Genetic Variation and Inheritance Principles

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Master Exam Manual: CB 1.3 Genetic Variation

Chapter 1: Genetic Variation

1. Cells and Chromosomes

  • The Nucleus as the Command Center: Eukaryotic cells isolate genomic material inside a double-membrane nucleus. This structure protects DNA from chemical damage caused by cellular metabolism.
  • Chromosome Condensation: DNA does not float as loose strands. It wraps around spool-like proteins called histones. This complex coils into chromatin. During cell division, chromatin packs into visible, X-shaped structures called chromosomes.
  • Karyotypes and Chromosome Pairs: Humans possess 46 chromosomes in total. They form 23 homologous pairs. One chromosome of each pair comes from the sperm (paternal); the other comes from the egg (maternal).
  • Autosomes vs. Sex Chromosomes: Pairs 1 to 22 are autosomes. They control general somatic traits. Pair 23 consists of sex chromosomes (XX for biological females, XY for biological males).

2. The DNA Model & Nucleotide Anatomy

  • The Double Helix Structure: DNA is a double-stranded polymer twisted into a spiral helix. James Watson, Francis Crick, and Rosalind Franklin discovered this structure.
  • The Monomer Subunit: Each strand consists of repeating units called nucleotides. A single nucleotide contains three covalently bonded parts:
    • A 5-carbon Deoxyribose Sugar ring.
    • A negatively charged Phosphate Group.
    • A nitrogenous Base (A, T, C, or G).
   Phosphate
       |
    [Sugar] --- Base (A, T, C, or G)
  • The Sugar-Phosphate Backbone: Nucleotides link vertically via strong covalent phosphodiester bonds. The sugar of one nucleotide connects to the phosphate of the next, forming a highly stable outer backbone.
  • Antiparallel Alignment: The two strands run in opposite directions. One strand runs from 5′ to 3′, while the complementary strand runs from 3′ to 5′.

3. The Genetic Code & Base Pairing

  • Nitrogenous Base Classes: The four bases divide into two chemical groups:
    • Purines: Adenine (A) and Guanine (G) (double-ring structures).
    • Pyrimidines: Thymine (T) and Cytosine (C) (single-ring structures).
  • Complementary Base Pairing Rules: Strands bind horizontally via weak hydrogen bonds. A purine must always bond with a specific pyrimidine:
    • Adenine bonds exclusively with Thymine (A=T) via two hydrogen bonds.
    • Cytosine bonds exclusively with Guanine (C≡G) via three hydrogen bonds.
  • Genetic Code Universality: The order of these bases forms the genetic language for almost all living organisms on Earth.

4. DNA to Protein Synthesis

  • The Triplet Code Definition: A triplet is a consecutive sequence of three bases along a DNA strand.
  • Amino Acid Specification: Each individual triplet serves as a chemical code for one specific amino acid. For example, the triplet TAC codes for the amino acid Methionine.
  • Polypeptide Assembly: The cell reads a gene's triplets in order. It links the corresponding amino acids together using peptide bonds to build a polypeptide chain.
  • Protein Folding Mechanics: The unique sequence of amino acids dictates how the polypeptide chain folds into a complex 3D shape. This shape forms a functional protein, such as an enzyme, hormone, or structural tissue.

5. Genes and Alleles

  • Gene Locus and Function: A gene is a specific segment of DNA located at a precise position (locus) on a chromosome. It contains the exact sequence of triplets needed to build one specific protein.
  • Allele Derivation: An allele is an alternative form of a gene. It features a slightly different sequence of nucleotide bases.
  • Phenotypic Impact: Small variations in the base sequence of an allele alter the triplet code. This changes the amino acid sequence, modifying the resulting protein's structure and function. For example, a functional protein may produce brown eye pigment, while a modified version may produce blue eye pigment.

6. Mitosis

  • Biological Purpose: Mitosis is cell division used for somatic (body) tissue growth, repair of damaged cells, and asexual reproduction.
  • The Process:
    1. The cell replicates its DNA, forming chromosomes made of two identical sister chromatids.
    2. Chromosomes line up individually along the cell's equator.
    3. Spindle fibers pull the sister chromatids apart to opposite poles.
    4. The cell divides its cytoplasm (cytokinesis).
  • Genetic Outcome: Mitosis produces two genetically identical, diploid (2n) daughter cells. Each contains a complete set of 46 chromosomes, acting as exact clones of the original parent cell.

7. DNA Replication Mechanics

  • Precise Phase Timing: Replication occurs during Interphase, right before a cell enters mitosis or meiosis. This ensures every new cell receives a complete copy of the genome.
  • Step 1: Unzipping: The enzyme DNA Helicase breaks the weak hydrogen bonds between complementary base pairs. This splits the double helix into two separate template strands, exposing their bases.
  • Step 2: Primer Attachment: Short RNA sequences called primers attach to the template strands, marking the starting points for synthesis.
  • Step 3: Elongation: The enzyme DNA Polymerase moves along the exposed template strands. It matches free-floating nuclear nucleotides to the templates using strict base-pairing rules (A-T and C-G).
  • Step 4: Ligation: The enzyme DNA Ligase seals the sugar-phosphate backbones together, forming continuous strands.
  • Semi-Conservative Replication: Each resulting DNA molecule contains one original parent strand and one newly synthesized daughter strand. This minimizes copying errors.
Template Strand 5' ---> [A][T][G][C] ---> New Strand 3'
New Strand 3'      <--- [T][A][C][G] <--- Template Strand 5'

8. Understanding Variation

  • Definition of Variation: Variation refers to the structural, physiological, and behavioral differences among individuals of the same species.
  • Somatic vs. Heritable Variation:
    • Somatic Variation: Changes acquired during an organism's lifetime due to environmental exposure (e.g., muscle development or scars). These changes do not alter gamete DNA and cannot be passed to offspring.
    • Heritable Variation: Differences caused by an organism's genetic makeup. These can be passed down across generations.

9. Types of Variation (Continuous vs. Discontinuous)

  • Discontinuous Variation:
    • Characteristics: Features divide into distinct, non-overlapping categories with no intermediate forms.
    • Genetic Architecture: Typically controlled by a single gene with a few distinct alleles (monogenic inheritance). The environment rarely alters these traits.
    • Examples: Blood types (A, B, AB, O), biological sex, or the ability to roll your tongue.
  • Continuous Variation:
    • Characteristics: Features show a continuous spectrum of phenotypes across a population. When plotted as a graph, they form a bell-shaped Normal Distribution Curve.
    • Genetic Architecture: Controlled by the combined, additive effects of multiple genes working together (polygenic inheritance).
    • Examples: Human height, skin color, or hair density.

10. Variation and Normal Distribution

  • Environmental Modification: Continuous traits are highly influenced by environmental factors.
  • The Genotype-Environment Interaction: An individual may inherit alleles for a tall height (high genetic potential). However, if they experience childhood malnutrition or illness (environmental stress), they will not reach that potential. This shifts their physical phenotype toward the lower end of the distribution curve.

11. Meiosis

  • Biological Purpose: Meiosis is a specialized form of cell division that occurs only in the gonads (testes and ovaries). Its sole function is to produce gametes (sperm and eggs) for sexual reproduction.
  • The Process:
    • Meiosis I: Homologous chromosome pairs align along the cell equator and separate into two intermediate cells. This reduces the chromosome number by half.
    • Meiosis II: In both intermediate cells, individual chromosomes line up along the equator. Spindle fibers pull sister chromatids apart to opposite poles.
  • Genetic Outcome: Meiosis produces four genetically unique, haploid (n) daughter cells. Each gamete contains exactly 23 single chromosomes, ensuring the normal chromosome count restores at fertilization.

12. Meiosis as a Source of Variation

Meiosis introduces genetic variation through two key processes during its first division:

  1. Independent Assortment: Homologous pairs line up randomly along the cell equator during Metaphase I. The direction each chromosome faces is entirely accidental. When they separate, the distribution of maternal and paternal chromosomes into the daughter cells is completely random. This creates over 223 (8.4 million) possible chromosome combinations in human gametes.
  2. Crossing Over (Recombination): During Prophase I, homologous chromosomes pair up tightly. Non-sister chromatids can overlap, break, and swap matching segments of DNA. This shuffles alleles between maternal and paternal chromosomes, creating brand-new combinations of alleles that did not exist in the parents.
Homologous Pair:   [A][B]  (Maternal)  --> Crossing -->  [A][b] (Recombinant)
                   [a][b]  (Paternal)  -->  Over    -->  [a][B] (Recombinant)

13. Asexual Reproduction

  • Mechanistic Basis: Relying entirely on mitosis, a single parent organism produces offspring without gamete fusion.
  • Evolutionary Advantages:
    • Extremely fast and energy efficient; organisms do not need to spend resources finding a mate.
    • Rapidly builds large populations to colonize stable environments.
  • Evolutionary Disadvantages:
    • Produces offspring that are exact genetic clones of the parent.
    • The population lacks genetic variation. If the environment changes, a new disease emerges, or a drought occurs, every individual reacts identically, which can wipe out the entire population.

14. Sexual Reproduction

  • Mechanistic Basis: Requires two distinct parents. Each produces unique haploid gametes via meiosis, which combine during fertilization to form a diploid zygote.
  • Evolutionary Advantages:
    • Generates massive genetic variation through meiosis and the random fusion of gametes.
    • Provides a survival buffer for the population; changing conditions or diseases will likely spare some individuals with advantageous allele combinations, allowing the species to survive.
  • Evolutionary Disadvantages:
    • Requires significant biological time and energy to locate, court, and secure a compatible mate.
    • Population growth is slower because it depends on mating cycles and gestation.

15. Mutation as the Source of Variation

  • Definition: A permanent, random change in the baseline nucleotide sequence of DNA.
  • The Ultimate Source of Variation: While meiosis and fertilization shuffle existing alleles, mutation is the only process that creates entirely new alleles.
  • Somatic Mutations: Occur in non-reproductive body cells (e.g., skin cells damaged by UV radiation). They affect only the individual organism and cannot be passed to future offspring.
  • Gametic Mutations: Occur in germline cells within the gonads during gamete production. If a mutated sperm or egg undergoes fertilization, the mutation will be present in every cell of the offspring, introducing a new allele into the family lineage.

16. Case Study: Hairless Rats

  • Genetic Mechanism: Hairlessness in specific laboratory rat lines stems from a spontaneous, recessive gametic mutation within a gene responsible for keratin production.
  • Phenotypic Outcome: Rats homozygous for this mutated allele (hr/hr) cannot form proper hair shafts, leaving them completely hairless. This demonstrates how a small change in DNA structure directly alters a visible, structural trait.

17. Passing on Genes

  • The Hereditary Link: Parents do not pass physical traits directly to their offspring. Instead, they pass down discrete chemical units of information: alleles via gametes.
  • Fertilization Dynamism: Any single sperm can fertilize any single egg. This random combination of unique gametes creates a completely new, unpredictable genotype in the resulting zygote.

Chapter 2: Tracking Genes

1. Predicting Inheritance

  • Genotype: The actual combination of alleles an organism inherits for a specific gene (e.g., AA, Aa, or aa).
  • Phenotype: The observable physical, biochemical, or behavioral trait produced by that genotype.
  • Dominant Alleles: Alleles that express their phenotype whenever at least one copy is present. They use capital letters (e.g., B).
  • Recessive Alleles: Alleles whose phenotypes are masked in the presence of a dominant allele. They express only when an organism inherits two copies. They use lowercase letters (e.g., b).

2. Case Study: Predicting Rat Fur Color

  • Allelic Hierarchy: In rats, black coat color is typically driven by a dominant allele (B), while brown coat color is driven by a recessive allele (b).
  • The Heterozygous State: A rat with the genotype Bb possesses both a black and a brown allele. Because the black allele is dominant, it produces enough pigment to mask the brown allele, resulting in a black coat phenotype.

3. Genotypic and Phenotypic Ratios

  • The Punnett Square: A mathematical grid used to predict the probability of genotypes and phenotypes in offspring from a specific genetic cross.
 Bb
BBB (Black)Bb (Black)
bBb (Black)bb (Brown)
  • Standard Heterozygous Monohybrid Cross Ratios (Bb × Bb):
    • Expected Genotypic Ratio: 1 BB : 2 Bb : 1 bb
    • Expected Phenotypic Ratio: 3 Dominant (Black) : 1 Recessive (Brown)
  • Probability vs. Reality: Punnett squares provide expected probabilities, not guaranteed outcomes. Actual offspring counts rarely match these ratios perfectly in small litters. This discrepancy occurs because each fertilization event is independent and governed entirely by random chance.

4. Pure Breeding

  • Definition: An organism is pure-breeding for a trait if it is homozygous for that gene (BB or bb).
  • Lineage Stability: When two pure-breeding organisms with the same phenotype mate, their offspring will always display that exact phenotype across generations. This is because they can only pass down one type of allele.
  • The Test Cross: To determine whether an organism with a dominant phenotype is homozygous (BB) or heterozygous (Bb), it is crossed with a homozygous recessive individual (bb). If any offspring display the recessive phenotype, the parent must be heterozygous.

5. Mendel’s Peas

  • Historical Context: Gregor Mendel established modern genetics by breeding pea plants (Pisum sativum).
  • Law of Segregation: Mendel proved that alleles exist as discrete units. When gametes form, these two alleles separate so that each gamete carries only one allele for each gene. Traits do not blend; they retain their distinct identities across generations.

6. Case Study: Hairless Guinea Pigs

  • Skin Phenotypes: Hairlessness in guinea pigs (the "skinny pig" phenotype) is an autosomal recessive trait (sk).
  • Cross Dynamics: Mating two haired guinea pigs that carry the gene (Sk/sk × Sk/sk) can produce hairless offspring (sk/sk). This demonstrates how a physical trait can skip generations, remaining hidden in heterozygous carriers until two recessive alleles combine.

7. Pedigree Charts

Pedigree charts are family trees used to track how a specific trait passes through multiple generations.

   Legend:  [ ] Male     ( ) Female      [X] / (X) Affected
   
     ( )======[ ]  Unaffected Parents
          |
        ( X )      Affected Daughter (Indicates trait must be Recessive)
  • Standard Symbols: Squares represent biological males; circles represent biological females. Horizontal lines connecting a square and a circle indicate a mating pair. Vertical lines point to their offspring.
  • Shading Identification: Shaded shapes represent individuals who physically display the trait. Unshaded shapes represent individuals with the normal or alternative phenotype.
  • Deducing Inheritance Patterns:
    • To prove a trait is recessive: Look for two unshaded (unaffected) parents who have a shaded (affected) child. The parents must be heterozygous carriers (Aa) who passed their recessive alleles (a) to the child (aa).
    • To prove a trait is dominant: Look for two shaded (affected) parents who have an unshaded (unaffected) child. The parents must be heterozygous (Aa) and both passed on their recessive alleles (a), resulting in an unaffected child (aa).

8. The Genetics of Freckles

  • Allelic Nature: Having prominent facial freckles is an autosomal dominant trait (F).
  • Phenotypic Expression: Individuals with genotypes FF or Ff will develop freckles when exposed to sunlight. Individuals who lack freckles must be homozygous recessive (ff).

9. Tracking Genes for Health: Cystic Fibrosis

  • Pathology: Cystic Fibrosis is an inherited disorder that causes thick, sticky mucus to build up in the lungs and digestive tract, leading to severe respiratory and digestive issues.
  • Inheritance Mechanism: It is an autosomal recessive condition. An individual must inherit a copy of the mutated CFTR gene from both parents (ff) to develop the disease. Heterozygous individuals (Ff) do not show symptoms but can pass the mutated allele to their children.

10. Tracking Genes for Health: Polydactylism

  • Pathology: Polydactylism is a genetic condition where an individual is born with extra fingers or toes.
  • Inheritance Mechanism: Unlike most health conditions, it is an autosomal dominant trait. Inheriting just a single copy of the mutated allele (Pp or PP) will cause the trait to express. It does not skip generations; an affected child must have at least one affected parent.

Chapter 3: Population Genetics

1. Introduction to Population Genetics

  • The Gene Pool Concept: A gene pool is the total collection of all alleles for all genes present within an interbreeding population at a given time.
  • Allele Frequency Calculations: Allele frequency is the proportion of a specific allele relative to the total number of alleles for that gene locus in the population. Population genetics tracks how these frequencies change over time due to evolutionary pressures.

2. Genetic Diversity

  • Definition: Genetic diversity refers to the total number of different alleles present within a population's gene pool.
  • High Genetic Diversity Benefits: A population with a broad mix of alleles is highly resilient. If environmental conditions change, some individuals will likely carry alleles that allow them to survive the new pressures, preventing extinction.
  • Low Genetic Diversity Risks: Populations with low diversity contain nearly identical alleles. A single new disease or environmental shift could wipe out the entire population, as no individuals possess alleles for resistance.

3. Natural Selection

  • The Core Mechanism: Natural selection is the process where organisms better adapted to their environment tend to survive and produce more offspring.
  • Step-by-Step Breakdown:
    1. Overproduction: Populations produce more offspring than the environment can support, leading to competition.
    2. Variation: Pre-existing genetic variation exists within the population due to mutations and meiosis.
    3. Selection Pressure: Environmental factors (e.g., predators, climate shifts, disease, resource limits) act as selection pressures.
    4. Differential Survival: Individuals with advantageous phenotypes survive these pressures. Those with poorly suited traits die before reproducing.
    5. Inheritance: Surviving individuals pass their advantageous alleles to their offspring. Over generations, the frequency of these helpful alleles increases in the gene pool.

4. Adaptations

  • Definition: An adaptation is an inherited feature that enhances an organism's ability to survive and reproduce in its specific environment.
  • Three Functional Classes:
    • Structural: Physical body features (e.g., a bird's beak shape or thick camouflage fur).
    • Behavioral: Actions organisms take to survive (e.g., seasonal migration or nocturnal hunting).
    • Physiological: Internal biochemical processes (e.g., snake venom production or cold-weather antifreeze proteins).

5. Types of Selection Pressures

Natural selection can change the distribution of phenotypes in a population in three ways:

  • Stabilizing Selection: Selection pressure favors intermediate phenotypes, acting against both extremes. This reduces genetic variation and maintains the average phenotype (e.g., human birth weight).
  • Directional Selection: Selection pressure shifts in favor of one extreme phenotype. This causes the population distribution to slide toward that trait over time (e.g., antibiotic resistance in bacteria).
  • Disruptive Selection: Selection pressure acts against intermediate phenotypes, favoring individuals at both extremes. This can split the population into two distinct morphs and may lead to speciation.

6. Population Bottlenecks

  • Definition: A population bottleneck occurs when a sudden, catastrophic event (e.g., a volcanic eruption, flood, or intense over-hunting) abruptly reduces a population's size.
  • Genetic Consequences: The few surviving individuals do not reflect the genetic diversity of the original population. Rare alleles are permanently lost, and overall genetic diversity decreases. Even if the population recovers in total number, it remains genetically vulnerable due to high inbreeding.

7. Case Study: Bottlenecks and New Zealand Birds

  • The Crisis: Native New Zealand birds like the K&amacron;k&amacron;p&omacron;, Black Robin, and Takah&emacr; experienced severe population bottlenecks due to human colonization, habitat loss, and introduced mammalian predators (e.g., stoats, rats, and possums).
  • The Black Robin Example: By 1980, the entire Black Robin species was reduced to just five individuals, with only one fertile female remaining ("Old Blue"). While conservation efforts successfully increased their numbers, every living Black Robin is highly inbred and genetically identical, leaving the species vulnerable to disease outbreaks.

8. Migration (Gene Flow)

  • Immigration: The movement of new individuals into a population. This introduces new alleles into the local gene pool, increasing genetic diversity.
  • Emigration: The movement of individuals out of a population. This can permanently remove rare alleles, lowering the local population's genetic diversity.

9. The Founder Effect

  • Definition: The founder effect occurs when a small group of individuals leaves a larger parent population to establish a new colony in an isolated location (e.g., a remote island).
  • Genetic Realities: This small "founder" group carries only a tiny fraction of the original population's alleles. Their new gene pool will have different allele frequencies and lower genetic diversity compared to the population they left behind.

Chapter 4: Using Knowledge of Genetics

1. DNA Sequencing

  • Definition: DNA sequencing is a laboratory method used to determine the exact order of nucleotide bases (A, T, C, and G) along a DNA strand.
  • Modern Impact: Sequencing allows scientists to locate mutations, identify disease-causing genes, compare evolutionary lineages, and map entire genomes.

2. A Genome

  • Definition: A genome is the complete set of genetic material or DNA contained within a single cell of an organism.
  • Sizing: Genomes vary in size across species. The human genome contains roughly 3 billion base pairs split across 23 pairs of chromosomes, coding for approximately 20,000 distinct functional proteins.

3. Genetic Screening

  • Clinical Purpose: Genetic screening involves testing an individual's DNA to look for specific mutations or disease-associated alleles before symptoms appear.
  • Applications: It is used to identify carriers of recessive disorders (like Cystic Fibrosis), diagnose genetic conditions in fetuses (prenatal testing), and assess an individual's risk for adult-onset conditions like hereditary breast cancers.

4. DNA Profiling

  • Methodology: Also known as DNA fingerprinting, this technique analyzes specific regions of non-coding DNA called Short Tandem Repeats (STRs). The number of these repeats varies widely among individuals.
  • Gel Electrophoresis: Scientists extract STR fragments, replicate them using Polymerase Chain Reaction (PCR), and separate them by size using an electrical current on a gel sheet. This creates a unique pattern of bands for each individual.
                  Crime Scene    Suspect 1    Suspect 2
                  ___________    _________    _________
        Band 1    ---=====---                 ---=====---  (Match)
        Band 2    ---  -  ---    ---  -  ---  ---  -  ---  (Match)

5. Forensic Use of DNA Profiling

  • Evidence Matching: Forensics labs extract DNA from biological samples left at a crime scene (e.g., blood, saliva, or hair). They generate a DNA profile from this sample and compare it to profiles from suspects.
  • Accuracy: If every band matches perfectly across multiple STR locations, the sample belongs to the suspect. The odds of two unrelated people sharing the same profile can be lower than one in a billion.

6. DNA Profiling and Paternity Testing

  • Inheritance Rules: A child inherits exactly half of their nuclear DNA from their biological mother and half from their biological father.
  • Band Matching Logic: When reading a paternity gel, every band in the child's profile must align with a matching band in either the mother's or the father's profile. If the child has a band that does not appear in the mother's profile, it must match a band in the biological father's profile. If it does not, the tested man is not the biological father.

7. DNA Profiling and Species Identification

  • Conservation Tracking: Wildlife biologists use DNA profiling to identify animal species from hair, scat, or bone fragments. This helps them track population sizes and monitor endangered wildlife without disturbing the animals.

8. Genetic Diversity and the Takah&emacr;

  • Management Strategy: The native Takah&emacr; (Porphyrio hochstetteri) was rediscovered in 1948 after being thought extinct. Because their population is small, they face risks from inbreeding, which can lead to low fertility and poor chick survival.
  • Controlled Mating: Conservationists use DNA profiling to map the genetic profiles of individual birds. They use this information to arrange pairings that avoid inbreeding, helping to maximize genetic diversity and improve the species' long-term survival.

9. DNA Barcoding

  • Definition: DNA barcoding is a technique that uses a short, standardized section of DNA from a specific gene to identify which species an unknown organism belongs to.
  • Target Genes: For animals, scientists typically use a specific region of the mitochondrial gene Cytochrome c Oxidase I (COI). This region varies enough between species to act as a unique identifier, much like a supermarket barcode.

10. DNA Barcoding and Food Fraud

  • Supply Chain Security: DNA barcoding is used to identify mislabeled products in the global food supply chain.
  • Examples:
    • Testing fish tissue to ensure cheap white fish is not being sold as premium snapper.
    • Verifying that expensive Manuka honey contains pollen from native New Zealand Manuka trees (Leptospermum scoparium) rather than cheaper clover alternatives.

11. Environmental DNA (eDNA)

  • Definition: eDNA is genetic material shed by organisms into their surrounding environment through skin flakes, hair, mucus, feces, or urine.
  • Sampling Advantage: Scientists can collect a sample of water or soil, extract the free-floating eDNA, and identify all the species present in that habitat without needing to capture or see the organisms.

12. eDNA and Invasive Marine Pests

  • Biosecurity Monitoring: eDNA sampling is used to detect invasive marine pests, such as the Mediterranean fanworm (Sabella spallanzanii) or sea squirts, in New Zealand harbors.
  • Early Detection: By filtering harbor water and checking for pest eDNA, biosecurity teams can catch new invasions early, when populations are small and easier to manage, rather than waiting for them to grow large enough to be seen by divers.

13. Phylogenetic Trees

  • Definition: A phylogenetic tree is a branching diagram that shows the evolutionary relationships and lines of descent among different species based on their genetic similarities.
                          Common Ancestor
                                 |
                        ________/ \________
                       /                   \
                 Split Node              Species C
                   ___/ \___
                  /         \
              Species A  Species B

14. Reading the Branches

  • Nodes: The points where branches split represent a shared common ancestor.
  • Closeness: Species that share a more recent node are more closely related. They share more identical DNA sequences because they diverged from each other more recently in evolutionary history.

15. Using DNA to Trace Origins

  • Mitochondrial DNA (mtDNA): Mitochondria contain their own small genome. Because sperm only contribute nuclear DNA during fertilization, mtDNA is inherited entirely from the biological mother. It does not undergo recombination, making it useful for tracing maternal lineages back through thousands of generations.
  • Y-Chromosome Analysis: Similarly, the Y-chromosome passes directly from biological fathers to sons without recombining, allowing researchers to trace paternal ancestry over time.
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Meiosis Mutation Inheritance Genetic Diversity Population Genetics Natural Selection Punnett Squares Mitosis DNA Chromosomes Alleles DNA Profiling