Heredity
A crop grown entirely from runners has the same useful trait in every plant. Why could a new pathogen spread through this crop particularly quickly?
The plants are genetically similar, so few may have resistance to the new pathogen.
A DNA change is found in a patient’s skin tumour but not in their sperm or egg cells. Is it likely to be passed to their children?
No. It is a somatic mutation; inherited changes must be present in the germ line.
A patient’s disease is caused by a virus. Why would an antibiotic that blocks bacterial cell-wall synthesis not directly treat it?
Viruses lack bacterial cell walls and the target of that antibiotic.
Body temperature rises above its usual range. Name two responses that help return it toward that range.
Sweating and vasodilation of skin blood vessels; both increase heat loss.
A species has 12 chromosomes in each body cell. Ignoring crossing over, how many chromosome combinations can independent assortment produce in its gametes?
64 combinations: there are six homologous pairs, so 2^6 = 64.
Why can ultraviolet radiation increase mutation rates even though it does not directly write a new DNA base?
UV can damage DNA, including forming pyrimidine dimers; errors may arise during repair or replication.
Cases occur among people who drank from one untreated water source, including people who never met another patient. What transmission route is most strongly supported?
Waterborne transmission through a contaminated common source.
A patient has high blood glucose and high circulating insulin, but muscle cells take up little glucose. Which process is failing?
Insulin responsiveness: target cells are resistant to insulin’s signal.
Bacteria with DNA labelled using heavy nitrogen are moved to light nitrogen. What proportions of their DNA molecules are hybrid and fully light after two rounds of replication?
50% hybrid and 50% fully light, as predicted by semiconservative replication.
One patient has a single base substitution; another has 47 chromosomes. Which patient has a chromosome-number mutation?
The patient with 47 chromosomes has aneuploidy, generally caused by nondisjunction.
Cells around an infected area of a plant leaf die rapidly, limiting pathogen spread. What is this defence response called?
The hypersensitive response.
A person inherits a faulty enzyme for breaking down a dietary amino acid, yet symptoms can be reduced by limiting that amino acid in food. What does this show about disease expression?
A genetic cause can interact with an environmental factor, such as diet, to determine phenotype and severity.
A DNA template strand reads 3′-TAC GGA CTT-5′. What mRNA sequence is transcribed from it?
5′-AUG CCU GAA-3′.
A coding DNA change alters an mRNA codon from GAA to GAG; both specify glutamate. What effect does this substitution have on the polypeptide sequence?
It is a synonymous or silent substitution: the amino acid sequence is unchanged.
An infected cut becomes red, warm and swollen. Which two changes in nearby blood vessels explain these signs?
Vasodilation increases blood flow; increased capillary permeability allows fluid and immune components into tissue.
A population of 1,000 starts the year with 50 existing cases. There are 20 new cases, 10 recoveries and no deaths. What are the year’s new-case count and the number living with the condition at year’s end?
Incidence count: 20 new cases. End-of-year prevalence count: 60 cases.
One parent has genotype Iᴬi and the other Iᴮi. What is the probability of a child with blood group AB, and of a child with blood group O?
25% AB and 25% O.
A mutation leaves a gene’s coding sequence intact but weakens its promoter. Predict the likely effect on protein production.
Less transcription, so usually less mRNA and less protein despite the unchanged coding sequence.
A second exposure to the same pathogen produces a faster, stronger antibody response. Which feature of the immune system accounts for this?
Immunological memory, involving memory B cells formed after the first exposure.
In a cohort study, 40 of 1,000 exposed people and 10 of 1,000 unexposed people develop a condition. What is the relative risk?
4: the observed risk is 4% versus 1%.
Two unaffected parents have an affected daughter. Assuming a single-gene Mendelian trait with no new mutation, what inheritance pattern fits, and what is the chance their next child is affected?
Autosomal recessive; 25%. Both parents must be carriers.
A population contains 80 A alleles among 100 allele copies. Migrants add 20 copies, four of which are A. What is the new frequency of A?
70%: (80 + 4) / (100 + 20) = 84/120.
A vaccine and an injection of ready-made antibodies can both protect someone. Which provides immediate but usually short-lived protection, and why?
Ready-made antibodies provide passive immunity; the recipient has not generated the same lasting memory response.
A study finds that coffee drinkers have more lung cancer, but smoking is also more common among coffee drinkers. What must researchers account for before attributing the difference to coffee?
Smoking as a confounding variable.
A mother carries an X-linked recessive allele; the father is unaffected. What proportion of their sons are expected to be affected, and what proportion of their daughters?
50% of sons; 0% of daughters. Half the daughters are expected to be carriers.
A storm kills most individuals on an island at random. An allele becomes common among the few survivors despite providing no survival advantage. What process explains the change?
Genetic drift, specifically a bottleneck effect.
Cases in equal-length transmission intervals rise 12 → 24 → 48, then after controls fall 48 → 30 → 18. What happened to the approximate number of new cases generated per case?
It fell from about 2 to about 0.6, below the level needed for sustained growth.
Classify these strategies: reducing exposure to tobacco smoke, and screening symptom-free people for early cancer.
Reducing smoke exposure is primary prevention; screening is secondary prevention.
A sharp rise in luteinising hormone is followed by a rise in progesterone. What event does the first change trigger, and which structure produces most of the progesterone immediately afterward?
The LH surge triggers ovulation; the corpus luteum produces progesterone.
Resistant bacteria are rare before antibiotic treatment but common afterward. Why is it incorrect to conclude that the antibiotic needed to create the resistance mutation?
Resistant variants may already have existed; the antibiotic selects them by removing susceptible bacteria.
During an outbreak, 10 of 200 vaccinated people and 40 of 200 unvaccinated people become ill. What is the relative risk for the vaccinated group, and why should an observational study be interpreted cautiously?
Relative risk is 0.25: 5% divided by 20%. The groups may differ in exposure or other factors, so the comparison alone does not prove causation.
A person’s cochlear hair cells are badly damaged but their auditory nerve still functions. How can a cochlear implant assist hearing?
It converts sound into electrical signals that stimulate the auditory nerve, bypassing damaged hair cells.
Genetically identical seedlings grown in sun and shade develop different leaf thicknesses. Why does this not show that their DNA sequences have changed?
The environment can alter gene expression and phenotype without changing DNA sequence.
In a recombinant-DNA procedure, what are the distinct roles of a restriction enzyme and DNA ligase?
A restriction enzyme cuts DNA at specified sequences; ligase joins DNA fragments.
After repeated antibiotic use, a resistant bacterial strain becomes dominant. Explain the change using variation and selection.
Resistance variants existed or arose through mutation; treatment favoured their survival and reproduction while susceptible bacteria declined.
Dialysis removes wastes and excess fluid from blood. Why should it not be described as restoring all kidney functions?
It does not fully replace other kidney functions, including hormone production, or correct the underlying cause of kidney failure.
At one DNA-profile location, a child has alleles 12 and 14 and the mother has 10 and 12. Candidate A has 14 and 15; Candidate B has 12 and 13. Which candidate is excluded as the father, and why does this alone not prove the other is the father?
Candidate B is excluded: the child’s 14 allele must come from the father. A is compatible, but one matching location cannot establish identity.
One genetically modified crop variety replaces many locally adapted varieties across a region. Name two different biodiversity concerns that should be investigated.
Reduced genetic diversity within the crop, and possible gene flow of the introduced allele into compatible wild relatives.
Disease cases cluster near standing water after rain. Design a comparison that would test the hypothesis that a mosquito transmits the pathogen.
Compare new case rates in otherwise similar areas with and without mosquito-control measures, while measuring mosquito numbers and accounting for other differences.
Region A has a higher crude death rate from a non-infectious disease than Region B, but Region A has a much older population. What comparison is needed before claiming residents of A have greater disease risk?
Compare age-specific or age-standardised rates so the difference in population age is accounted for.