Biology - Genetics, Physiology and Evolution
Core concepts
GS biology spans genetics (Mendelian inheritance, monohybrid and dihybrid crosses, sex-linked traits, meiosis vs mitosis, DNA structure and protein synthesis), human physiology (nervous and hormonal regulation, immunity, blood glucose regulation, gas exchange) and evolution and ecology (natural selection, adaptation, population dynamics, energy flow). Experimental reasoning - interpreting a protocol and its controls - is increasingly tested.
Traps
- Confusing genotype ratios (1:2:1) with phenotype ratios (3:1) in a monohybrid cross.
- Forgetting that sex-linked recessive traits appear more often in males (one X chromosome).
- Mixing up mitosis (diploid → identical diploid) and meiosis (diploid → haploid gametes with recombination).
- Describing a feedback loop in the wrong direction: insulin lowers blood glucose, glucagon raises it.
- In immunology, confusing the innate (non-specific) and adaptive (specific, memory) responses.
Worked example
Cross two heterozygotes Aa × Aa. Set up a 2×2 Punnett square: offspring are 1 AA : 2 Aa : 1 aa, i.e. genotypic ratio 1:2:1 and phenotypic ratio 3:1 if A is dominant. For a dihybrid AaBb × AaBb, the ratio is 9:3:3:1. Rule of thumb: count alleles per parent, multiply probabilities for independent traits, and always state whether the trait is autosomal or sex-linked before concluding.