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Genetics – Unit 2 Area of Study 1 Comprehensive Notes

Key Knowledge Outline

  • Distinction among genes, alleles, genome
  • Homologous chromosomes, autosomes vs sex chromosomes
  • Size & number variability of chromosomes across organisms
  • Karyotypes to diagnose chromosomal abnormalities
  • Formation of haploid gametes by meiosis; significance of crossing-over & independent assortment
  • Genotype notation conventions
  • Patterns of dominance: dominant, recessive, codominance, incomplete dominance
  • Proportionate influence of genetic, environmental & epigenetic factors on phenotype

DNA Recap

  • DNA (deoxyribonucleic acid)
    • Located in nucleus of eukaryotic cells
    • Polymer of repeating nucleotides
    • Double-helix; complementary strands held by H-bonds
  • Nucleotide structure
    • Phosphate group attached to 5' carbon
    • Pentose (deoxyribose) sugar numbered 1' \text{–} 5'
    • Nitrogenous base at 1' carbon: Adenine (A), Thymine (T), Cytosine (C), Guanine (G)
  • Complementary base-pair rule
    • A \leftrightarrow T, C \leftrightarrow G
    • In RNA: A \leftrightarrow U
  • Antiparallel orientation: one strand 5' \to 3', partner 3' \to 5'

Genes, Alleles & Genome

  • Gene
    • Discrete DNA segment encoding instructions for a protein (or functional RNA)
    • Occupies a specific locus (plural loci) on chromosome
  • Allele
    • Alternative DNA sequence of same gene producing variant protein/function
    • Individuals typically possess two alleles per locus (maternal & paternal)
    • Example: eye-colour gene \textit{OCA2} has alleles for brown vs blue pigmentation
  • Genome
    • Complete haploid set of chromosomes and all genetic information
    • Human genome ≈ 3 \times 10^{9} base pairs, ~25 000 genes

Chromosomes

  • Somatic human cell: 46 chromosomes (diploid 2n)
  • Gamete: 23 chromosomes (haploid n); fusion of two gametes restores 2n
  • Structural features
    • Telomeres: repetitive DNA protecting ends
    • Centromere: constriction joining sister chromatids; attachment site for spindle
    • DNA wound around histone proteins → chromatin
  • Homologous chromosome pair
    • Same length, centromere position, gene loci; one maternal, one paternal

Karyotypes & Chromosome Abnormalities

  • Karyotype: ordered visual display of metaphase chromosomes
    • Pairs 1–22 = autosomes; pair 23 = sex chromosomes (XX or XY)
  • Aneuploidy = abnormal number
    • Monosomy: 2n-1 (e.g.
    • Turner syndrome: XO; incidence 1:2000; symptoms: infertility, short stature, webbed neck)
    • Trisomy: 2n+1 (e.g.
    • Down syndrome: Trisomy 21; incidence 1:1000; symptoms: growth delay, heart defects, flat face, mild–moderate ID)
    • Tetrasomy: 2n+2 (e.g.
    • Tetrasomy X: XXXX; ~100–150 known cases; mild physical & speech delays, learning difficulties)
  • Polyploidy
    • >2 complete sets (e.g. 3n, 4n)
    • Lethal in humans; common in plants (may confer size & vigour advantages)

Chromosome Number Variation Across Species

  • Examples (diploid numbers 2n):
    • Jack Jumper Ant 2, Housefly 12, Cat 38, Dog 78, Butterfly 190
    • Garden pea 14, Cabbage 18, Corn 20, Coconut 32, Pineapple 50
    • Fern 1440

Cell Types

  • Somatic cells
    • Diploid, produced via mitosis; genetically identical to parent
  • Germ cells (gametes)
    • Haploid, produced via meiosis; genetically unique

Meiosis Overview

  • Purpose: generate haploid gametes and genetic diversity for sexual reproduction
  • Reduction division: 2n \to n
  • Two sequential divisions
    • Meiosis I (reductional)
    • Meiosis II (equational)
  • Outcomes: 4 non-identical haploid daughter cells

Detailed Stages of Meiosis

  • Interphase I: DNA replication → each chromosome = pair of sister chromatids (still 2n)

Meiosis I

  • Prophase I
    • Chromosomes condense; nuclear envelope dissolves
    • Homologous chromosomes pair (synapsis) forming tetrads
    • Crossing-over: reciprocal exchange between non-sister chromatids → recombinant chromatids
  • Metaphase I
    • Tetrads align randomly at metaphase plate
    • Independent assortment: paternal vs maternal homolog orientation random
  • Anaphase I
    • Homologous chromosomes segregate to opposite poles; sister chromatids remain joined
  • Telophase I & Cytokinesis
    • Two haploid (n) cells, chromosomes still duplicated (chromatids)

Meiosis II (resembles mitosis)

  • Prophase II: spindle forms, nuclear envelope breakdown
  • Metaphase II: chromosomes align singly at plate
  • Anaphase II: sister chromatids separate
  • Telophase II & Cytokinesis: chromatids decondense; four haploid cells produced

Sources of Genetic Variation

  • Crossing-over (Prophase I): new allele combinations along chromatids
  • Independent assortment (Metaphase I): 2^{n} possible gamete chromosome combinations (humans: 2^{23} \approx 8.4\,\text{million})
  • Random fertilization further multiplies diversity

Meiotic Errors

  • Nondisjunction: failure of homologs (Anaphase I) or chromatids (Anaphase II) to separate
    • Produces gametes with n+1 or n-1 chromosomes → aneuploid zygotes

Genotype vs Phenotype

  • Genotype: allele constitution of an organism
    • Written with symbols (e.g. BB, Bb, bb)
  • Phenotype: observable trait expression (e.g. brown eyes, blue eyes)

Allelic Relationships

  • Dominant allele: expressed when at least one copy present (symbol uppercase, e.g. B)
  • Recessive allele: expressed only when homozygous (lowercase, e.g. b)
  • Homozygous dominant: BB
  • Heterozygous: Bb (dominant phenotype)
  • Homozygous recessive: bb

Codominance & Multiple Alleles

  • Codominance: both alleles fully expressed in heterozygote
    • Example: human ABO blood — I^{A}, I^{B} codominant; i recessive
    • Roan cattle: C^{R}C^{W} produces intermixed red & white hairs
  • Multiple alleles: >2 alleles at a locus in population (ABO has 3)

Incomplete Dominance

  • Neither allele completely dominant → heterozygote phenotype intermediate (“blending”)
    • Example: red \times white snapdragons → pink F_{1}

Sex Determination & Sex-Linked Inheritance

  • Females: XX; produce only X-bearing ova
  • Males: XY; spermatogenesis yields 50% X, 50% Y sperm → sperm determines sex
  • Sex-linked (X-linked) traits
    • Males hemizygous for X genes; recessive alleles expressed unmasked
    • Disorders: colour-blindness, haemophilia, pattern baldness

Nature, Nurture & Phenotypic Expression

  • Phenotype = genes + environment + epigenetic modulation
  • Identical twin case study: malnutrition alters bone structure despite identical DNA
  • Hydrangea flower colour varies with soil pH (acidic → blue, alkaline → pink); illustrates proportionate heritability

Epigenetics

  • Definition: heritable changes in gene expression without DNA sequence alteration
  • Mechanism (diagram narrative)
    • External experience (stress, nutrition, toxins) → neural signals
    • Signals trigger gene-regulatory proteins
    • Proteins attract/repel enzymes adding/removing epigenetic markers (e.g. methyl groups)
    • Markers modulate chromatin accessibility, turning genes “on/off”
  • Outcomes
    • Adjust protein production, influencing development & health
    • Potentially transmissible across cell generations (and sometimes to offspring)

Ethical, Philosophical & Practical Implications

  • Prenatal karyotyping enables early diagnosis; raises questions on selective termination & genetic counselling
  • Epigenetic plasticity underscores importance of early-life environment, informing public-health strategies
  • Agricultural manipulation of polyploidy used to create seedless fruits & larger crops

Key Formulae & Numerical References

  • Human chromosome combinations per gamete: 2^{23}
  • Possible zygote combinations (random fertilization): 2^{23} \times 2^{23} = 2^{46} (~70\,\text{trillion})
  • Diploid ↔ haploid: \text{somatic} = 2n = 46, \text{gamete} = n = 23
  • Aneuploid notation examples: XO\ (2n-1), 21^{+}\ (2n+1), XXXX\ (2n+2)