MTTC Biology (017) common mistakes
MTTC Biology (017) · Understand the traps and how to avoid them.
Analyzing a genetics cross as if alleles assort independently whenever two traits appear in one question, even when the genes are stated or implied to be linked on the same chromosome.
Most practice crosses involve unlinked genes, so candidates default to a 9:3:3:1 Punnett square and ignore recombination frequency, then pick a ratio that matches no listed option and guess.
What to do instead
Before computing, write the parental genotypes and ask whether the genes are on the same chromosome. If linkage is given, compute expected gamete frequencies from the recombination percentage first, then build the cross from those gametes rather than from independent assortment.
Reading a pedigree and declaring a trait dominant or recessive based on the number of affected individuals rather than on whether the pattern is possible under each model.
Candidates count affected squares and circles and use majority rule, forgetting that a rare recessive trait can appear in many descendants of a carrier couple, and that unaffected parents with an affected child is decisive evidence.
What to do instead
Apply elimination tests in a fixed order: unaffected parents with an affected child means recessive; affected parent with unaffected child rules out certain dominant models; check sex-linked possibilities last by asking whether an affected daughter has an unaffected father (which rules out X-linked recessive).
Treating transcription and translation as one blended process and placing the wrong molecule at the wrong step, such as saying tRNA carries the genetic message or that ribosomes make mRNA.
The processes are taught together and share vocabulary (template, codon, anticodon), so under time pressure the roles of DNA, mRNA, tRNA, rRNA, and the ribosome collapse into one script.
What to do instead
Build a one-line chain and rehearse it: DNA to mRNA (RNA polymerase, nucleus) to ribosome (rRNA plus protein) to tRNA delivering amino acids matched by anticodon to codon. Then answer each question by locating exactly where in the chain it sits.
Confusing the products of mitosis and meiosis, particularly claiming meiosis produces four identical haploid cells or that crossing over happens in mitosis.
Both are described as nuclear division with chromosome segregation, and the word haploid gets attached to the wrong outcome; candidates also forget that meiosis II resembles mitosis.
What to do instead
Memorize a contrast table with the four axes examiners test: number of divisions, ploidy of products, genetic identity of products, and where variation arises (crossing over in prophase I, independent assortment in metaphase I). Recall that a single meiosis yields four genetically distinct haploid cells.
Describing enzyme behavior as if the enzyme is consumed or as if raising temperature always raises reaction rate, instead of recognizing denaturation and the plateau at saturation.
Everyday intuition says more heat means faster reactions, and the lock-and-key diagram makes the enzyme look like a reactant that gets used up.
What to do instead
State the two rules explicitly when studying: enzymes are catalysts recovered unchanged, and activity rises with temperature only until the optimum, after which the tertiary structure unfolds and activity falls irreversibly. Also separate the effects of substrate concentration (plateaus at Vmax) from temperature and pH (bell-shaped).
Answering questions about a diagrammed cell by naming the organelle that looks prominent rather than the one whose function the stem describes, and misassigning functions such as protein synthesis to the Golgi or ATP production to the chloroplast in a nonphotosynthetic context.
Organelle shapes are visually similar in simplified drawings, and function lists are memorized as a block without linking each function to its structure.
What to do instead
Create a function-first list: for each function (ATP synthesis, protein synthesis, modification and packaging, lipid synthesis, waste breakdown, storage), name the organelle and one structural cue that identifies it. Practice answering from the function side, not the picture side.
Assuming that natural selection produces what an organism needs, or that individuals adapt during their lifetime and pass the change on.
Everyday language uses adapt to mean adjust, and Lamarckian phrasing appears in popular science writing, so the distinction between acquired change and heritable variation is blurred.
What to do instead
Use a fixed sentence frame in every evolution answer: variation already exists in the population, the environment differentially affects survival and reproduction, allele frequencies shift across generations. Reject any option that says an individual changed in order to survive or that a trait arose because it was needed.
Treating a food web as a set of independent chains and ignoring the arithmetic of energy transfer, for example claiming a top predator receives most of the producer's energy.
Arrows in food webs are read as flow of biomass rather than flow of energy, and the roughly ten percent transfer efficiency is remembered as a phrase without being applied.
What to do instead
Whenever a trophic question appears, sketch the pyramid and apply the ten percent rule numerically: if producers fix a given amount, each successive level retains about a tenth. Also state that arrows point in the direction energy flows, from consumed to consumer.
Describing homeostasis as a fixed set point with no feedback, or reversing negative feedback so that the response amplifies the deviation.
The term feedback sounds like reinforcement, and diagrams with arrows looping back are easy to read in either direction.
What to do instead
For each example (body temperature, blood glucose, water balance), write the sensor, the integrating control, and the effector response, then confirm the response opposes the initial change. Contrast with one positive feedback example such as oxytocin during childbirth to fix the difference.
Answering experimental-design and data-interpretation items from the conclusion in the stem rather than from the variables and controls, such as accepting a result with no control group or confusing correlation with causation.
Biology items increasingly embed data and experimental scenarios, and candidates trained on recall questions read for the fact they recognize instead of identifying the independent variable, dependent variable, and control.
What to do instead
Before choosing, label the independent variable, dependent variable, constants, and control group in the scenario. Reject conclusions that exceed the data shown, and treat any claim of cause without a manipulated variable and a control as unsupported.