Worked Solutions
Biology — Worked Solutions (Year 10 Science)
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Worked examples for Year 10 Science biology. Each shows where the marks are awarded, the key idea, and the full solution explained by your choice of tutor — Stella, Ella or Cassie.
How to use these
Try each question first, then check your working. Use the tutor tabs to read the full 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.
Use clear genetics terms — allele, genotype and phenotype — and set out Punnett squares neatly.
Example 1 — Punnett square for flower colour
Question
In pea plants, the allele for purple flowers ($P$) is dominant over the allele for white flowers ($p$). Two heterozygous purple-flowered plants ($Pp$) are crossed. Draw a Punnett square for this cross and state the expected ratio of purple to white offspring.
Solution
Both parents are $Pp$, so each can pass on either $P$ or $p$. Set up the Punnett square with $P$ and $p$ on each side:
| P | p | |
|---|---|---|
| P | $PP$ | $Pp$ |
| p | $Pp$ | $pp$ |
Genotypes: $1\ PP : 2\ Pp : 1\ pp$. The first three carry a dominant $P$, so they show purple; only $pp$ is white.
Phenotype ratio = 3 purple : 1 white. Don't confuse genotype and phenotype ratios — the question asks for the flower colour.
Each parent is $Pp$, which means it carries one of each allele and passes just one to each offspring — so the possible gametes are $P$ or $p$. We put these along the top and side of a Punnett square.
| P | p | |
|---|---|---|
| P | $PP$ | $Pp$ |
| p | $Pp$ | $pp$ |
Reading the boxes gives genotypes $PP$, $Pp$, $Pp$ and $pp$. Because $P$ is dominant, any plant with at least one $P$ looks purple — that's three of the four. Only $pp$, with no dominant allele, is white.
So the expected ratio is 3 purple : 1 white. The 3:1 pattern is the classic result whenever two heterozygotes are crossed.
Parents $Pp \times Pp$; gametes $P$ or $p$.
| P | p | |
|---|---|---|
| P | $PP$ | $Pp$ |
| p | $Pp$ | $pp$ |
- Genotypes: $1\ PP : 2\ Pp : 1\ pp$
- $P$ dominant → $PP, Pp, Pp$ purple; $pp$ white
Phenotype ratio: 3 purple : 1 white.
Where the marks go
- 1 mark: Correct gametes ($P$ and $p$) for both parents
- 1 mark: Correctly completed Punnett square ($PP$, $Pp$, $Pp$, $pp$)
- 1 mark: Identifies that any plant with a $P$ allele is purple
- 1 mark: Correct phenotype ratio 3 purple : 1 white
Key idea
A cross of two heterozygotes ($Pp \times Pp$) gives a 3:1 phenotype ratio because the dominant allele is expressed whenever it is present.
Example 2 — Natural selection in moths
Question
A population of moths contains both light and dark individuals. Pollution darkens the tree trunks where they rest. Over many generations the dark moths become far more common. Explain this change using the theory of natural selection.
Solution
There is natural variation in the population — some moths are light, some dark.
On darkened trunks the dark moths are better camouflaged, so birds eat fewer of them. The light moths stand out and are eaten more — they are selected against.
Surviving dark moths reproduce and pass the dark-colour alleles to their offspring. Over many generations the proportion of dark moths rises.
Hit all four steps — variation, selection pressure, survival/reproduction, inheritance — to earn full marks.
Natural selection starts with variation: within the moth population some individuals are light and some are dark, due to differences in their genes.
When pollution darkens the trunks, that variation suddenly matters. Dark moths blend in and are harder for predatory birds to spot, so more of them survive. Light moths are now easy to see and are eaten more often — this is the selection pressure.
The moths that survive go on to breed, passing their dark-colour alleles to the next generation. Because survival depends on colour, each generation has a higher proportion of dark moths, and over many generations the population becomes mostly dark.
The key is that the environment didn't change the moths — it changed which moths were most likely to survive and reproduce.
- Variation: light and dark moths exist in the population.
- Selection pressure: darkened trunks camouflage dark moths; light moths more visible to predators.
- Survival/reproduction: dark moths survive and breed more.
- Inheritance: dark-colour alleles passed on; proportion of dark moths rises over generations.
Where the marks go
- 1 mark: Identifies natural variation in moth colour
- 1 mark: Explains the selection pressure (camouflage/predation favours dark moths)
- 1 mark: Links survival and reproduction to inheritance of the allele over generations
Key idea
Natural selection acts on existing variation: individuals best suited to the environment survive, reproduce and pass on their alleles, shifting the population over generations.