Reproduction in Animals

Q1 — Short answer (4 marks)

Sample top-band response: Gametes (sperm and eggs) are haploid, meaning they each contain one set of chromosomes (n) [1]. At fertilisation, a male and a female gamete fuse, combining their two sets of chromosomes to form a diploid zygote (2n) [1]. This restores the species' normal chromosome number in the next generation [1]. Restoring diploidy is essential because correct chromosome number is required for normal development, gene dosage and meiosis in adulthood — without it, embryos cannot develop into reproductively functional adults, and the species cannot continue from one generation to the next [1].

Marking notes:

• Band 3 (1–2 marks): Identifies that fertilisation joins sperm and egg but does not use "haploid" or "diploid" precisely.
• Band 4 (3 marks): Uses the haploid → fusion → diploid sequence correctly but does not connect diploidy to continuity of species.
• Band 5–6 (4 marks): All of the above, plus explicit link to continuity of species via correct chromosome number, normal development and ability of offspring to reproduce.

Q2 — Long answer (8 marks) sample top-band response, annotated

Neither external nor internal fertilisation is universally superior; each is the more effective strategy in the environmental context to which it is adapted, and both successfully maintain continuity of species. [1 mark — clear comparative claim that rejects a single "winner"]

External fertilisation, used by mass-spawning corals such as Acropora on the Great Barrier Reef, involves the simultaneous release of millions of haploid gametes into seawater on synchronised environmental cues such as lunar cycle and sea-surface temperature. [1 mark — named external example with mechanism] Because gametes are diluted, eaten and damaged by currents, the probability of any one sperm meeting any one egg is very low, so corals compensate by producing enormous numbers of gametes and synchronising release across the reef to push local sperm concentration into a productive range. [1 mark — explains low per-gamete success and the role of numbers and synchrony]

Internal fertilisation, used by mammals such as the eastern grey kangaroo, transfers sperm directly into the female reproductive tract where it meets the egg in a moist, protected internal environment. [1 mark — named internal example with mechanism] Because gametes are sheltered from desiccation, predation and dilution, the probability of fertilisation per gamete is much higher, so fewer gametes are required and energy can be redirected into much greater parental investment per offspring — gestation, lactation and prolonged care. [1 mark — explains high per-gamete success and links to parental investment]

The two strategies therefore differ on three trade-offs: gamete number (many vs few), reproductive environment (aquatic vs terrestrial) and per-offspring investment (low vs high). [1 mark — explicit comparison on multiple criteria] External fertilisation is the more effective strategy in stable aquatic habitats where huge gamete release and synchronisation can offset losses; internal fertilisation is the more effective strategy on land, where free gametes would desiccate almost immediately and parental protection is needed. [1 mark — environment-dependent judgement] In both cases the strategy ensures that enough offspring survive to reach reproductive age to maintain the species' chromosome complement, genetic variation and population — i.e. continuity of species. [1 mark — link back to continuity of species]

Marking criteria summary: 8/8 = balanced comparative judgement + 2 specific real examples + at least 3 trade-off criteria (gamete number, environment, parental investment) + explicit link to continuity of species. 6–7 marks = strong evidence but unbalanced judgement or only one trade-off criterion. 4–5 marks = mostly description with limited evaluation. 1–3 marks = recall only, no evaluation.

Q3 — Evaluate the claim (4 marks)

Sample response: The claim is partly correct but largely flawed. [clear evaluative judgement]

Correct elements: Internal fertilisation does generally have a higher per-gamete success rate, because sperm are placed close to the egg in a protected environment, and it is used by all "advanced" terrestrial vertebrates (reptiles, birds, mammals). [acknowledges what is right]

Wrong elements: (i) "Always produces more offspring" is incorrect — internal fertilisation typically produces fewer offspring per reproductive event, not more, because energy is invested in protection rather than gamete number; mass-spawning corals can release millions of gametes in a single event. (ii) Calling external fertilisation "primitive" is wrong — the lesson explicitly identifies this as a common error. External fertilisation is a strategy well adapted to aquatic environments and remains highly successful in many fish, amphibians and marine invertebrates today. Frogs and fish are not evolutionarily primitive; they are simply adapted to different reproductive environments. (iii) "Biologically superior" presupposes a single ranking — but in biology a strategy's effectiveness is judged against its environment. [corrects each false claim using lesson content]

Justified judgement: A better claim would be: "Internal fertilisation is the more effective strategy in terrestrial environments, while external fertilisation is the more effective strategy in aquatic environments where gametes can be released in large numbers and dispersed." [offers a better-formed alternative]

Marking notes per band:

• Band 3 (1 mark): States the claim is wrong but does not specify which parts or why.
• Band 4 (2 marks): Identifies at least one specific error (e.g. "external fertilisation is not primitive") with a correct reason.
• Band 5 (3 marks): Identifies multiple errors, distinguishes correct from incorrect parts, and uses lesson terminology accurately (per-gamete success, parental investment, environment-dependent effectiveness).
• Band 6 (4 marks): All of the above, plus reformulates the claim into a biologically defensible alternative that acknowledges environmental context and the trade-offs identified in the lesson.