Living World: systems, reproduction, genetics, and homeostasis
This domain connects organ systems, reproduction, inheritance, and internal balance. Exam questions often ask you to distinguish mitosis from meiosis, interpret a Punnett square, explain a feedback loop, or relate a system's structure to its function.
Reproduction and systems
Sexual reproduction involves gametes and produces genetic variation; asexual reproduction produces offspring genetically identical to the parent. Mitosis makes genetically identical body cells and supports growth and repair. Meiosis produces haploid gametes with half the chromosome number and creates variation through crossing over and independent assortment. Organ systems work together: digestive, respiratory, and circulatory systems exchange and transport materials.
Genetics
DNA is organized into genes on chromosomes. A gene has alleles; genotype is the allele combination, and phenotype is the observable trait. Dominant alleles are expressed when present; recessive alleles are expressed only when two recessive alleles are present. A Punnett square predicts probabilities, not guaranteed outcomes. A monohybrid cross of Bb × Bb gives genotype probabilities 1 BB : 2 Bb : 1 bb and phenotype probabilities 3 dominant : 1 recessive.
Homeostasis
Homeostasis maintains a stable internal environment around a set point. Negative feedback detects a change, triggers a response, and reduces the change. For example, when blood glucose rises, insulin promotes glucose uptake; when it falls, glucagon promotes glucose release. Positive feedback amplifies a change and is less common in normal homeostasis.
Worked example
Cross two heterozygous parents, Bb × Bb. Each parent can pass B or b. The Punnett square gives BB, Bb, Bb, bb. Genotype ratio = 1 BB : 2 Bb : 1 bb. Phenotype ratio = 3 dominant : 1 recessive. The probability of a recessive offspring is 1/4, or 25%, for each offspring; previous offspring do not change that probability.
Traps
- Saying meiosis produces identical cells; it produces haploid gametes.
- Confusing genotype with phenotype.
- Assuming a dominant allele is more common in a population.
- Treating Punnett-square results as certain rather than probabilistic.
- Reversing the roles of insulin and glucagon in blood-glucose regulation.