Biological Science: Structure, Function, and Development in Organisms; Patterns of Heredity
This domain asks how multicellular organisms build and run themselves - transport, gas exchange, regulation, development - and how traits pass from parents to offspring.
Structure, function, and homeostasis
Structure follows function: fish gills use countercurrent exchange to maximize oxygen uptake; the nephron filters, reabsorbs, and secretes; xylem moves water under tension and phloem translocates sugars. Homeostasis relies on negative feedback (thermoregulation, blood glucose via insulin and glucagon); positive feedback is rare and self-amplifying (childbirth, clotting).
Signaling
A resting neuron is polarized by the Na+/K+ pump; an action potential is a depolarizing wave of Na+ influx followed by K+ efflux. Hormones travel in blood to specific receptors; neurotransmitters act across synapses.
Development
Fertilization → cleavage → blastula → gastrula, forming ectoderm (skin, nerves), mesoderm (muscle, bone, blood), and endoderm (gut lining). Differentiation reflects differential gene expression, not loss of genes; induction is signaling between tissues. Plants grow from meristems.
Patterns of heredity
Mendel's law of segregation: alleles separate during gamete formation. Independent assortment applies to genes on different chromosomes. Monohybrid crosses give 3:1; dihybrids give 9:3:3:1. Extensions include incomplete dominance, codominance (AB blood), multiple alleles (ABO), polygenic traits, and X-linked inheritance - males are hemizygous, so recessive X-linked traits appear far more often in males.
Worked example
A colorblind man (XbY) marries a carrier woman (XBXb). Father's gametes: Xb or Y. Mother's: XB or Xb. Daughters get Xb from father plus XB or Xb from mother: half carriers, half colorblind. Sons get Y from father plus XB or Xb from mother: half unaffected, half colorblind. Probability of an affected child is 1/2. Reasoning: the father's X always goes to daughters, never to sons.
Traps
Assuming dominant alleles are more common; confusing genotype with phenotype; applying independent assortment to linked genes; forgetting a father cannot pass an X-linked allele to his son; treating acquired traits as heritable.