BSEH SS Biology common mistakes
BSEH SS Biology · Understand the traps and how to avoid them.
Writing 'sperm mother cell undergoes meiosis to give two spermatids' or otherwise stating the wrong chromosome number or product count per cell, instead of tracking 2n to n through meiosis I and II to yield four haploid spermatids from one primary spermatocyte.
Candidates memorise the spermatogenesis diagram as a picture rather than as a chromosome-count argument, so crossing-over, the two divisions, and the one-to-four expansion blur together; they also confuse the number of cells with the number of chromatids.
What to do instead
Redraw the pathway from memory with chromosome number written beside each stage: spermatogonium (2n) → primary spermatocyte (2n, after S phase) → two secondary spermatocytes (n each) → four spermatids (n each). Then answer any count question by asking 'which stage, and how many cells of that stage exist per original cell?'
On genetics crosses, giving the phenotype ratio without stating the genotype ratio, or reporting a 3:1 ratio for a dihybrid cross or for a test cross, because the candidate is pattern-matching the ratio they last memorised rather than deriving it.
Ratio questions are usually practised as recall of standard numbers (3:1, 9:3:3:1, 1:1), so under time pressure a candidate grabs a familiar ratio without checking the cross type, linkage, or whether the question asks for genotype or phenotype.
What to do instead
Before computing, write three labels on the page: cross type (monohybrid/dihybrid), test cross or self cross, and what is asked (genotype, phenotype, or gametes). Derive the ratio with a Punnett square or branch diagram from the stated parental genotypes, then read off the requested category only.
Describing natural selection as 'organisms adapt in order to survive' or 'they develop resistance because they need it', rather than as differential survival of pre-existing heritable variation that shifts allele frequencies across generations.
Everyday language uses 'adapt' as an active verb for individuals, and textbook summaries are compressed; candidates then write teleological sentences that sound biological but are mechanistically wrong and lose marks on explanation questions.
What to do instead
Use a fixed template for every natural selection answer: (1) variation already exists in the population; (2) it is heritable; (3) a selective pressure acts; (4) individuals with the favourable variant leave more offspring; (5) allele frequency in the population changes over generations. Never write 'in order to' or 'needs to'.
Naming the vector or the enzyme in a biotechnology scenario wrongly, for example calling the Ti plasmid a gene gun, calling EcoRI a ligase, or claiming PCR uses DNA ligase for extension, instead of matching each named tool to its actual function in the workflow.
Biotechnology questions are dense with tool names, and candidates revise from lists rather than from a process narrative, so the names stay attached to the chapter heading rather than to a step in the procedure.
What to do instead
Build one linear workflow and place every tool on it: isolate DNA → cut with restriction endonuclease (sticky ends) → insert into vector (plasmid/Ti) → join with DNA ligase → transform host → select with marker → amplify by PCR (Taq polymerase, primers, thermal cycling). Rehearse by covering the labels and naming each tool from its function.
Confusing restriction enzyme recognition sites with promoters or selectable markers, or describing a vector as needing 'a restriction enzyme gene', when a vector needs an origin of replication, a selectable marker, and a recognition site for the chosen enzyme.
The three vector features are taught as a list, and candidates memorise the list without understanding what each element does, so they substitute one for another when the question is phrased differently.
What to do instead
For each vector feature, write the question it answers: origin of replication - can it replicate in the host? selectable marker - how do I find transformed cells? recognition site - where does my insert go? Answer vector questions by checking each feature against its question.
Writing a food chain or energy-flow answer that gives energy as recycled or that omits the 10 percent transfer rule, or that states decomposers are the primary consumers, instead of treating energy as unidirectional and matter as recyclable.
Nutrient cycling and energy flow are taught together, and candidates merge them into one statement; the difference between matter (recycled) and energy (lost as heat at each level) is the point most often lost.
What to do instead
State the rule explicitly in every answer: energy flows one way and is lost as heat at each trophic level (roughly 10 percent transferred), while nutrients are recycled by decomposers. If asked about a food chain, draw the arrow in the direction of energy flow and label each level.
Describing the light reactions and the Calvin cycle with their products swapped, for example saying ATP and NADPH are made in the Calvin cycle or that oxygen comes from carbon dioxide, rather than assigning each product to the correct stage and source.
Both stages produce and consume the same molecules, so a candidate who has memorised names without the location and inputs conflates them; the oxygen-source fact (photolysis of water) is also often reduced to 'water is used'.
What to do instead
Draw the chloroplast as two compartments: thylakoid (light reactions: water split, O2 released, ATP and NADPH made) and stroma (Calvin cycle: CO2 fixed by RuBisCO, ATP and NADPH consumed, sugar made). For every product asked about, name the compartment that makes it.
Answering a hormone or homeostasis question by naming the gland that secretes a hormone but not its target or its feedback control, or reversing the roles of insulin and glucagon, instead of giving the secretion, target, effect, and feedback loop.
Hormone questions are revised as gland-hormone pairs, so the candidate can name the source but cannot explain the physiological response or the negative feedback that regulates it, which is what the marks are for.
What to do instead
For each hormone, write four lines: secreted by, target tissue, effect on blood parameter, and feedback direction. Example: insulin from pancreatic beta cells, target liver and muscle, lowers blood glucose, high glucose stimulates and low glucose inhibits it. Answer in that order every time.
Stating the products of mitosis and meiosis as if they were interchangeable, for example saying mitosis produces four haploid cells or that meiosis occurs in somatic tissue, instead of tying each process to its product and its role.
Both processes are drawn as similar division diagrams, and candidates revise them side by side without a comparison table, so the distinguishing features (crossing over, homologous pairing, chromosome number of products, tissue location) collapse together.
What to do instead
Keep a two-column comparison in your notes: mitosis - somatic cells, one division, no pairing, two diploid identical cells, growth and repair; meiosis - germ cells, two divisions, pairing and crossing over, four haploid non-identical cells, gamete formation. Before answering, name the tissue and the product required.
Giving a one-line answer to a 'describe' or 'explain' question, for example writing 'vaccination gives immunity' or 'biodiversity is important' without the mechanism, the named example, or the consequence, instead of matching the answer to the command word.
Candidates under time pressure answer the topic rather than the instruction, and because the topic is familiar they assume the marker will infer the rest; marks are awarded for stated mechanism and specific example, not for the topic name.
What to do instead
Underline the command word first. For 'describe', give the sequence of steps; for 'explain', give cause and effect; for 'compare', give both sides of at least two criteria. Then add one named example from the syllabus. Check that the number of distinct points you wrote matches the marks available.