Biology: Systems, Inheritance, and Experimental Logic
Core concepts
Class XII Biology covers reproduction, genetics and evolution, human physiology, biotechnology, and ecology. Understand processes rather than lists: how meiosis produces variation, how Mendelian ratios arise, how enzymes and hormones regulate the body, and how energy flows through ecosystems. Diagrams - the heart, nephron, flower, and DNA replication - are frequently assessed.
Traps in questions
- Genetics notation: distinguish genotype from phenotype and read the cross carefully; a 3:1 ratio needs heterozygous parents.
- Mitosis vs meiosis: meiosis halves chromosome number and creates variation; mitosis does not.
- Hormone sources: match each hormone to its gland and target; adrenaline and insulin are common confusions.
- Ecological terms: biomass, energy, and numbers pyramids differ; energy always decreases up a trophic level.
- "Explain" vs "state": explanation requires a mechanism, not a label.
Worked example
Cross two heterozygous tall pea plants ($Tt \times Tt$). Punnett square gives $TT$, $Tt$, $Tt$, $tt$ - a 1:2:1 genotype ratio and 3:1 phenotype ratio (tall:short). If 400 offspring are produced, expect about 300 tall and 100 short, because $tt$ alone is short.
Rule of thumb
For inheritance questions, write parental genotypes, draw the square, then state genotype and phenotype ratios separately. For physiology, describe the stimulus–response pathway step by step.