CB1.3 Genetic Variation Comprehensive NCEA Level 1 Study Guide

Standard CB1.3 Overview & Assessment Information

• Assessment Details: Achievement Standard 92022 (CB1.3) is an NCEA Level 1 Chemistry and Biology standard covering genetic variation in relation to an identified characteristic (e.g., CFTR or CCR5 gene mutations).

• Examination Schedule: Held as an online examination worth $5\text{ credits}$ on Thursday, 19 November 2026, during the afternoon session at $2:00\text{ pm}$.

• Requirements for Excellence: Candidates must integrate, link, and justify biological concepts across four main pillars:

  • DNA & Alleles: The molecular relationships between DNA bases, genes, and alleles.

  • Sources of Variation: The origin of new alleles through permanent mutations and new allele combinations created via sexual reproduction (meiosis and fertilization).

  • Gene Tracking: Tracking alleles through successive generations to determine genotypes and phenotypes using pedigree charts and Punnett squares.

  • Population Advantages: Evaluating why genetic variation is necessary for population survival and environmental adaptability.

Molecular Structure of DNA & Genetic Hierarchy

• Scale and Physical Metrics of DNA:

  • Total DNA length across all cells in the human body is approximately $16\text{ billion kilometers}$ ($16,000,000,000\,\text{km}$), equivalent to a round trip from Earth to Pluto.

  • Uncoiled DNA from a single human cell nucleus spans approximately $2\,\text{m}$ in length.

• Genomic Statistics & Shared Sequences:

  • Humans share $99.9\%$ of their DNA sequence with other humans, $99.1\%$ with chimpanzees, and $50.0\%$ with bananas (and parents).

  • The human genome contains approximately $23,000$ to $25,000$ distinct genes.

  • Sequencing the first human genome cost approximately $\$2.7\text{ billion}$, whereas modern sequencing costs around $\$1,000$.

  • Human DNA stores roughly $700\text{ Terabytes}$ ($700,000\text{ Gigabytes}$) of biological data.

  • Cellular DNA undergoes damage over $1,000$ times per day; unrepaired damage can lead to mutations and conditions such as cancer.

  • DNA was first isolated in $1869$.

  • Approximately $8\%$ of human DNA originates from ancient viral insertions.

• Molecular Architecture of DNA:

  • DNA (Deoxyribonucleic Acid) is a double-stranded nucleic acid configured in a double-helix shape.

  • The double helix model was named following X-ray crystallography images taken by Rosalind Franklin.

  • DNA polymers are formed from repeating monomer units called nucleotides.

  • Each nucleotide consists of three subunits:

    1. A phosphate group.

    2. A deoxyribose sugar.

    3. A nitrogenous base.

  • The sugar and phosphate units bond to form the sugar-phosphate backbone of each strand.

• Nitrogenous Bases & Complementary Base Pairing:

  • Four nitrogenous bases exist in DNA: Adenine ($A$), Thymine ($T$), Cytosine ($C$), and Guanine ($G$).

  • Bases pair across strands via hydrogen bonding following strict complementary base-pairing rules:

    • Adenine always bonds with Thymine ($A-T$).

    • Guanine always bonds with Cytosine ($G-C$).

  • Base Proportion Calculations: If Guanine accounts for $10\%$ of the bases in a DNA sample, Cytosine must also account for $10\%$. The remaining $80\%$ is split equally between Thymine ($40\%$) and Adenine ($40\%$).

• Hierarchical Genetic Definitions:

  • DNA: The nucleic acid molecule containing the coded genetic instructions responsible for the development, function, and characteristics of an organism.

  • Chromosome: A long, continuous length of DNA coiled around proteins that contains many genes along its length.

  • Gene: A specific section of a chromosome/DNA sequence that codes for a functional protein and determines a particular trait.

  • Locus: The specific physical location of a gene on a chromosome.

  • Allele: An alternate form or version of a specific gene (e.g., the gene for eye colour may have a brown allele or a blue allele).

• Gene Expression (DNA to Protein to Trait):

  • The coding sequence of a gene dictates the assembly of amino acids into a polypeptide chain.

  • The polypeptide chain folds into a specific three-dimensional protein.

  • Proteins determine an organism's physical characteristics or traits (e.g., structural proteins determining straight vs. curly hair texture).

• Protein Structural Levels:

  • Primary Structure: The linear sequence of amino acids in a polypeptide chain.

  • Secondary Structure: Local folding into $\alpha$-helices or $\beta$-pleated sheets held by hydrogen bonds.

  • Tertiary Structure: Complete 3D folding driven by side-chain interactions.

  • Quaternary Structure: Assemblies containing more than one amino acid chain.

• Dual Allele Origin: Organisms possess two alleles for every gene because one chromosome is inherited from the mother via the egg cell and one from the father via the sperm cell.

Chromosomes, Karyotypes, and Genetic Disorders

• Chromosomal Organization:

  • Human somatic (body) cells contain $46\text{ chromosomes}$ arranged in $23\text{ pairs}$.

  • Each homologous pair contains one maternal chromosome and one paternal chromosome possessing identical genes at corresponding loci.

  • Human cells contain $22\text{ pairs}$ of autosomes and $1\text{ pair}$ of sex chromosomes ($XX$ for biological females, $XY$ for biological males).

  • Species Variations: Domestic dogs possess $78\text{ chromosomes}$ ($39\text{ pairs}$) per cell; domestic cows possess $60\text{ chromosomes}$ ($30\text{ pairs}$) per cell.

• Karyotype Diagrams:

  • A karyotype diagram is a visual photograph of the complete set of chromosomes within a cell nucleus during mitosis, arranged by size, shape, and matching pairs.

  • Used clinically to evaluate chromosome number, structure, and biological sex.

  • Diagnostic testing tissues include amniotic fluid, blood, bone marrow, and placental tissue.

  • Chemotherapy treatments can cause chromosome breaks that alter karyotype results.

• Genetically Linked Disorders & Abnormalities:

  • Down Syndrome: Caused by an extra copy of chromosome 21 (Trisomy 21).

  • Turner Syndrome: Caused by a missing X chromosome in females ($45, X$).

  • Other Disorders: Klinefelter syndrome, Philadelphia chromosome, Trisomy 18, Albinism, Cystic Fibrosis.

  • Methemoglobinemia: An extremely rare blood disorder affecting $0.035\%$ of the global population. Blood loses normal oxygen-carrying capacity and turns brown. Affected individuals with light skin display blue-tinted skin and purple lips.

Sources of Genetic Variation & Mutation Dynamics

• Definition of Variation: The structural, physiological, or functional differences present between individuals of the same species.

• Continuous vs. Discontinuous Variation:

  • Continuous Variation: Variation displaying a continuous range of quantitative values without distinct categories (e.g., height, foot size, hand span). Controlled by multiple genes and environmental factors.

  • Discontinuous Variation: Variation displaying distinct, non-overlapping categories with no intermediate values (e.g., ABO blood groups, left vs. right handedness, ability to roll the tongue, earlobe attachment).

• Inheritable vs. Non-Inheritable Variation:

  • Inheritable Variation: Variations encoded in the base sequence of DNA. Can be passed down to offspring if present in gametes (e.g., natural eye colour, dimples, red leaf pigmentation in Venus flytraps).

  • Non-Inheritable Variation: Variations acquired during an individual's lifetime due to environmental factors or somatic cell changes. Cannot be passed to offspring (e.g., dyed hair colour, acquired scars, surgical tissue alterations, long leaf growth in shaded Venus flytraps, or COVID-19 lung damage).

• Phenotype Determination:

  • An organism's phenotype is determined by its genotype interacting with environmental factors:         $\text{Phenotype} = \text{Genotype} + \text{Environment}$

  • Example: Skin colour is genetically determined, but exposure to UV sunlight (environmental factor) stimulates melanin production, darkening the skin phenotype (tan).

• Mutations:

  • A mutation is a permanent, random change in the base sequence of DNA.

  • Mutations are the ultimate source of all new genetic variation because they create entirely new alleles.

  • Causes: Spontaneous errors during DNA replication or exposure to mutagens.

  • Mutagens: Physical agents (e.g., UV light, radiation) or chemical agents that damage DNA and increase mutation rates.

• Somatic vs. Gametic Mutations:

  • Somatic Mutations: Occur in regular body cells. Impact only the individual organism during its lifetime (e.g., localized tissue dysfunction or cancer). Cannot be passed to offspring.

  • Gametic Mutations: Occur in sex cells (sperm or egg). Can be passed to future generations provided the mutated gamete is fertilized. These introduce new alleles into population gene pools.

• Categories of Mutation Effects:

  • Beneficial: Enhances survival/reproductive success (e.g., antibiotic resistance in bacteria, white fur in polar bears, super-dense bone mutations, lactase persistence).

  • Harmful: Decreases survival (e.g., sickle cell anemia, cystic fibrosis, cancer).

  • Silent/Neutral: Has no immediate effect on phenotype or survival.

  • Environmental Context: Whether a mutation is beneficial or harmful depends on environmental selection pressures.

• Lactose Persistence Case Study:

  • Globally, approximately $65\%$ of human adults are lactose intolerant.

  • A DNA mutation in coding/regulatory sequences created a functional allele enabling adult lactase production.

  • This gametic mutation was transmitted to offspring, providing a metabolic advantage in cultures with dairy farming.

Human Geographic Diversity & Evolutionary Migration

• Human Evolutionary Origins:

  • Out of Africa Theory: Asserts that modern humans evolved in Africa and subsequently migrated across the globe, replacing existing hominids. Supported by fossil records, language/tool distributions, and genomic data.

  • Multiregional Theory: Asserts that modern humans evolved concurrently from regional hominid populations globally.

• Geographic Allele Distribution Trends:

  • Genetic diversity decreases as physical distance from Africa increases.

  • Populations native to Africa display the highest overall genetic variation because human populations have resided in Africa longest, allowing more time to accumulate spontaneous mutations.

• Geographic Adaptations: Phenotypic adaptations aligned with geographic origin include nose morphology, hair texture, skin pigmentation, and pathogen resistance (e.g., the sickle cell allele conferring protection against malaria in equatorial regions).

DNA Replication & Cell Division (Mitosis vs. Meiosis)

• Semi-Conservative DNA Replication Process:

  • Occurs prior to cell division to produce two identical copies of DNA from a single double-stranded molecule.

  • Step 1: Unwinding/Unzipping: The enzyme helicase unwinds the double helix and breaks hydrogen bonds between base pairs, exposing template strands and forming a replication fork.

  • Step 2: Base Pairing: Free nucleotide monomers in the cell nucleus align along exposed template strands following complementary rules ($A-T, C-G$).

  • Step 3: Joining: The enzyme DNA polymerase joins adjacent nucleotides to construct the new complementary sugar-phosphate backbone.

  • Step 4: Formation: Two identical DNA double helices result, each containing one original parent strand and one newly synthesized strand.

• Mitosis:

  • Purpose: Growth, tissue repair, cell replacement, and asexual reproduction.

  • Location: Somatic (body) cells.

  • Daughter Cells: Produces $2\text{ daughter cells}$ that are genetically identical to each other and to the parent cell ($2n = 46\text{ chromosomes}$ in humans).

  • Variation: Does not create genetic variation.

  • Phases:

    • Interphase: DNA replicates; cell carries out routine metabolic functions.

    • Prophase (Stage 1): DNA condenses into visible chromosomes (two chromatids joined at a centromere); nuclear membrane dissolves; centrioles migrate and form spindle fibers.

    • Metaphase (Stage 2): Chromosomes align along the cell equator attached to spindle fibers.

    • Anaphase (Stage 3): Centromeres split; sister chromatids are pulled to opposite poles.

    • Telophase & Cytokinesis (Stages 4 & 5): Nuclear membranes reform around separate chromosome sets; cell cytoplasm divides into two discrete cells.

• Meiosis:

  • Purpose: Production of gametes (sex cells: sperm and egg in animals; pollen and ovule in plants).

  • Location: Gonads (testes and ovaries in animals; anthers and ovaries in plants).

  • Daughter Cells: Produces $4\text{ daughter cells}$, each possessing half the original chromosome number ($n = 23\text{ chromosomes}$ in humans; haploid state).

  • Variation: Creates substantial genetic variation; no daughter cell is identical to another or to the parent cell.

• Meiotic Mechanisms Generating Genetic Diversity:

  1. Crossing Over: During Meiosis I, homologous chromosome pairs line up side-by-side. Non-sister chromatids break and swap equivalent segments of DNA. This recombines maternal and paternal alleles on single chromatids, creating unique allele combinations.

  2. Independent Assortment: During Meiosis I, homologous chromosome pairs align randomly at the cell equator. The distribution of maternal and paternal chromosomes into gametes is entirely random ($2^{23}$ possible combinations in human gametes).

  3. Random Fertilization: The random union of a unique sperm with a unique egg cell further multiplies genetic variation among offspring.

Comparative Reproduction Strategies (Asexual vs. Sexual)

• Asexual Reproduction:

  • Process: Involves a single parent producing genetically identical clones via mitosis.

  • Plant Mechanisms: Tubers (potatoes), runners (strawberries), cuttings, and grafting.

  • Animal Mechanisms: Parthenogenesis (development of unfertilized eggs in honeybees, wasps, and rattlesnakes; unfertilized eggs become males, fertilized become females), body regeneration (starfish), and live aphid virgin births.

  • Artificial Animal Cloning: Somatic Cell Nuclear Transfer (SCNT) removes a somatic cell nucleus, fuses it into an enucleated egg cell, and implants the embryo into a surrogate mother (e.g., Dolly the sheep).

  • Advantages: Requires only one individual; fast; conserves energy (no mate selection required); guarantees passing on favorable phenotypes in stable environments.

  • Disadvantages: Zero genetic variation; entire populations are uniformly susceptible to environmental changes, disease outbreaks, or sudden catastrophic shifts.

• Sexual Reproduction:

  • Process: Involves two parents combining haploid gametes (produced via meiosis) through fertilization to produce a genetically unique diploid zygote.

  • Advantages: High genetic variation; increases population resilience and evolutionary potential under changing selection pressures.

  • Disadvantages: Slower process; high energy cost associated with finding and securing mates; potential to produce individuals with less favorable combinations of traits.

Inheritance Patterns, Monohybrid Crosses, Pedigrees, & Test Crosses

• Genetics Terminology:

  • Dominant Allele: An allele that is always expressed in the phenotype when present ($B$).

  • Recessive Allele: An allele that is expressed in the phenotype only when two copies are present ($b$).

  • Homozygous: Possessing two identical alleles for a gene ($BB$ = homozygous dominant; $bb$ = homozygous recessive / purebred).

  • Heterozygous: Possessing two different alleles for a gene ($Bb$).

  • Genotype: The specific combination of alleles possessed by an organism (e.g., $BB, Bb, bb$).

  • Phenotype: The physical appearance or observable characteristic of an organism (e.g., brown hair, blue eyes).

• Test Crosses & Purebred Status:

  • Purebred: An individual homozygous for a specific trait ($BB$ or $bb$). Breeding two purebred individuals yields $100\%$ identical phenotypes.

  • Purpose of Test Cross: To determine whether an individual showing a dominant phenotype is homozygous dominant ($BB$) or heterozygous ($Bb$).

  • Procedure: Cross the dominant individual ($B\_$) with a known homozygous recessive individual ($bb$).

  • Interpreting Outcomes:

    • If any offspring display the recessive phenotype ($bb$), the tested parent must be heterozygous ($Bb$).

    • If all offspring show the dominant phenotype across a large sample size, the parent is likely homozygous dominant ($BB$ / purebred).

  • Probabilistic Nature: Each fertilization event is independent and random. A single cross producing dominant offspring does not prove purebred status; large numbers of offspring are required for statistical certainty.

• Pedigree Chart Interpretation:

  • Standard symbols: Squares represent males; circles represent females; shaded symbols represent affected individuals; unshaded symbols represent unaffected individuals.

  • Carriers: Heterozygous individuals ($Bb$) who carry a recessive allele without displaying the recessive phenotype.

  • Determining Recessive Traits: If two unaffected parents produce an affected offspring ($B\_ \times B\_ \rightarrow bb$), the trait must be recessive, and both parents must be heterozygous carriers ($Bb$).

  • Determining Dominant Traits: If two affected parents produce an unaffected offspring, the trait must be dominant, and both parents must be heterozygous ($Bb$).

• Polycystic Kidney Disease Case Study:

  • Caused by an autosomal dominant allele ($D$); unaffected status is recessive ($d$).

  • Non-heritable kidney failure caused by environmental infections affects somatic cells and cannot be inherited by future children.

• Chicken Comb Inheritance Case Study:

  • Rose comb allele ($R$) is dominant over single comb ($r$).

  • Two rose comb chickens producing a single comb chick must both be heterozygous ($Rr$):         $Rr \times Rr \rightarrow 25\%\,RR,\,50\%\,Rr,\,25\%\,rr$

  • Phenotypic ratio: $75\%$ Rose comb, $25\%$ Single comb.

Phylogenetic Trees, Human Genome, and Applications of Biotechnology

• Phylogenetic Trees:

  • Diagrammatic hypotheses representing evolutionary relationships and shared common ancestry among species.

  • Branch points (nodes) represent common ancestors.

  • Two species are closely related if they share a recent common ancestor on the tree.

• Genomes & Comparative Gene Numbers:

  • A genome is the complete sum of all genetic information (DNA base sequence) in an organism.

  • Comparative Gene Counts Across Species:

    • Haemophilus influenzae (Pathogenic bacterium): $1,700\text{ genes}$

    • Escherichia coli (Gut bacterium): $3,200\text{ genes}$

    • Saccharomyces cerevisiae (Yeast): $6,000\text{ genes}$

    • Drosophila melanogaster (Fruit fly): $14,000\text{ genes}$

    • Caenorhabditis elegans (Roundworm): $19,000\text{ genes}$

    • Homo sapiens (Human): $23,000\text{ genes}$

    • Arabidopsis thaliana (Thale cress plant): $26,000\text{ genes}$

    • Daphnia pulex (Water flea): $31,000\text{ genes}$

    • Oryza sativa (Rice): $41,000\text{ genes}$

    • Populus trichocarpa (Black cottonwood tree): $46,000\text{ genes}$

    • Trichomonas vaginalis (Unicellular parasite): $60,000\text{ genes}$

• Human Genome Project (HGP):

  • International scientific project conducted from $1990$ to $2003$.

  • Sequenced the entire 3 billion base-pair human DNA sequence and mapped all human genes.

• Practical Applications of DNA Sequencing:

  1. Paternity and Maternity Testing: Evaluates genetic marker overlaps ($16\text{ standard markers}$) between parents and offspring ($50\%$ inherited from each parent).

  2. Forensics: Matches DNA profiles from crime scene evidence to potential suspects.

  3. Medical Diagnostics: Identifies inherited disease alleles (e.g., screening newborns for Phenylketonuria / PKU).

  4. Preventive Oncology Interventions: Detecting disease alleles allows preemptive interventions (e.g., detecting the $CDH1$ gene mutation for stomach cancer or $BRCA1/BRCA2$ gene mutations for breast cancer).

  5. Conservation Breeding Programs: Sequencing endangered species (e.g., cheetahs) helps breeders manage mating choices to maximize overall genetic diversity.

Population Genetics & Evolutionary Mechanics

• Population Definition: A group of organisms belonging to the same species living in the same geographic area at the same time, capable of interbreeding.

• Natural Selection Process:

  1. Overproduction: Species produce more offspring than the environment can support.

  2. Variation: Pre-existing inheritable genetic variation exists within the population.

  3. Selection Pressure & Competition: Environmental factors (predators, disease, climate) create survival challenges.

  4. Differential Survival: Individuals possessing advantageous phenotypes/alleles are better adapted, surviving and reproducing at higher rates ("survival of the fittest").

  5. Adaptation: Favourable alleles become more frequent in the population gene pool over successive generations.

• Peppered Moth Selection Example: Dark morphs gain selection advantages in soot-polluted environments via camouflage, whereas light morphs are favoured in unpolluted lichen covered environments.

• Hoiho Penguin & Avian Malaria Case Study:

  • In a Hoiho (yellow-eyed penguin) population exposed to avian malaria, natural genetic variation provides malaria resistance alleles to certain individuals.

  • Malaria infection acts as a selection pressure, killing non-resistant penguins.

  • Resistant penguins survive, reproduce, and transmit resistance alleles to offspring, increasing resistance allele frequency in the gene pool over time.

• Factors Affecting Allele Frequencies in Populations:

  • Migration (Gene Flow): Movement of individuals into (immigration) or out of (emigration) a population. Introduces new alleles or removes alleles, reducing genetic divergence between populations. Examples: NZ Longfin Eels migrating to spawn, Godwits migrating globally, NZ Fur Seals.

  • Population Bottleneck: A catastrophic event (e.g., natural disaster, disease outbreak, habitat loss) drastically reduces population size. Survivors represent a small sample of the original genetic diversity, leaving the rebuilt population with low variation (e.g., Takahē population declines due to introduced predators).

  • Non-Random Mating (Sexual Selection): Occurs when individuals choose mates based on specific phenotypic traits (e.g., peahens selecting peacocks with large, bright tail feathers; assortative mating for similar phenotypes; or geographic proximity limitations). Reduces overall genetic diversity and increases homozygosity for selected traits.