(a) Green house effect. (10 marks) (b) Sign stimuli in animals and their significance. (10 marks) (c) Gene therapy and its…

(a) Greenhouse effect (10 marks)

The greenhouse effect is the warming of the lower atmosphere because gases such as water vapour, CO₂, CH₄, N₂O and ozone absorb the infrared radiation emitted by the Earth and re-radiate part of it downward.

Natural effect. It keeps the average surface temperature at about 15°C instead of about −18°C, and so makes life possible.

Enhanced effect. Human activity adds gases and traps more heat.

• CO₂: fossil fuels and deforestation.
• CH₄: rice fields, cattle and landfills; about 28 times the warming potential of CO₂ over 100 years.
• N₂O: fertilisers; about 265 times.
• CFCs, HFCs and SF₆: industry.

Consequences. Global warming, glacier melt, sea-level rise, altered rainfall, floods, droughts, coral bleaching and lower crop yields.

Mitigation. The Kyoto Protocol (1997) and the Paris Agreement (2015), with renewable energy and afforestation.

(b) Sign stimuli in animals and their significance (10 marks)

A sign (key) stimulus is a simple feature of an object or situation that triggers an innate fixed action pattern through an innate releasing mechanism, without learning (Lorenz and Tinbergen).

Examples.

• The red belly of a male three-spined stickleback releases attack by rival males, even on a crude red model.
• The red spot on a herring gull's beak releases begging and pecking in the chick.
• A greylag goose rolls back an egg lying outside its nest.

Supernormal stimulus. An exaggerated artificial stimulus, such as a larger or more speckled egg, is preferred to the natural one, showing that a particular feature releases the response.

Significance.

• It gives quick, correct responses in feeding, mating, defence and parental care without experience.
• It helps species recognition and reproductive isolation.
• It explains mimicry and deception.

(c) Gene therapy and its significance (10 marks)

Gene therapy treats disease by introducing a functional gene, or correcting or silencing a faulty one, in the patient's cells.

Types.

• Somatic therapy affects body cells and is not inherited; germline therapy would be heritable and is prohibited in most countries.
• Ex vivo: cells are modified outside the body and returned, as in ADA-SCID. In vivo: the vector is given directly, as in Luxturna for retinal dystrophy.

Vectors. Retroviruses, lentiviruses, adenoviruses, adeno-associated virus, and liposomes. CRISPR–Cas9 editing is used for sickle-cell disease and β-thalassaemia.

Significance. It offers cure for single-gene disorders such as SCID, haemophilia and sickle-cell disease, and treatment for cancer (CAR-T cells).

Limitations. Immune reactions, insertional mutagenesis (leukaemia in early SCID-X1 trials), delivery, high cost and ethical concerns.

(d) Null hypothesis (10 marks)

The null hypothesis (H₀) states that there is no effect or difference, for example that two means are equal or that counts fit an expected ratio. The alternative hypothesis (H₁) states that there is an effect. A test asks whether the data are consistent with H₀.

Logic.

• A test statistic (t, χ² or F) is calculated.
• The P-value is the probability of a result at least as extreme as that observed if H₀ is true.
• With a significance level α (usually 0.05), H₀ is rejected if P < α; otherwise we fail to reject it.

Errors. Type I error: rejecting a true H₀ (probability α). Type II error: failing to reject a false H₀ (probability β); power = 1 − β.

Example. H₀: a drug does not change the mean weight of mice. In genetics, H₀: the F₂ ratio fits 3:1, tested by χ².

(e) Principle and applications of gel electrophoresis (10 marks)

Principle. Charged molecules migrate in an electric field through a gel that acts as a molecular sieve: agarose for DNA, polyacrylamide for proteins.

• DNA is uniformly negatively charged, so it moves to the anode, and small fragments run faster; distance travelled is inversely related to log size.
• Proteins differ in charge, so SDS-PAGE coats them with SDS and they separate by mass.
• Bands are seen with ethidium bromide under UV, or with Coomassie stain.

Applications.

• Research: PCR products, restriction mapping, sequencing, and Southern and Western blots.
• Diagnosis: haemoglobin variants in sickle-cell disease and thalassaemia.
• Forensics: DNA fingerprinting and paternity testing.