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NSWBiologySyllabus dot point

Inquiry Question 1: How does mutation introduce new alleles into a population?

Investigate the causes of genetic variation relating to the changes and conservation of the DNA sequence including: variations in gametes due to crossing over and segregation in meiosis, the cell replication processes that allow the conservation, variation and mutation of DNA, and the contribution of mutation to genetic variation and evolution

A focused answer to the HSC Biology Module 6 dot point on the sources of genetic variation. Meiotic shuffling (independent assortment, crossing over, random fertilisation), DNA replication fidelity, mutation as the ultimate source of new alleles, and the link to natural selection and evolution.

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

NESA wants you to explain where genetic variation comes from: the shuffling that happens in meiosis, the fidelity of DNA replication, and the role of mutation as the ultimate source of new alleles. The evolutionary significance ties Modules 5 and 6 together.

The answer

Genetic variation has two distinct sources: recombination of existing alleles in meiosis and fertilisation, and new alleles introduced by mutation.

Variation from meiosis

Meiosis produces haploid gametes from a diploid parent, reducing chromosome number by half. Three processes generate variation.

1. Independent assortment
During metaphase I, the homologous chromosome pairs line up at the spindle equator independently of one another. Either the maternal or the paternal chromosome of each pair can face either pole. With nn chromosome pairs, this produces 2n2^n possible combinations. In humans (n = 23), that is 2232^{23} = 8,388,608 combinations per gamete.
2. Crossing over
During prophase I, homologous chromosomes pair up (synapsis) and exchange segments at chiasmata. This recombines maternal and paternal alleles within each chromosome, generating combinations that were not present in either parent. Crossing over essentially makes the 2n2^n figure an underestimate; the actual number of unique gametes is astronomically larger.
3. Random fertilisation
Any one of the millions of possible sperm can fertilise any one of the possible eggs, multiplying the variation across the population.

These mechanisms generate enormous variation within a generation, but they all act on existing alleles. They cannot create alleles that are not already present in the parents.

Variation from DNA replication and conservation

Semi-conservative replication
Each daughter DNA molecule retains one original strand and one newly synthesised strand. This conserves the parental sequence.
Proofreading
DNA polymerase has a 3' to 5' exonuclease activity that removes incorrectly added bases during synthesis, reducing the error rate from about 1 in 10510^5 (unaided) to 1 in 10710^7.
Mismatch repair
After replication, mismatch repair proteins recognise base mismatches and excise the wrongly inserted base, reducing the error rate further to about 1 in 101010^{10}.

The net effect is that DNA replication is extraordinarily faithful. This conservation is essential for maintaining the genetic information across generations and across the trillions of cell divisions within a single body.

Variation from mutation

Even with proofreading and repair, mistakes accumulate. Mutagens (UV, chemicals, viruses) further increase the rate. In humans, each newborn carries roughly 60 to 100 new mutations not present in either parent. Most are in non-coding regions and have no effect; some are mildly deleterious; a small fraction are advantageous.

Mutation is the only source of completely new alleles. Meiosis can only shuffle what already exists.

The link to evolution

Natural selection requires three things: heritable variation, differential reproduction and inheritance of the advantageous variant.

  • Meiosis generates the variation natural selection acts on in the short term.
  • Mutation supplies new alleles for selection to act on in the long term.
  • Selection, drift and gene flow then change allele frequencies across generations.

Without mutation, evolution would stall once the existing alleles were sorted by selection. With mutation, the genetic toolkit is continually replenished and new traits (and new species) can arise.

Summary table

Source Mechanism Scale New alleles?
Independent assortment Random alignment in metaphase I 2232^{23} in humans No
Crossing over Chiasmata exchange in prophase I Multiplies meiotic variation No
Random fertilisation Any sperm meets any egg Multiplies variation No
DNA replication errors Mis-incorporation by polymerase 60 to 100 per human generation Yes
Mutagens UV, chemicals, viruses, ROS Variable; high in some environments Yes

Examples in context

Example 1. Peppered moth and industrial melanism. Before the Industrial Revolution, almost all Biston betularia (peppered moth) populations near Manchester were the pale "typica" form, well camouflaged on lichen-covered tree bark. A spontaneous mutation in the cortex gene (originally arising in a single moth in the early 1800s, dated by SNP analysis in 2016) produced a melanic "carbonaria" form. As coal soot killed lichens and darkened trees, the carbonaria form became cryptic against soot-stained bark and pale moths became conspicuous. From 1848 to 1900, the carbonaria allele rose from under 2 percent to over 95 percent. This is the canonical example of how a single new mutation, combined with strong directional selection, drives rapid evolution.

Example 2. Bacterial antibiotic resistance in Sydney hospitals. Methicillin-resistant Staphylococcus aureus (MRSA) emerged when a spontaneous mutation in the mecA gene of a Staphylococcus strain altered the penicillin-binding protein's structure so beta-lactam antibiotics no longer bound. NSW Health surveillance data show MRSA bloodstream infection rates rose from negligible in the 1970s to a peak of about 1.4 per 10 000 patient-days by 2005 before infection-control measures pushed them down. Every dose of antibiotic in a Sydney ICU creates strong selection: susceptible bacteria die; resistant bacteria carrying the mecA mutation survive and reproduce. The new allele was rare to begin with, but mutation supplied the variation and selection did the rest.

Try this

Q1. Distinguish between the sources of genetic variation in meiosis and the source of new alleles in a population. [3 marks]

  • Cue. Meiosis (crossing over, independent assortment, random fertilisation) shuffles existing alleles; only mutation creates new alleles.

Q2. A population of 10 000 antibiotic-susceptible bacteria has a spontaneous mutation rate of 1 in 10 million per replication. Calculate the expected number of resistant mutants after 10 generations, assuming each cell divides once per generation. [3 marks]

  • Cue. Cells after 10 generations: 10 000 by 2^10 = ~10 million. Expected mutants: 10 million by 1/10 million = approximately 1 to a few resistant bacteria, enough to seed a resistant lineage under antibiotic pressure.

Q3. Explain why mutation, rather than meiotic shuffling, is the ultimate source of variation in evolution. (a) Define mutation. (b) Distinguish a new mutation from a recombinant gamete. (c) Justify why a closed population without mutation would eventually stop evolving. [1+2+3 marks]

  • Cue. (a) Heritable change in DNA sequence. (b) Recombinant gamete is a new combination of existing alleles; a new mutation creates a never-before-seen allele. (c) Selection and drift only redistribute existing alleles; without new ones, allelic diversity decays.

Exam-style practice questions

Practice questions written in the style of NESA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

2022 HSC7 marksAn allele of a gene is associated with increased lung inflammation and increased chance of death from a virus. Its frequency is South Asian 60.3%, European 15.1%, African 2.4%, East Asian 1.8%. Explain how mutation, natural selection, genetic drift and gene flow could have led to these differences in the gene pools of populations with differing ancestry.
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Address all four mechanisms and tie each to the data. Sample answer: Mutation is likely the underlying origin of the allele, but mutation alone is unlikely to produce such large frequency differences unless environmental mutagens differed between populations (most prevalent in South Asia, ~60%). Natural selection occurs when certain genotypes are more likely to survive and reproduce; if a population was challenged by the virus causing severe inflammation, selection could reduce the allele frequency, as seen in Africans and East Asians. Gene flow: if mixing between populations was rare, limited gene flow (little migration across wide geographic areas) would maintain the different frequencies. Genetic drift: allele frequencies can change by chance, especially in small populations where which individuals breed can shift frequencies; over large geographic areas chance effects average out. Marks: 7 = extensive understanding with thorough explanations of all four processes, using the stimulus; 6 = thorough; 4-5 = sound understanding of processes affecting gene pools; 2-3 = some understanding; 1 = relevant information. Common error: not distinguishing gene flow from genetic drift and not using cause-and-effect tied to the table.

2023 HSC7 marksThe mountain pygmy possum at Mt Buller is critically endangered. A graph shows its population following bushfires (1998-2002) and the introduction of 6 males from Mt Bogong (2007 and 2012). Evaluate how bushfires and the introduction of males from other locations have affected the population size and gene pool of the Mt Buller population.
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Evaluate both factors against population size and gene pool, using the timeline and a judgement. Sample answer (key points): The Mt Buller population was already low (~90) in 1996, so the gene pool had low allele diversity. Bushfires in 1998, 2000 and 2002 further reduced the population and restricted the alleles present; the surviving population formed a new gene pool in which some allele frequencies were lost or amplified - this can cause genetic drift / a bottleneck effect, reducing the population further (2002-2007). Introducing 6 males from the distant, isolated Mt Bogong population produced gene flow, bringing new/different alleles, increasing genetic diversity and the number of suitable adaptations, and the population rose from about 8 individuals in 2007 to ~150 in 2016. Judgement: the increased diversity (genetic rescue) improved the sub-population's capacity to adapt, whereas bushfires reduced both size and diversity. Marks: 7 = extensive understanding of the relationships AND an informed judgement; 5-6 = sound understanding and suitable judgement; 3-4 = understanding of bushfires OR males on size/gene pool; 2 = identifies one relationship; 1 = relevant information. Common error: not using scientific terminology to explain trends or not engaging with all the stimulus.

2025 HSC4 marksTwo graphs show changes in the frequency of an introduced allele in a small population (n = 20) and a large population (n = 2000) over 50 generations. Evaluate the effects of gene flow on the gene pools of the two populations.
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Compare gene flow's effect in small vs large populations and reach an overall judgement. Sample answer: Small populations have limited genetic diversity. Gene flow can benefit the small gene pool by introducing genetic variation; in favourable conditions the new allele can spread quickly and improve adaptability, but in unfavourable conditions it is rapidly removed by natural selection - shown in the small-population graph where the introduced allele's frequency fluctuates dramatically. In the larger population the impact is less pronounced because high genetic variation is already present; gene flow generally still increases diversity, but the graph shows the allele frequency changes little because the introduced allele is a smaller proportion of the gene pool. Judgement: the effect of gene flow can be positive or negative depending on population size and environmental conditions. Marks: 4 = evaluates effects for both small and large populations; 3 = describes effects for both; 2 = describes an effect for either; 1 = relevant information. Common error: misstating the direction of gene flow's effect, or not explaining how population size influences the impact.

2019 HSC5 marksA map shows the percentage of adult Indigenous populations able to digest lactose. The gene producing lactase is usually switched off between ages 2 and 5, but some people remain able to digest lactose for life. With reference to evolution and DNA, provide possible reasons for the distribution shown in the map.
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Use natural selection with a clear selective pressure plus a DNA-level (mutation) explanation. Sample answer: The map shows lactose-digestion ability varies between populations (e.g. much lower in Australia than in Northern Europe). This variation is likely due to natural selection where the presence of milk in the diet is the selective pressure. A mutation to the lactase gene causes continued production of lactase past age five. Adults with this mutation have an increased chance of survival because of the extra nutrition, so they reproduce and pass the mutation on, making it more common in dairying populations. Populations that remained largely lactose-intolerant were less likely to have milk available, so the mutation offered no advantage and did not become common. Marks: 5 = identifies the variation from the stimulus AND gives reasons with detailed reference to evolution and DNA; 4 = some reference to evolution and DNA; 3 = reference to evolution OR DNA; lower bands = less detail. Common error: invoking Lamarckian inheritance instead of natural selection.

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