Biology — Cell Division, Genetics, Plant Physiology & Human Body Systems — Practice

Question 1

A man with blood group A (genotype IAi) marries a woman with blood group B (genotype IBi). What are the possible blood groups of their children? Give the ratio.

Accepted Answer

Father gametes: IA and i. Mother gametes: IB and i. Punnett square: IAIB (blood group AB), IAi (blood group A), IBi (blood group B), ii (blood group O). Possible blood groups: AB : A : B : O in ratio 1:1:1:1. So all four blood groups are possible, each with 25% probability. This is a classic ICSE genetics question. Note: Blood group AB shows CODOMINANCE — both IA and IB are expressed simultaneously.

Question 2

A plant is kept at 25°C with adequate water. When light intensity is increased from 0 to 10,000 lux, the photosynthesis rate increases. But beyond 10,000 lux, the rate stays constant at the same CO₂ concentration. Explain what is happening and what you would do to increase the rate further.

Accepted Answer

At low light intensity: LIGHT is the limiting factor — increasing light increases photosynthesis rate. At 10,000 lux: the rate plateaus because LIGHT is no longer limiting — now CO₂ CONCENTRATION or temperature has become the new limiting factor. To increase the rate further: INCREASE the CO₂ concentration (e.g., add CO₂ to the greenhouse atmosphere). This is why greenhouses pump in extra CO₂ to increase crop yield. Alternatively, slightly increasing temperature (up to the enzyme's optimum, around 30–35°C) could also increase the rate if CO₂ is increased simultaneously.

Question 3

Describe the three processes that occur in the nephron to produce urine. State ONE substance present in the glomerular filtrate that is NOT present in urine and explain why.

Accepted Answer

THREE PROCESSES in the nephron: (1) GLOMERULAR FILTRATION: Blood at high pressure in the glomerulus forces small molecules (water, glucose, urea, salts, amino acids) through the capillary walls into Bowman's capsule. Large molecules (proteins, blood cells) remain in blood. (2) TUBULAR REABSORPTION: As filtrate passes through PCT (proximal convoluted tubule), loop of Henle, and DCT: ALL glucose, amino acids, vitamins, and most water and salts are actively or passively reabsorbed back into the blood. (3) TUBULAR SECRETION: Additional waste products (creatinine, H⁺ ions, drugs) are actively secreted from blood into the tubular filtrate. SUBSTANCE present in filtrate but NOT in urine: GLUCOSE. Reason: All glucose that is filtered is completely reabsorbed by active transport in the proximal convoluted tubule. Exception: In diabetes mellitus, blood glucose is so high that the reabsorption capacity is exceeded → glucose appears in urine (glucosuria).

Practice — Biology — Cell Division, Genetics, Plant Physiology & Human Body Systems

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