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: $XO\ (2n-1)$, $21^{+}\ (2n+1)$, $XXXX\ (2n+2)$