Biology • Year 12 • Module 5 • Lesson 5
Manipulating Reproduction in Agriculture
Apply selective breeding, artificial insemination, embryo transfer and controlled pollination to real yield data, a real case study, and a real prediction problem.
1. Interpret real-yield data — Australian dairy breeding programmes
The table below shows mean annual milk yield per cow in three Australian commercial dairy herds run on different reproductive strategies, recorded over five lactation seasons. 7 marks Band 4
Stylised data adapted from Dairy Australia "InCalf" benchmarking reports (2019–2023) and Pryce et al. (2014), J. Dairy Sci. 97: 1342–1356.
| Season | Herd A — natural service only (L/cow/yr) |
Herd B — AI from local sires (L/cow/yr) |
Herd C — AI from imported elite sires + ET (L/cow/yr) |
|---|---|---|---|
| 1 (baseline) | 5 800 | 6 100 | 6 350 |
| 2 | 5 850 | 6 480 | 7 050 |
| 3 | 5 900 | 6 760 | 7 720 |
| 4 | 5 920 | 7 010 | 8 240 |
| 5 | 5 940 | 7 240 | 8 690 |
1.1 Describe the trend in mean annual milk yield for Herd A, Herd B and Herd C from Season 1 to Season 5. 2 marks
1.2 Using lesson content, explain why Herd C's yield rose much faster than Herd A's even though all three herds began at a similar baseline. Refer to at least two reproductive techniques. 3 marks
1.3 Identify one biological cost that Herd C is most likely to experience by Season 10 if this strategy continues with no other changes, and justify your answer. 2 marks
2. Interpret real data — gene-pool diversity in elite Holstein-Friesian sires
The figure below shows effective population size (Ne) of the Holstein-Friesian dairy cattle breed over the past 60 years, a measure of genetic diversity in the breeding stock. The decline coincides with the worldwide adoption of artificial insemination and frozen-semen storage from chosen elite sires. 6 marks Band 4–5
Figure adapted from Stachowicz et al. (2011), J. Dairy Science 94: 5160–5175 and FAO (2015), The Second Report on the State of the World's Animal Genetic Resources.
2.1 Describe the trend in effective population size of Holstein-Friesian cattle between 1960 and 2020. 2 marks
2.2 Estimate Ne in 1975 and again in 2020, and calculate the approximate percentage decrease. 2 marks
2.3 The FAO classifies a livestock breed as "at risk" once Ne falls below 50. Use the graph and lesson content to explain how the same technique (AI from elite sires) can both raise milk yields and push a breed across this threshold. 2 marks
3. Case study — Dolly the sheep and the limits of cloning in livestock
On 5 July 1996, scientists at the Roslin Institute near Edinburgh used somatic-cell nuclear transfer to produce Dolly, a Finn-Dorset lamb genetically identical to the donor ewe. Of 277 reconstructed embryos transferred into surrogate ewes, only 29 implanted, three reached late gestation, and one lamb was born alive. Dolly developed early-onset arthritis at five years and died at six and a half — about half the typical Finn-Dorset lifespan. Despite the technical breakthrough, no major commercial sheep herd has since adopted whole-animal cloning: most producers continue to use artificial insemination and embryo transfer from elite naturally-bred parents instead. 5 marks Band 4–5
In 4–6 sentences, explain why this case shows that techniques for "manipulating reproduction" in agriculture span a spectrum from low-risk (AI) to high-risk (whole-animal cloning), and why most commercial breeders currently choose the lower-risk end of that spectrum. Use lesson terminology including gene pool, elite sire and animal welfare.
4. Predict-and-justify — wheat monoculture meets a new rust strain
A regional wheat industry uses controlled pollination over twenty years to fix a single high-yielding, drought-tolerant cultivar across 90% of its growing area. In Year 21, a new strain of stem rust (Puccinia graminis) arrives that overcomes the resistance gene carried by this cultivar. 4 marks Band 5
Predict the likely outcome for total regional wheat yield in the following three growing seasons, and justify your prediction using lesson content on the trade-off between uniformity and gene-pool diversity.
Q1.1 — Trend description (2 marks)
Herd A (natural service) shows essentially flat yield (5800 → 5940 L/cow/yr; ~2.4% rise over 5 seasons). Herd B (AI from local sires) shows a steady increase (6100 → 7240 L; ~19% rise). Herd C (imported elite AI + ET) shows the steepest rise (6350 → 8690 L; ~37% rise) — almost double Herd B's gain over the same period.
Q1.2 — Why Herd C accelerates fastest (3 marks)
Artificial insemination from imported elite sires lets Herd C concentrate genes from internationally proven high-yield bulls into every breeding event, instead of being limited to whichever local bull is available [1]. Embryo transfer further multiplies offspring from the herd's own elite donor cows — every season many surrogates can produce calves from a single top-genetic-merit donor, so favoured alleles spread through the herd faster than the donor could carry them herself [1]. Combined, these two techniques compress what would otherwise be several generations of selective breeding into one or two, accelerating the rise in mean yield [1].
Q1.3 — Biological cost by Season 10 (2 marks)
The most likely cost is a narrower gene pool and rising homozygosity, because so many calves descend from a small number of imported sires and elite donor cows [1]. This typically appears as a measurable rise in inbreeding-related problems — for example reduced fertility, mastitis susceptibility or lameness — and a fall in the herd's resilience to any new disease that targets the shared genotype [1]. Accept also: welfare problems linked to extreme milk-yield selection (e.g. metabolic stress).
Q2.1 — Trend description (2 marks)
Effective population size Ne in the Holstein-Friesian breed has fallen steadily over the 60-year period, from ~150 in 1960 to ~40 in 2020 [1]. The steepest decline coincides with the worldwide adoption of frozen-semen AI from ~1975 onward [1]. The 2020 value sits below the FAO "at-risk" threshold (Ne = 50).
Q2.2 — Read-off values + percentage change (2 marks)
1975 Ne ≈ 120; 2020 Ne ≈ 40 [1]. Percentage decrease ≈ (120 − 40) / 120 × 100 ≈ 67% [1]. Accept ±5 either side of each read-off and ±5% on the final answer.
Q2.3 — Same technique, opposite outcomes (2 marks)
AI from elite sires spreads the alleles of a few high-merit bulls across the global breed very efficiently, which is why average milk yield per cow has roughly doubled over the same period [1]. But because almost every modern dairy cow now traces back to a tiny number of ancestor bulls (in some lineages just 2–3 individuals), the breed's effective gene pool collapses — meaning the same technique that raised yield also pushed Ne below the FAO "at-risk" threshold by reducing how many genetically distinct individuals are contributing to the next generation [1].
Q3 — Case study sample response (5 marks)
Dolly the sheep shows that reproductive manipulation is not a single technique but a spectrum. At the low-risk end, AI and embryo transfer simply move existing gametes or embryos between animals — fertilisation is normal, the offspring are genetically variable mixes of two parents, and welfare costs are well understood. At the high-risk end, somatic-cell nuclear transfer cloning produces a calf or lamb genetically identical to a single donor; success rates are low (Dolly took 277 attempts to make 1 live birth), surrogate ewes carried many failed pregnancies, and the clone itself had a much shortened lifespan and early-onset arthritis — clear animal-welfare costs. Whole-animal cloning also collapses the gene pool to a single individual, the opposite of healthy population genetics. Most commercial breeders therefore continue to use AI from elite sires and embryo transfer rather than cloning, because these techniques still concentrate favoured alleles and increase the spread of elite genetics, but they retain meaningful genetic variation, accept much lower welfare risk, and remain economically viable at scale. The lesson's Boundary callout is reflected in industry practice: agriculture mostly chooses the lower-risk end of the manipulation spectrum.
Marking notes: 1 mark for placing AI/ET vs cloning on a risk spectrum; 1 mark for citing the Dolly statistics (success rate or lifespan) to evidence welfare cost; 1 mark for using gene pool correctly in the cloning context; 1 mark for using elite sire correctly in the AI/ET context; 1 mark for the explicit "commercial breeders choose lower-risk end" judgement.
Q4 — Predict-and-justify sample response (4 marks)
Total regional wheat yield is predicted to collapse sharply across the three following seasons. Because controlled pollination has fixed a single cultivar across 90% of the growing area, the regional crop is genetically uniform and shares the same resistance gene; a new rust strain that overcomes that gene therefore meets almost no resistance and can spread rapidly across the whole region [1]. By the end of Season 1 after arrival, infected fields will lose a significant fraction of yield to rust pustules and grain shrivelling; by Season 2 the rust is likely to be endemic and yields may fall to a fraction of baseline; by Season 3, without a new resistance cultivar, the region remains highly vulnerable [1]. This is the exact trade-off described in Card 4 — uniformity raised yield in the favourable years but reduced resilience when conditions changed [1]. A more resilient strategy would have been to maintain a wider gene pool (multiple cultivars or controlled pollination programmes that include diverse resistance backgrounds) so that no single pathogen could affect the whole crop simultaneously [1].