Worked Solutions
Module 5: Heredity — Worked Solutions (HSC Biology)
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Worked examples for HSC Biology Module 5: Heredity. Each shows where the marks are awarded, the key idea, and a full model answer explained by your choice of tutor — Stella, Ella or Cassie.
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Attempt each question first, then check your answer against the model responses. Use the tutor tabs to read the solution in the style that suits you: Stella is direct and challenging, Ella is warm and explains the why, and Cassie is concise and analytical.
Heredity questions reward genetic reasoning. Define your allele symbols, show your Punnett square or pedigree logic, and always link DNA back to the protein it codes for.
Example 1 — Autosomal inheritance and Punnett squares
Question
In pea plants, purple flower colour ($\text{P}$) is dominant over white ($\text{p}$). Two heterozygous purple-flowered plants are crossed. Using a Punnett square, determine the expected phenotypic ratio of the offspring, and explain why a white-flowered offspring can arise from two purple-flowered parents.
Solution
Both parents are heterozygous purple, so $\text{Pp} \times \text{Pp}$.
The Punnett square gives offspring genotypes $\text{PP} : \text{Pp} : \text{pp}$ in a $1 : 2 : 1$ ratio.
Phenotypes: $\text{PP}$ and $\text{Pp}$ are purple (3), $\text{pp}$ is white (1), so the ratio is 3 purple : 1 white.
A white offspring appears because each parent carries one recessive $\text{p}$ allele hidden behind the dominant $\text{P}$. When two $\text{p}$ alleles combine in one offspring ($\text{pp}$), the recessive phenotype is expressed.
Let's set up the cross. Both parents show the dominant purple phenotype but are heterozygous, so their genotype is $\text{Pp}$ — they each carry a hidden recessive allele. The cross is $\text{Pp} \times \text{Pp}$.
Drawing the Punnett square with gametes $\text{P}$ and $\text{p}$ from each parent gives $\text{PP}$, $\text{Pp}$, $\text{Pp}$, $\text{pp}$ — a genotypic ratio of $1 : 2 : 1$.
Now think about what each genotype looks like: $\text{PP}$ and $\text{Pp}$ both show purple (because $\text{P}$ is dominant), and only $\text{pp}$ shows white. That gives a phenotypic ratio of 3 purple : 1 white.
The reason a white offspring can come from two purple parents is the key idea of recessive inheritance — the recessive allele can be carried silently. Each parent passes on a $\text{p}$, and when an offspring inherits two of them it has no dominant allele to mask the white phenotype.
Cross: $\text{Pp} \times \text{Pp}$.
- Gametes: $\text{P}$, $\text{p}$ from each parent
- Offspring genotypes: $1\ \text{PP} : 2\ \text{Pp} : 1\ \text{pp}$
- Phenotypes: purple ($\text{PP}$, $\text{Pp}$) vs white ($\text{pp}$)
- Ratio: 3 purple : 1 white
White from two purple parents:
- Both parents heterozygous → carry recessive $\text{p}$
- $\text{pp}$ offspring inherits one $\text{p}$ from each
- No dominant $\text{P}$ present → recessive white phenotype expressed
Where the marks go
- 1 mark: Correctly identifies parental genotypes as $\text{Pp} \times \text{Pp}$
- 1 mark: Completes a correct Punnett square showing genotypes
- 1 mark: States the correct phenotypic ratio of 3 purple : 1 white
- 1 mark: Explains that white offspring arise when two recessive alleles combine in a $\text{pp}$ genotype
Key idea
A heterozygous cross ($\text{Pp} \times \text{Pp}$) gives a 3 : 1 phenotypic ratio; recessive phenotypes appear when two hidden recessive alleles combine.
Example 2 — DNA and polypeptide synthesis
Question
Explain how the sequence of nucleotides in DNA is used to synthesise a polypeptide, referring to both transcription and translation.
Solution
Two stages: transcription in the nucleus, translation at the ribosome.
Transcription: the DNA double helix unwinds and one strand acts as a template. RNA polymerase pairs free RNA nucleotides to the template, producing messenger RNA (mRNA). The mRNA carries the base sequence as a complementary copy and leaves the nucleus.
Translation: the mRNA binds to a ribosome. Each group of three bases (a codon) specifies one amino acid. Transfer RNA (tRNA) molecules with complementary anticodons deliver the matching amino acids. The ribosome joins the amino acids with peptide bonds in the order set by the codons, building the polypeptide.
So the nucleotide sequence in DNA determines the codon order in mRNA, which determines the amino acid sequence — and that sequence is the polypeptide.
The whole point of this process is that DNA stores the instructions, but the actual working molecules are proteins (polypeptides). Getting from one to the other happens in two steps.
First, transcription. This happens in the nucleus. The DNA helix unwinds and one strand serves as a template. RNA polymerase reads it and assembles a complementary strand of messenger RNA, matching RNA nucleotides to the DNA bases. This mRNA is a portable copy of the gene, and it moves out of the nucleus to the cytoplasm.
Then, translation, at the ribosome. The mRNA is read in groups of three bases called codons, and each codon corresponds to a specific amino acid. Transfer RNA molecules each carry an amino acid and an anticodon that pairs with the codon, so the right amino acid is brought to the right place. The ribosome links these amino acids together with peptide bonds in sequence.
The reason this matters is that the order of nucleotides in DNA directly sets the order of amino acids in the polypeptide — and the amino acid order determines how the protein folds and functions.
Transcription (nucleus):
- DNA unwinds; one strand is the template
- RNA polymerase pairs RNA nucleotides to template
- Produces mRNA (complementary copy); exits nucleus
Translation (ribosome):
- mRNA read in codons (3 bases = 1 amino acid)
- tRNA anticodons pair with codons, deliver amino acids
- Ribosome joins amino acids by peptide bonds in codon order
Result: DNA sequence → mRNA codon sequence → amino acid sequence = polypeptide.
Where the marks go
- 1 mark: Identifies transcription and translation as the two stages
- 1 mark: Describes transcription producing mRNA from a DNA template
- 1 mark: States that codons (triplets) on mRNA each specify an amino acid
- 1 mark: Describes the role of tRNA and the ribosome in assembling amino acids
- 1 mark: Links the DNA nucleotide sequence to the resulting amino acid sequence of the polypeptide
Key idea
DNA is transcribed into mRNA, then translated at the ribosome where codons specify amino acids — so the nucleotide order determines the polypeptide's amino acid sequence.