Biology and Life Sciences
Core concepts
Understand cell structure (prokaryote versus eukaryote, organelles and their functions), the central dogma of molecular biology (DNA to RNA to protein), mitosis versus meiosis, Mendelian inheritance, and the basics of ecology and evolution by natural selection. Know the difference between genotype and phenotype and how dominant and recessive alleles interact.
Common traps
- Claiming mitochondria are present in prokaryotes; they are not.
- Confusing mitosis (two identical diploid cells) with meiosis (four genetically varied haploid gametes).
- Mixing transcription (DNA to mRNA) with translation (mRNA to protein).
- Assuming a dominant allele is more common in a population; dominance is about expression, not frequency.
- Forgetting that natural selection acts on phenotypes, not directly on genotypes.
Worked example and rule of thumb
Rule of thumb: draw a Punnett square for any two-parent cross. Example: a cross between two heterozygotes (Aa × Aa) gives genotype ratios 1 AA : 2 Aa : 1 aa, and phenotype ratios 3 dominant : 1 recessive. If a question asks for the probability of a recessive phenotype, the answer is one quarter. Always state whether the question asks about genotype or phenotype, and whether alleles are autosomal or sex-linked, because that changes the ratios entirely.