Control and Coordination, Reproduction and Heredity
This domain joins two chapters: control and coordination through nerves and hormones, and reproduction with basic genetics.
Nervous system
The neuron has a cell body, dendrites and an axon; a synapse passes the signal chemically. A reflex action is a rapid, involuntary response through a reflex arc: receptor → sensory neuron → spinal cord → motor neuron → muscle. The brain has the cerebrum (thinking), cerebellum (balance) and medulla (breathing, heartbeat). Plants respond by tropism: in phototropism auxin accumulates on the shaded side, bending the shoot towards light.
Endocrine system
Hormones are made in glands and travel in the blood: pituitary (growth; the master gland), thyroid (thyroxine, needs iodine), pancreas (insulin lowers blood sugar), adrenal (adrenaline in emergencies), testes and ovaries (sex hormones).
Reproduction
Asexual: binary fission (Amoeba), budding (Hydra, yeast), fragmentation and regeneration, vegetative propagation. Sexual in plants: stigma, style and ovary; anther and filament. Pollination is followed by fertilisation, forming the seed, while the ovary becomes the fruit. In humans the male system is testes, vas deferens and sperm; the female has ovaries, fallopian tube and uterus; menstruation is the shedding of the uterine lining.
Heredity
Mendel's pea crosses revealed dominant and recessive traits and a 3:1 ratio in the F₂ of a monohybrid cross. Genotype is the allele pair (TT, Tt, tt); phenotype is the appearance. Human sex is determined by the father's X or Y sperm.
Worked example
Cross Tt × Tt. Each parent gives T or t, producing TT, Tt, Tt, tt - three tall to one short (3:1 phenotype), with a genotypic ratio of 1:2:1.
Traps
Saying the mother determines the child's sex; confusing dominant with common; treating reflex actions as brain-controlled; forgetting that the thyroid needs iodine.