR Gene Editing

CRISPR-Cas9 is a gene editing tool adapted from bacterial immune systems.

How it works:
1. Guide RNA (gRNA) is designed to match the target DNA sequence.
2. Cas9 protein uses the gRNA to locate and cut both DNA strands at that site.
3. Cell repair mechanisms allow gene knockout or insertion of new genetic material.

Current applications:
• Sickle cell disease and beta-thalassaemia: curative treatments approved (UK, 2023).
• Cancer immunotherapy: editing T-cells to attack tumour cells.
• Disease-resistant crops and improved livestock.

Ethical issues:
• Germline editing: changes inherited by all future descendants.
• Off-target cuts: Cas9 may cut at unintended genomic locations.
• Designer babies: potential to select non-medical traits.
• Equity of access: expensive therapies only available to wealthy nations.

Australia: OGTR (Office of the Gene Technology Regulator) oversees all genetic modification research under strict guidelines.

Summary

Replication is semi-conservative: each new DNA molecule has one original and one new strand.
Central dogma: DNA → mRNA (transcription, nucleus) → protein (translation, ribosome).
Mutations: substitution (silent/missense/nonsense), frameshift (insertion/deletion).
X-linked inheritance: males (XY) hemizygous — one recessive allele → trait expressed.
Punnett squares earn method marks even when the final answer contains an error.