NCEA 91157: genetic variation and change (Level 2 Biology)

What NCEA 91157 assesses

NZQA says the standard (version 2) covers sources of variation within a gene pool, and factors that change the allele frequency in a gene pool. The sources are selected from:

• mutation as a source of new alleles
• independent assortment, segregation and crossing over during meiosis
• monohybrid inheritance with co-dominance, incomplete dominance, lethal alleles and multiple alleles
• dihybrid inheritance with complete dominance
• the effect of crossing over and linked genes on dihybrid inheritance
• and the factors that change allele frequencies are natural selection, migration and genetic drift

The three grades go up in steps. Achieved defines, describes with annotated diagrams or models, and describes characteristics of or gives an account of genetic variation and change. Merit gives reasons as to how or why genetic variation and change occurs. Excellence links biological ideas about genetic variation and change, which may involve justifying, relating, evaluating, comparing and contrasting, or analysing.

The NCEA 91157 exam in 2026 and 2027

NCEA 91157 is one of three Level 2 Biology externals, with 91156 (life processes at the cellular level) and 91159 (gene expression). The 2026 exam is on Friday 20 November, in the morning session. NZQA's specifications for both years say the exam is on printed paper and 91157 is a 4-credit external.

• Three questions: you answer all three. NZQA's 2025 report says each question had bullet points to guide the structure of your answer, and the strongest candidates answered every bullet.
• Question style: you may need to interpret diagrams and new information, draw diagrams, and write responses of one or more paragraphs. Some questions are resource-based, and you may use annotated diagrams as evidence.
• Use the resource and go beyond it: the report says candidates who linked the resource material to their answers did better, and the highest grades added relevant detail that the resource did not give.
• Stay on the question: the report says material that is related to biology but outside what the question asks cannot be credited.
• Punnett squares: you may need to draw or interpret Punnett squares for any of the specified monohybrid or dihybrid patterns and give the expected proportions of genotype and phenotype as a ratio, fraction, percentage or decimal.
• Included ideas: NZQA says linked genes include sex linkage, mutation as a source of new alleles needs the effect of the mutation on the proteins produced, and genetic drift includes the founder effect and genetic bottlenecks.

How NCEA 91157 is marked

Three questions are each marked N0 to E8, then added to a total out of 24. NZQA's report says the 2025 questions used linked and unlinked genes, crossing over, segregation, migration, founder effect, bottlenecks and inheritance patterns, with bullet points guiding each answer.

NCEA 91157 cut scores

From 2021 to 2023 Achieved started at 8, falling to 7 in 2024 and 2025. Merit has been 13 and Excellence 19 throughout. The scores are where each grade begins, out of 24.

Cut scores 2025

2025 schedule: Achieved from 7, Merit from 13, Excellence from 19. The same as 2024.

Cut scores 2024

2024 schedule: Achieved from 7, Merit from 13, Excellence from 19. Compared with 2023, Achieved came down by 1.

Cut scores 2023

2023 schedule: Achieved from 8, Merit from 13, Excellence from 19. The same as 2022.

Cut scores 2022

2022 schedule: Achieved from 8, Merit from 13, Excellence from 19. The same as 2021.

Cut scores 2021

2021 schedule: Achieved from 8, Merit from 13, Excellence from 19.

Worked examples at each grade

Achieved: a dihybrid cross and a co-dominance cross

Two plants, both AaBb, are crossed. A and B are dominant, and the genes are on different chromosomes. Each parent makes four equally likely gametes: AB, Ab, aB and ab. The Punnett square has 16 boxes, which give the phenotype ratio 9 : 3 : 3 : 1. From 160 seeds you expect about 90, 30, 30 and 10.

For multiple alleles and co-dominance, a parent with blood group AB (genotype IᴬIᴮ) and a parent with group O (genotype ii) can only have children who are Iᴬi (group A) or Iᴮi (group B), in a 1 : 1 ratio. Both Iᴬ and Iᴮ show in the heterozygote, which is what co-dominance means.

Merit: linked genes and crossing over

A plant heterozygous for two genes (AB on one chromosome, ab on the other) is crossed with a plant that is ab/ab. If the genes were on different chromosomes the offspring would be in a 1 : 1 : 1 : 1 ratio. The observed offspring from 100 seeds are 42 AaBb, 42 aabb, 8 Aabb and 8 aaBb. The two parental types are far more common, so the genes are linked: they sit close together on the same chromosome and tend to be inherited together. The 16 recombinant offspring (8 + 8) come from crossing over in meiosis, when the homologous chromosomes swap segments and make new combinations of alleles:

Genes that are far apart on the same chromosome cross over often and behave as if unlinked, which the report says some candidates did not recognise. Mutation, independent assortment and segregation add to the variation, because each gamete gets a different mix.

Excellence: drift or selection?

A population of 1000 mice has the allele a at a frequency of 0.20. A storm leaves 10 survivors by chance, and 8 of their 20 alleles are a:

The frequency has doubled, but a is not shown to be helpful. This is genetic drift through a population bottleneck: the survivors are a small random sample, so their allele frequencies differ from the original and rare alleles can be lost or become common by chance. Compare natural selection, where individuals with a beneficial allele survive and reproduce more, so the allele rises in frequency over generations in a predictable direction. Excellence compares the two, links them to the size of the population, and explains what would be needed to tell them apart. Migration can also change the frequency, by adding alleles from another population.

Now predict some inheritance odds in our Genetics and DNA course, then press Mark to check your answer in the course:

Exam answers

The answers and a worked mark scheme come with today's paper once it is marked. We do not publish official or past paper answers. Every question on the daily paper is our own.

Use the daily paper as a practice exam

The Level 2 Biology track above is a short timed mock of 8 questions, not a full exam and not written by NZQA (NCEA Level 2 Biology). It covers genetic diversity and classification, nucleic acids and DNA replication, and mitosis, which touch 91157, but it has no Punnett square or allele frequency questions, and none on genetic drift. For those use our Genetics and DNA course and the Genetics Cross Calculator.

Is NCEA 91157 changing?

NZQA's 2027 specification keeps Punnett squares, linked genes including sex linkage and drift including bottlenecks in this standard. After 2027, Year 12 moves to a new certificate in 2029.

Common mistakes in NCEA 91157

• Wrong definitions. NZQA's 2025 report lists incorrect definitions, such as for natural selection and genetic drift, as a common Not Achieved error.
• Wrong gametes in the Punnett square. Work out the gametes first, then fill in the grid.
• Thinking unlinked genes must be on different chromosomes. The report says they can be on the same chromosome if far apart.
• Missing multiple alleles. Know the difference between dominant and recessive, and between complete dominance and co-dominance.
• Answering part of each question. Respond to every bullet point and every part.

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