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I. The Basics of Heredity and Conception

  • Conception: Zygote forms when sperm and ovum unite.
  • Genetic Code:
    • 46 chromosomes in zygote (23 pairs).
    • About 21,000 genes, 3 billion base pairs.
    • Gametes have 23 chromosomes.
  • Mathematical Variation: Parents can create over 64 trillion combinations of chromosomes.
  • Microbiome: Trillions of microbes in the body (150 times more genes than human genome) influence immunity, weight, and mood.

II. Sex Determination (The 23rd Pair)

  • Autosomes vs. Sex Chromosomes: 1st 22 pairs = autosomes; 23rd pair = biological sex.
  • Females (XX): Inherit X chromosome from both parents.
  • Males (XY): Inherit X from mother, Y from father.
  • SRY Gene: Triggers male organ development after 8 weeks.
  • Determination: Biological father determines sex of zygote (X or Y).

III. Genotype, Phenotype, and Epigenetics

  • Genotype: Entire genetic inheritance (hidden blueprint).
  • Phenotype: Observable characteristics (appearance, personality).
  • Epigenetics: Environmental factors influencing gene expression (e.g. nutrition).
    • Plasticity: Traits can change due to experiences.
    • Social Impact: Cultural changes can activate genetic potential.

IV. Patterns of Genetic Interaction

  • Additive Heredity: Traits depend on many genes (e.g., height).
  • Dominant-Recessive:
    • Dominant genes mask recessive ones.
    • Recessive appears only if inherited from both parents.
    • Carrier: Has recessive gene but doesn't express it.
  • X-Linked Genes: Males more likely to show X-linked disorders (e.g., color blindness).
  • Genetic Advantage: Some harmful genes provide protection (e.g., sickle-cell gene and malaria).

V. Twins and Multiples

  • Monozygotic (MZ) Twins: Identical twins from one zygote.
  • Dizygotic (DZ) Twins: Fraternal twins from two separate ova.
  • Twinning Factors: Genetic and age-related; influenced by mother's ovulation.

VI. Prenatal Development Periods

  1. Germinal Period (Days 0–14): Cell division and implantation.
  2. Embryonic Period (Weeks 3–8): Development of major organs and systems, sensitive to teratogens.
  3. Fetal Period (Week 9–Birth): Significant growth; age of viability at about 22 weeks.

VII. Birth and the Newborn

  • Labor: Triggered by oxytocin from the fetal brain.
  • Skull Development: Fontanels allow head to deform during birth.
  • Apgar Scale: Assesses health at 1 and 5 minutes post-birth.
    • Score of 7+ is healthy.
  • Birth Options: C-sections and doulas for support.

VIII. Postpartum and Bonding

  • Postpartum Depression: Affects 5–27% of parents; mitigated by support.
  • Kangaroo Care: Skin-to-skin contact benefits newborn.
  • Bonding: Important but not strictly necessary right after birth.

IX. Developmental Challenges

  • Chromosomal Abnormalities:
    • Trisomy: Three chromosomes instead of two; Down syndrome is common.
  • Teratogens: Substances that risk abnormalities (e.g., Fetal Alcohol Syndrome).
  • Low Birthweight (LBW): Under 2,500 grams (5.5 lbs); immigrant paradox may apply.

Must Know for the Exam

  • The 23rd Pair: Father's sperm determines sex; SRY gene triggers male development.
  • Survival Odds: Only about 27% of zygotes survive.
  • Critical Period: Structural defects are likely during embryonic period.
  • Age of Viability: 22 weeks necessary for survival outside womb.
  • Epigenetics: Environmental factors influence gene expression.
  • Apgar Scale: Assesses newborn health; 7 is healthy.
  • MZ vs. DZ Twins: MZ = 1 zygote (100% DNA); DZ = 2 zygotes (50% DNA).