ILTS Science: Biology (239) common mistakes
ILTS Science: Biology (239) · Understand the traps and how to avoid them.
Solving genetics probability problems by reasoning word-by-word through the cross instead of setting up a Punnett square or product rule, then reporting the probability of one specific birth order when the question asks for the probability of a family composition (e.g., 'two of three children affected').
The candidate learned monohybrid crosses visually but never internalized that multi-child or multi-gene problems require the binomial expansion or the multiplication rule applied to unordered outcomes; under time pressure they default to the single-square answer.
What to do instead
Before computing, classify the question: (a) single offspring genotype/phenotype - use the Punnett square; (b) family composition - use binomial coefficients C(n,k) p^k q^(n-k); (c) two independent genes - multiply per-gene probabilities. Practice writing the coefficient explicitly (e.g., 3 for 'two of three') before multiplying.
Treating meiosis and mitosis as interchangeable when answering questions about crossing over, independent assortment, or chromosome number in gametes, and mislabeling the products of meiosis I versus meiosis II.
Candidates memorize the sequence of phases but not the functional consequence of each division; they conflate 'sister chromatids separate' with 'homologs separate' and lose track of when ploidy halves.
What to do instead
Build a one-page contrast table: homolog separation and reduction to haploid at meiosis I (with crossing over at prophase I), sister-chromatid separation at meiosis II. For every question, ask 'which division is this, and are the separating units homologs or sisters?' before answering.
Confusing the roles of the light-dependent reactions and the Calvin cycle, and placing ATP/NADPH consumption or O2 release in the wrong stage.
Textbook diagrams are memorized as a single blur; candidates do not anchor each stage to its location (thylakoid membrane vs. stroma) or to its inputs/outputs, so distractor options that swap products look plausible.
What to do instead
Write the two half-reactions from memory: light reactions consume H2O and light, produce O2, ATP, NADPH at the thylakoid membrane; the Calvin cycle consumes ATP and NADPH and fixes CO2 in the stroma. Then answer by eliminating any option that puts a product in the wrong compartment.
Reading a graph of enzyme activity versus temperature or pH and selecting the point of maximum rate as the 'optimum' without checking whether the question asks for optimum conditions, denaturation onset, or the rate at a specified condition.
Candidates skim the stem and pattern-match 'peak = answer'; they do not re-read the exact quantity requested, and they confuse the shape of the curve (sharp drop = denaturation) with its maximum.
What to do instead
Underline the requested variable in the stem before looking at the graph. State in words what the x-axis and y-axis measure, then locate the value asked for. For denaturation questions, identify the steep decline, not the peak.
Answering natural-selection items by describing individual organisms 'adapting' or 'mutating in response to a need' rather than describing change in allele frequencies in a population across generations.
Everyday language ('the plant adapted to shade') conflicts with the technical meaning; candidates carry the Lamarckian framing into the exam and choose the distractor that sounds intuitive.
What to do instead
Use a fixed sentence template for every evolution item: 'Variation exists → differential reproductive success → allele frequencies shift.' Reject any option in which an individual acquires a heritable trait during its lifetime or where selection acts on anything other than heritable variation.
Miscounting trophic levels and energy transfer by treating a food-web diagram as a set of pairwise predator-prey links, so that a secondary consumer is labeled a primary consumer or the 10% rule is applied to the wrong step.
Candidates trace arrows as 'eats' relationships without establishing the producer baseline; arrow direction conventions (energy flow vs. consumption) are also misread.
What to do instead
Annotate the diagram before answering: mark producers as level 1, then number each organism by the longest path from a producer. Apply the 10% rule only between adjacent numbered levels, and check whether the question asks about a single chain or the whole web.
Using the wrong equation or unit in osmosis and water-potential problems, e.g., computing solute potential without the ionization factor i, or reporting water potential in the wrong sign convention.
Candidates memorize ψ = ψs + ψp but forget that ψs = −iCRT and that i depends on dissociation; they also forget that water moves toward more negative water potential.
What to do instead
Write the full formula with units (bars or MPa) and identify i for the solute (1 for glucose, 2 for NaCl, etc.) before substituting. Sanity-check the sign: pure water at atmospheric pressure has ψ = 0, and water always moves from higher (less negative) to lower (more negative) ψ.
Confusing correlation with causation and misidentifying independent, dependent, and controlled variables when interpreting an experimental scenario or a data table.
The stem describes a realistic classroom experiment and candidates answer from general impressions of the result rather than from the design; control groups and constants are not distinguished.
What to do instead
For each scenario, list the manipulated variable, the measured variable, the constants, and the control group in a four-line table before reading the options. Reject any conclusion that goes beyond the manipulated variable or that lacks a control comparison.
Answering cell-transport items by labeling any movement down a concentration gradient as 'active transport' or any protein-mediated movement as 'active', and failing to distinguish facilitated diffusion from primary active transport.
Candidates key on the presence of a membrane protein as the defining feature of active transport, ignoring the ATP requirement and the direction relative to the gradient.
What to do instead
Apply a two-question test: does the substance move down or against its gradient, and is ATP (or an electrochemical gradient established by ATP) consumed? Down + no ATP = diffusion (simple or facilitated); against + ATP = active transport. Verify with the specific example (Na+/K+ pump vs. GLUT transporter).
Misreading pedigree symbols and inheritance patterns - assigning autosomal recessive to a trait that appears in every generation, or missing that an affected daughter of unaffected parents rules out X-linked dominant.
Candidates memorize 'skips a generation = recessive' as a shortcut without applying the diagnostic rules for sex-linked and dominant inheritance, and they overlook carrier status.
What to do instead
Run a checklist on every pedigree: (1) Does the trait skip generations? (2) Are affected individuals predominantly one sex? (3) Do affected fathers pass it to all daughters (X-linked dominant) or no sons? (4) Do two affected parents have an unaffected child (dominant)? State which rule each option violates before choosing.