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Chemistry & Biology · NCEA Level 1

92022 · Genetic variation

Understand how genetic variation arises and is inherited for a characteristic.

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Key concepts

  • DNA, chromosomes, genes and alleles fit inside each other

    DNA is the long molecule that carries the coded instructions for building and running an organism. A chromosome is one very long DNA molecule packaged up with protein; humans have 46 of them, arranged in 23 pairs, with one of each pair inherited from each parent. A gene is a section of a chromosome carrying the instruction for one characteristic. An allele is one of the different versions a gene can come in - the fleece-colour gene in sheep, for example, has a white allele and a black allele. Because chromosomes come in pairs, an organism carries two alleles of each of those genes, one on each chromosome of the pair. Getting this nesting right - DNA, then chromosome, then gene, then allele - clears up most of the confusion in this standard.

  • Genotype and phenotype are not the same thing

    The genotype is the pair of alleles an organism carries for a characteristic, written as two symbols such as Ww. The phenotype is the characteristic you can actually observe. The genotype sets what is possible, and the environment often decides where within that range the organism ends up: two lambs with identical alleles for growth rate will still differ in weight if one had better feed. The relationship also runs one way only. A phenotype can be changed without touching the genotype - dyed wool, a pruned tree, a well-fed calf - and none of those changes are passed on to the next generation.

  • Dominant, recessive, homozygous, heterozygous - and carriers

    An organism is homozygous when its two alleles are the same (WW or ww) and heterozygous when they differ (Ww). A dominant allele shows in the phenotype whenever it is present, so WW and Ww sheep both have white fleece. A recessive allele only shows when there is no dominant allele masking it, so only ww sheep are black. That is what makes a heterozygous animal a carrier: it looks exactly like the dominant type but can still pass the recessive allele to its offspring, which is how a black lamb appears in a flock of white sheep. Dominant does not mean common, stronger or better - the polled (hornless) allele in cattle is dominant, yet whole breeds are horned.

  • Meiosis, independent assortment and random fertilisation

    Body cells carry chromosomes in pairs, but a gamete must carry only one of each pair - otherwise the number would double every generation. Meiosis is the cell division that makes gametes: it halves the chromosome number, so each gamete receives just one allele from each pair, and it produces four genetically different cells from one. Two further steps multiply the variety. Independent assortment means each chromosome pair lines up and separates independently of the others, so the mix of chromosomes inherited from each parent is different in every gamete - with 23 pairs that alone allows over 8 million combinations. Random fertilisation then means any one sperm may fertilise any one egg. Between them these processes explain why siblings differ, without a single new allele being created.

  • Mutation is where new alleles actually come from

    A mutation is a change in the base sequence of an organism's DNA. This matters because meiosis and fertilisation only reshuffle alleles that already exist; mutation is the one process that produces a version of a gene that was not there before. Most mutations have no noticeable effect, some are harmful, and occasionally one turns out to be useful in a particular environment. Mutagens such as ionising radiation and some chemicals raise the rate at which they occur. Two points are worth stating carefully in an answer: only a mutation in a gamete, or in the cells that produce gametes, can be inherited - a mutation in a skin cell cannot - and a mutation is never produced because an organism needs it. It happens at random, and the environment decides afterwards whether it helps.

  • Punnett squares: what they predict, and what they don't

    A Punnett square is simply a table of every way the parents' gametes can combine. Write each parent's genotype, work out the two gamete types each can make, put one parent's along the top and the other's down the side, then fill each box with the pair of alleles that combination produces. Every box is one equally likely outcome. A Ww × Ww cross gives WW, Ww, Ww and ww - a genotype ratio of 1 : 2 : 1 and a phenotype ratio of 3 white to 1 black. But the 3 : 1 is a probability that applies to each offspring separately, not a promise about a particular litter: a ewe with four lambs can easily have none black, or two. Predicted ratios only show up reliably across large numbers.

  • Reading a pedigree

    A pedigree is a diagram of descent across generations - squares for males, circles for females, shading for individuals showing the characteristic, and one row per generation. The single most useful move is this: if two parents that do not show a characteristic produce offspring that does, the characteristic must be recessive and both parents must be heterozygous carriers. From there, work outwards from the individuals you can be certain about, because anyone showing a recessive characteristic must be homozygous recessive. Recording descent this carefully is not only a scientific habit; whakapapa traces the same kind of relationships between people, and the reasoning about who is descended from whom works in exactly the same way.

  • Discontinuous and continuous variation

    Discontinuous variation falls into distinct categories with nothing in between: a beast is horned or polled, a fleece is black or white, a blood group is A, B, AB or O. It is usually controlled by one gene, or a very small number, and the environment barely affects it - so it is displayed on a bar chart. Continuous variation can take any value within a range: height, mass, milk yield, leaf length. It is controlled by many genes acting together and is strongly influenced by conditions such as feed, light and shelter, so it is displayed on a histogram and usually forms a spread with most individuals near the middle. Deciding which of the two you are looking at also tells you whether a Punnett square is even the right tool - it is not, for a continuous characteristic.

  • Variation plus selection changes a population over time

    Every population already contains variation. When conditions change - a new disease arrives, a chemical is used, a predator appears - the individuals whose alleles happen to suit the new conditions survive and reproduce more often, and pass those alleles on. Generation by generation the proportion of the population carrying them rises. Note carefully what has changed: the population, not any individual. No animal became resistant during its lifetime; the resistant ones simply left more offspring. The same mechanism explains drench resistance in sheep parasites, antibiotic resistance in bacteria, and the differences that build up between isolated populations of a native species. Selective breeding works the same way with a grower choosing the parents instead of the environment - which is how distinct kūmara cultivars and taewa varieties were selected and kept true over many generations.

  • One response, judged as a whole

    Because 92022 is external and graded by grade-score marking, a question is answered with one connected response that is read and judged as a single piece of work, rather than being scored line by line. That changes what a strong answer looks like. Achieved is describing the genetics accurately - naming the alleles, drawing the cross, reading the ratio. Merit is explaining how and why: linking the alleles in the gametes to the genotypes in the offspring, and the genotypes to the phenotypes. Excellence is pulling the ideas together and justifying a conclusion using the specific evidence in the question - saying what the data does and does not prove, and why. Length is not the difference between the bands; the depth of the link between evidence and conclusion is.

Assessment

External · grade-score marked.

This is an external achievement standard. Whetū offers original exam-style questions marked on the NCEA grade-score scale. Grades are estimates, not official results.

A mock paper is available in Whetū: Mock paper · Genetic variation.

Learn

4 authored Learn units for this standard.

  • DNA, genes and alleles

    This unit is about the words that sit underneath every later question in 92022. An identified characteristic is a trait that shows differences or similarities in how organisms look or are built — fleece colour, rust response, whether a person produces a working membrane protein. The standard always asks about a characteristic like that, and the first job is to name the inherited material that carries it.

  • Where genetic variation comes from

    A rust-scarred mānuka beside a clean one, a worm that survives a drench its neighbours cannot, a lamb whose fleece texture has never been seen in the flock: each of those stories needs a source. A source is an origin or factor that significantly contributes to variation. For one organism the official sources are mutation and sexual reproduction. For a group they are small population size, differing rates of survival, migration, and non-random mating.

  • Tracking genes through generations

    Once variation exists, the next job is to follow it. Gene tracking is any method that identifies the presence or absence of genes, genetic markers or DNA sequences in an individual or a group. A purpose comes first: family planning, a health decision, matching a profile, spotting an inbred group, or planning a conservation or orchard cross so that a characteristic is kept or avoided.

  • Variation in populations over time

    A population is a group of individuals of the same species, in the same area, that can interbreed. Allele frequency is the share of all copies of a gene in that group that are a particular allele. Those two terms are required knowledge for this standard. They turn a pile of genotypes into a number the group can be compared with later.

Practise

32 Practise questions in “Genetic variation”. Feedback here is formative and is not an official NCEA grade.

  • DNA, genes and alleles

    Fit DNA, chromosomes, genes and alleles inside one another correctly.

  • Genotype and phenotype

    Separate the alleles an organism carries from the characteristic you can see.

  • Sources of variation

    Name what shuffles existing alleles and what creates new ones.

  • Meiosis and fertilisation

    Explain how gametes halve the chromosome number and mix the alleles.

  • Mutation

    Say what a mutation is, when it happens and when it can be inherited.

  • Punnett squares

    Build a cross from gametes and read its genotype and phenotype ratios.

  • Inheritance patterns

    Work out dominance, carriers and the offspring a cross should give.

  • Reading pedigrees

    Deduce genotypes from a family diagram across generations.

  • Variation in populations

    Tell discontinuous from continuous variation and explain what causes each.

  • Selection over time

    Explain how a selection pressure changes a population across generations.

Sample questions

  1. Which statement correctly describes how chromosomes, genes and alleles relate to each other?
  2. Match each term to what it means.
  3. Why does a sheep carry two alleles of the gene for fleece colour rather than one?
  4. Which of these describes a genotype rather than a phenotype?
  5. Which student is correct?
  6. Explain how this difference is possible.
  7. Which process is the only source of a genuinely new allele?
  8. Put these stages in order, from where an allele originally comes from to the offspring that inherits it.

Practise 92022 in Whetū

Exam-style questions

373 original exam-style questions in Whetū. These are practice papers, not official NZQA assessments.

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