Chapter 16 — Excretion, reproduction and heredity

Excretion is removal of metabolic waste

Cells produce waste as they work. In humans, breakdown of nitrogen-containing molecules leads to urea formation, mainly in the liver. Blood carries urea to the kidneys, which help regulate water and dissolved substances while removing wastes in urine. This route distinguishes three organs with different jobs: liver makes much of the urea, kidneys filter and adjust the blood-derived fluid, and the bladder stores urine before release. The kidneys do not simply “store urine,” and the bladder does not manufacture it. A question may intentionally swap these roles.

The nephron is a functional unit of a kidney. In a simplified sequence, blood-derived fluid is filtered in the glomerular region, useful substances and water are selectively reabsorbed along tubules, other substances can be secreted into the tubule, and the remaining fluid becomes urine. Calling filtration “the whole process” omits regulation and reabsorption. The kidney does not throw away every small molecule that enters the initial filtrate: much water and needed material return to blood. The amount and composition of urine vary with intake, physiology and conditions, so a single fixed volume is not an invariant fact for all people.

The ureters carry urine from kidneys to bladder. The urethra carries it from bladder to outside. The similar names are a common exam trap. Write “kidney→ureter→bladder→urethra” and then attach the function to the route. Exhaled carbon dioxide is also a metabolic waste, removed through the respiratory system. Sweat can carry some water and salts, but the skin is not a substitute for kidney regulation. Egestion of undigested food from the digestive tract is not the same as excretion of cellular metabolic waste.

Dialysis is a medical process that can remove some wastes and excess fluid from blood when kidney function is severely impaired. It does not exactly duplicate every endocrine and regulatory role of a healthy kidney. The book uses the term only as a conceptual match for an examination question, without giving treatment advice. For medical conditions, diagnosis and care depend on a qualified professional and current clinical guidance; no static MCQ should be interpreted as an individual treatment plan.

Reproduction passes information to a new generation

Asexual reproduction can produce offspring from one parent through processes such as fission, budding or vegetative propagation, depending on organism. Sexual reproduction combines genetic contributions from two gametes. These categories describe how genetic material is passed on; they do not guarantee that every asexual offspring is forever identical or every sexual offspring is visibly different in every trait. Mutation can introduce change, and environment influences appearance. An exam stem naming an organism should identify the process actually described rather than assuming one mode is exclusive to the whole species.

In humans, sperm are male gametes produced in the testes; ova are female gametes produced in the ovaries. Fertilisation normally combines one sperm and one ovum to form a zygote. The zygote divides and develops; implantation occurs in the uterus in the usual pregnancy route. A gamete is haploid, carrying one set of chromosomes; a typical body cell is diploid, with two sets. In the common human count, body cells have 46 chromosomes arranged in 23 pairs, while ordinary sperm and eggs have 23 each. Fertilisation restores the two-set count. The numbers are useful, but a question about chromosome abnormalities or unusual cell types requires more nuance than this basic route.

The placenta is an organ of exchange between maternal and developing fetal circulations in pregnancy. It supports transfer of oxygen and nutrients and removal of some wastes. It does not mean maternal and fetal blood simply mix as one open pool in the normal anatomy. The umbilical cord connects fetus to placenta. A stem that asks which structure enables exchange should not be answered “the mother's stomach”; digestion and placental exchange occur in different systems. This chapter teaches the structure–function map without attempting clinical or personal guidance.

In flowering plants, sexual reproduction follows pollination and fertilisation, after which ovule and ovary usually develop into seed and fruit respectively. That plant route differs from human reproductive anatomy but shares the idea of gametes combining and information passing to offspring. Asexual vegetative propagation in plants can use stem pieces or tubers, and it does not require seed formation. This comparison can appear in a mixed biology test. Do not call pollen itself a plant “seed”; it carries or produces male gametes, while the seed develops later from an ovule.

DNA, genes and chromosomes

DNA stores hereditary information in a sequence of nucleotides. A gene is a region of DNA with a biological product or regulatory role in the modern broad definition. Chromosomes are long DNA-containing structures packaged with proteins in eukaryotic cells. They are not three interchangeable sizes of a single object in the sense that a whole chromosome equals one gene: a chromosome can contain many genes. An inherited trait can involve one or many genes, environment and developmental processes. A four-option question with “one gene always causes one visible trait” should be checked for overstatement.

An allele is a version of a gene at a particular locus. In a simple two-allele Mendelian model, an organism can have two alleles for a gene, one inherited through each gamete. Dominant means an allele's specified effect appears in the heterozygote under the model; recessive means it does not appear in that heterozygote. Dominant does not mean “more common,” “stronger” or “better.” The label describes a relationship for a defined trait and context. Many human traits are not adequately explained by one dominant–recessive pair, so the Punnett-square method must be used only when the question explicitly provides that model.

Mendel's pea-plant experiments illustrated predictable ratios for selected traits under controlled crosses. A classic monohybrid cross of two heterozygotes, Tt × Tt, gives gametes T and t from each parent. The four equally represented combinations in the simple grid are TT, Tt, Tt and tt. Genotype ratio is 1:2:1. If T is completely dominant for the stated trait, phenotype ratio is 3:1. These are expected proportions over many offspring, not a promise that every family of four has exactly three showing one trait. Small samples can deviate by chance.

Sex determination in the familiar human chromosomal account involves an egg typically carrying an X chromosome and a sperm carrying X or Y. Fertilisation with an X-bearing sperm commonly yields XX; a Y-bearing sperm commonly yields XY. Human biological sex development has additional pathways and exceptions, so this model is a simple exam account, not a complete definition of every individual. Crucially, the egg does not choose X versus Y in this common model. Blaming a mother for a child's sex is scientifically unsound. An MCQ should be answered with the chromosomal contribution specified in the stem.

Cell division and variation

Mitosis produces daughter cells used in growth and repair in multicellular organisms, preserving the chromosome-set number in the standard account. In humans, meiosis is part of producing gametes with a reduced chromosome-set number and creates variation through processes including recombination and assortment. The two divisions differ in purpose and outcome. A question asking why fertilisation does not double the human chromosome number every generation should point to meiosis reducing the gamete number first. Mitosis does not produce the ordinary haploid gametes in this comparison.

Variation arises from mutation, genetic recombination and environmental influences. An acquired skill, such as learning to ride a bicycle, is not automatically encoded as a DNA change that offspring inherit. Conversely, an inherited predisposition does not guarantee the same outcome in every environment. Heredity and variation need both mechanisms and context. Do not infer a person's full genetic makeup from one outward observation when several genetic routes could produce it.

Worked Punnett route

For the stated model T dominant over t, place T and t across the top of a two-by-two grid and T and t down the side. Fill each cell by combining one allele from each parent: TT, Tt, Tt, tt. Count three genotypes with at least one T, hence three expected dominant phenotypes, and one tt recessive phenotype. If instead the question asks the probability of a heterozygous offspring, count the two Tt cells out of four: 1/2. Do not answer 3/4 merely because three offspring show the dominant phenotype in the model.

In a separate anatomy match, given kidney, ureter, bladder and urethra, trace fluid from where it is formed to where it leaves the body. The path gives the answer even if the options are shuffled. If the question instead asks where urea is generated from nitrogen metabolism, choose liver, not the organ that later removes it. This distinction between production of waste and removal of waste is more valuable than memorising an organ list.

Kidney processes: four terms and two directions

Filtration, reabsorption, secretion and excretion describe different events. Filtration moves fluid and small dissolved substances from blood into the initial nephron filtrate under the relevant pressure and barrier conditions. Reabsorption returns substances from the tubule towards the blood. Secretion moves substances from blood or surrounding cells into tubular fluid. Excretion is the final loss from the body in urine. A question about direction can often be answered before naming the chemical: tubule to blood indicates reabsorption, while blood to tubule beyond the initial filter indicates secretion.

The renal corpuscle contains the glomerular capillary network and the surrounding capsule. Under ordinary conditions, blood cells and most large plasma proteins do not pass into the initial filtrate in significant amounts. Small substances can enter, but their presence in filtrate does not mean they must all appear in final urine. Glucose is a useful teaching example: it is normally filtered and then reabsorbed to a very large extent. The kidney therefore performs selection and adjustment after filtration, rather than simply pouring a fixed fraction of blood into the bladder.

A simplified mass-balance equation is amount excreted = amount filtered − amount reabsorbed + amount secreted over the same time interval. Suppose a fictional substance has 100 units filtered, 80 reabsorbed and 5 secreted during one minute. The resulting excretion is 100 − 80 + 5 = 25 units per minute. This is a bookkeeping exercise, not a measurement of a particular normal metabolite. Keep the time intervals consistent. If one quantity is given per hour and the others per minute, convert before applying the equation.

Water balance helps show why urine concentration varies. When the body conserves water, hormonal regulation can increase renal water reabsorption, producing a smaller volume of more concentrated urine under appropriate conditions. With different intake and physiological signals, more water may be excreted. Antidiuretic hormone is one regulator; the kidney also contributes to electrolyte balance and acid–base regulation. A statement that kidneys remove only urea leaves out these functions. A statement that drinking a fixed quantity of water always produces the same urine volume ignores losses through breathing, skin, faeces and other physiological factors.

Nitrogenous waste and dialysis comparisons

Nitrogen in amino acids cannot simply be treated as extra carbon dioxide when amino acids are metabolised. Different animals eliminate nitrogenous waste mainly in different forms. Ammonia is highly water-demanding to eliminate safely in the usual comparative account; many aquatic animals can release it into surrounding water. Mammals mainly use urea, and birds commonly excrete nitrogen largely as uric acid. These are broad dominant patterns, not claims that an animal produces only one nitrogen-containing waste substance. Associate the named animal group and main waste form with the level of detail requested.

In haemodialysis, blood and dialysis fluid are separated by a membrane that permits selected exchanges. Concentration differences and controlled pressure conditions help remove certain wastes and excess fluid. Blood cells are retained within the blood circuit in the intended functioning system. The fluid outside the membrane is not simply plain water chosen without regard to blood composition. As an exam concept, dialysis illustrates selective transfer across a membrane; it does not recreate all functions of a healthy kidney, including its full hormonal contribution. No operational treatment settings are needed to understand this comparison.

Human reproduction: sequence and site

Keep the reproductive route separate from the urinary route even where anatomical structures have relationships. The testes produce sperm; sperm mature and are transported through associated ducts, and fluids from accessory glands contribute to semen. The ovaries contain developing egg cells and release an oocyte during ovulation in the ordinary cycle. The uterine tubes provide the usual site of fertilisation. The developing early embryo travels towards the uterus, where implantation normally occurs. Fertilisation and implantation are separate events at different locations in this standard route.

The uterine lining changes during the menstrual cycle in response to hormonal regulation. If pregnancy is not established, part of the lining is shed during menstruation. Cycle length and timing vary; the convenient 28-day teaching diagram is not a universal schedule for every individual. For a basic matching question, associate ovulation with release from an ovary, fertilisation with gamete union, and implantation with attachment of the developing embryo to the uterine lining. These definitions remain clearer than trying to assign a fixed day to every event without information from the question.

The placenta allows exchange between maternal and fetal systems across specialised tissues. The umbilical vein carries relatively oxygen-rich blood from the placenta towards the fetus, while the umbilical arteries carry blood from the fetus towards the placenta in the usual fetal circulation. As in pulmonary circulation, artery and vein names refer to direction relative to the fetal heart rather than a universal oxygen-content rule. The placenta is not a perfect barrier to every chemical or infectious agent. A school statement that it allows useful exchange does not justify assuming complete protection from all exposures.

Build a genetic cross from the parent's genotype

A genotype records the allele combination in the model; a phenotype records the expressed characteristic being studied. Homozygous means two matching alleles at the specified locus, such as AA or aa. Heterozygous means two different alleles, such as Aa. A capital letter is a notation convention for a dominant allele in a simple model, not evidence that the allele is physically larger or more frequent in the population. The problem must define dominance or provide enough information to infer it.

Start a cross by listing possible gametes, not by filling boxes at random. Under ordinary segregation, an Aa parent produces gametes carrying A or a at equal expected frequencies in the simple model. An AA parent produces only A gametes at this locus, while an aa parent produces only a gametes. Thus AA × aa gives all Aa offspring. If A is completely dominant, all show the dominant phenotype, although none is AA. This contrast tests whether the student can separate an appearance from an allele combination.

For Aa × aa, one parent contributes A or a while the other contributes a. The expected offspring are half Aa and half aa. This produces a 1:1 phenotype ratio under complete dominance. Crossing an individual with the dominant phenotype to a homozygous recessive individual is a test-cross strategy in this elementary setting. If recessive offspring occur, the dominant-phenotype parent must carry the recessive allele under the assumptions. If only dominant offspring appear in a small sample, however, that alone does not prove the parent is homozygous; chance can hide the recessive outcome.

Probability is not a quota for a family

In an Aa × Aa cross with complete dominance, the probability of aa for each offspring is 1/4 in the stated model. If successive outcomes are independent, the probability of two aa offspring in succession is 1/4 × 1/4 = 1/16. The probability of no aa offspring among two is 3/4 × 3/4 = 9/16, so the probability of at least one aa is 1 − 9/16 = 7/16. “At least one” is different from “exactly one”: exactly one occurs in two possible orders and has probability 2 × 1/4 × 3/4 = 6/16.

For 80 offspring under this model, the expected number with the recessive phenotype is 80 × 1/4 = 20. The observed count need not be exactly 20. Expected counts describe a probabilistic model and become useful in comparisons across samples; they are not a schedule that forces the next offspring to compensate for previous outcomes. If several dominant offspring appear in succession, the next offspring's probability remains 1/4 for aa under the same independent model. This is the same reasoning used for independent probability questions in Arithmetic.

Not all inheritance shows complete dominance. In an explicitly stated incomplete-dominance example, the heterozygote has an intermediate phenotype. Crossing two heterozygotes then gives a 1:2:1 phenotype ratio as well as a 1:2:1 genotype ratio. In codominance, both specified allele effects are expressed in the heterozygote. These relationships cannot be inferred solely from whether the problem uses upper-case letters. Read the description of phenotype expression, then decide whether the 3:1 shortcut applies.

Independent assortment and linkage: use the stated model

A two-locus model may describe AaBb parents and independent assortment. Under that assumption, each produces AB, Ab, aB and ab gametes with equal expected frequency. With complete dominance at each locus, an AaBb × AaBb cross has expected phenotype proportions 9:3:3:1. One can derive the combined dominant phenotype probability as 3/4 × 3/4 = 9/16 rather than memorising sixteen boxes. The double-recessive aabb probability is 1/4 × 1/4 = 1/16. This multiplication depends on the independence assumptions supplied by the model.

Genes close together on the same chromosome can show linkage, so the independent-assortment ratio is not a universal prediction for every pair of genes. Recombination during meiosis can rearrange combinations, with outcomes depending on the genetic context. For police-level preparation, the important distinction is between a model's assumptions and a law applied without limits. If the question explicitly states independent assortment, use it. If it provides evidence of linkage, do not force a 9:3:3:1 ratio merely because two traits are named.

DNA sequence, mutation and environment

A DNA sequence carries information using four bases conventionally abbreviated A, T, G and C. In the standard double-stranded model, A pairs with T and G with C. If a question gives 30% adenine in a double-stranded DNA sample, thymine is also 30%; the remaining 40% is split equally between guanine and cytosine, giving 20% each. The inference depends on double-stranded complementary DNA. It should not be transferred without thought to a single-stranded molecule or to RNA with a different base composition.

A mutation is a change in genetic sequence. Its effect can be neutral, harmful or beneficial depending on the change and context; it is not automatically a visible deformity. A mutation in a body cell is not necessarily passed to offspring, whereas changes that enter a reproductive lineage can be inherited. Environmental conditions can influence phenotype without changing the inherited DNA sequence. A plant's short height under nutrient shortage does not prove it carries a recessive dwarfing allele. Genetic inference requires evidence beyond one outward observation when several explanations are possible.

Retrieval practice with reasons

Problem 1: Filtered amount is 60 units, reabsorbed amount 45 and secreted amount 3 over the same interval. Excretion is 18 units. Problem 2: AA × aa produces Aa offspring at the named locus, not a mixture of AA and aa. Problem 3: Under Aa × aa with complete dominance, the recessive phenotype has probability 1/2. Problem 4: Under independent assortment, the probability of aabb from AaBb × AaBb is 1/16.

Problem 5: A double-stranded DNA sample has 22% guanine. Cytosine is 22%, leaving 56% shared equally by adenine and thymine, so each is 28%. Problem 6: An embryo implants in the uterus, whereas the usual fertilisation site is the uterine tube. Problem 7: A vessel carries oxygenated blood from placenta towards the fetus. It is the umbilical vein in the conventional fetal route. In each answer, state the model or anatomical direction that makes the conclusion valid.

Recall and review

Trace urea from liver through blood to kidney and urine through ureter, bladder and urethra. Distinguish filtration from reabsorption. Compare asexual and sexual reproduction by source of gametes and variation, without absolute claims. Connect DNA, gene and chromosome. Fill a Tt × Tt grid and distinguish genotype from phenotype ratio. Finally, explain why the simple human XX/XY account attributes the X-or-Y sperm contribution to the father, while recognising that real development is more complex. Attempt the chapter practice set, then use the explanations to revisit any uncertain concepts.

Figure 16. Original chapter diagram.

Practice 31 questions

Choose one answer for each item. The answer key and explanations follow this chapter set.

1. What is the average length of the human menstrual cycle?

A. 14 days B. 21 days C. 28 days D. 35 days

2. Who are credited with working out the double helix structure of DNA in 1953?

A. James Watson and Francis Crick B. Gregor Mendel and William Bateson C. Thomas Morgan and Walter Sutton D. Charles Darwin and Alfred Wallace

3. Which nitrogenous base is present in RNA in place of thymine?

A. Uracil B. Guanine C. Cytosine D. Adenine

4. Arrange the following in the order in which urine flows through them after it is formed:
(A) Urethra (B) Urinary bladder (C) Nephron (D) Ureter

A. C, D, B, A B. C, A, B, D C. D, C, B, A D. C, B, D, A

5. Assertion (A): In humans, fertilisation of the ovum normally takes place in the uterus.
Reason (R): The foetus develops inside the uterus during pregnancy.
Choose the correct option.

A. Both A and R are true and R explains A B. Both A and R are true but R does not explain A C. A is true but R is false D. A is false but R is true

6. Which organ converts toxic ammonia into the less toxic compound urea?

A. Liver B. Kidney C. Pancreas D. Spleen

7. Match List-I (Structure) with List-II (Role):
List-I: (a) Scrotum (b) Fallopian tube (c) Uterus (d) Seminal vesicles
List-II: (i) Contribute fluid to semen (ii) Site of development of the foetus (iii) Usual site of fertilisation (iv) Keeps the testes at a lower temperature than the body

A. a-iv, b-ii, c-iii, d-i B. a-i, b-iii, c-ii, d-iv C. a-iii, b-iv, c-ii, d-i D. a-iv, b-iii, c-ii, d-i

8. Approximately how many nephrons are present in each human kidney?

A. 10 thousand B. 1 lakh C. 10 lakh (one million) D. 1 crore

9. Birds excrete their nitrogenous waste mainly in the form of:

A. Urea B. Uric acid C. Ammonia D. Creatinine

10. In a nephron, the initial filtration of blood under pressure takes place in the:

A. Loop of Henle B. Glomerulus C. Collecting duct D. Ureter

11. Kidney stones (renal calculi) in humans are most commonly made up of:

A. Calcium oxalate B. Sodium chloride C. Potassium nitrate D. Magnesium sulphate

12. Which tube carries urine from each kidney to the urinary bladder?

A. Ureter B. Urethra C. Vas deferens D. Renal vein

13. ADH (antidiuretic hormone) acts on the kidney tubules to:

A. Filter more blood at the glomerulus B. Reabsorb more water, concentrating the urine C. Convert ammonia into urea D. Increase the loss of sodium in the urine

14. Humans convert toxic ammonia into urea before excreting it. On this basis, humans are described as:

A. Ammonotelic B. Uricotelic C. Ureotelic D. Aminotelic

15. In the usual human XX/XY chromosome model, which gamete may contribute either an X or a Y chromosome at fertilisation?

A. The egg, which always contributes Y B. The egg, which contributes X or Y equally C. The sperm, which may contribute X or Y D. Neither gamete; blood group sets the sex chromosomes

16. Which blood test measures the amount of urea nitrogen in blood and helps assess kidney function?

A. Blood urea nitrogen (BUN) test B. Urine glucose dipstick C. Haemoglobin estimation D. Blood group typing

17. The cup-shaped structure of a nephron that surrounds the glomerulus and collects the filtrate is called:

A. Loop of Henle B. Renal pelvis C. Bowman's capsule D. Collecting duct

18. Consider the following statements:
Statement I: In humans, the liver converts ammonia into urea.
Statement II: Ammonia is less toxic than urea.
Which of the above is/are correct?

A. Only I B. Only II C. Both I and II D. Neither I nor II

19. For typical terrestrial forms, which animal–main nitrogen-waste pair is incorrectly matched?

A. Humans – urea B. Insects – uric acid C. Terrestrial lizards – mainly urea D. Many aquatic bony fishes – ammonia

20. Which of the following is NOT an example of excretion?

A. Removal of urea by the kidneys B. Removal of undigested food as faeces C. Removal of carbon dioxide by the lungs D. Removal of salts and water in sweat

21. Which hormone, secreted by the testes, controls the development of male secondary sexual characteristics such as facial hair and a deeper voice?

A. Progesterone B. Oestrogen C. Testosterone D. Prolactin

22. Arrange the following milestones in genetics in chronological order:
A. Watson and Crick propose the double-helix model of DNA
B. Mendel carries out his pea-plant breeding experiments
C. William Bateson coins the term 'genetics'
D. Nobel Prize awarded for the discovery of DNA's structure

A. B, C, D, A B. C, B, A, D C. B, A, C, D D. B, C, A, D

23. Which European monarch is well known as a carrier of the haemophilia allele, which she passed on to several royal houses of Europe?

A. Marie Antoinette B. Queen Victoria C. Queen Elizabeth I D. Catherine the Great

24. Two heterozygous tall pea plants (Tt x Tt) are crossed and produce 400 offspring. Assuming an exact Mendelian 3 : 1 ratio, how many dwarf plants are expected?

A. 200 B. 100 C. 50 D. 300

25. When F1 hybrids (Tt) of a pure tall and a pure dwarf pea plant are self-pollinated, what is the genotypic ratio of the F2 generation?

A. 2 TT : 1 Tt : 1 tt B. 3 TT : 1 tt C. 1 TT : 1 Tt : 2 tt D. 1 TT : 2 Tt : 1 tt

26. Match each condition with its characteristic textbook chromosomal or inheritance pattern. A Down syndrome; B Turner syndrome; C Klinefelter syndrome; D haemophilia A. 1 47,XXY; 2 X-linked recessive; 3 trisomy 21; 4 45,X.

A. A-3, B-1, C-4, D-2 B. A-4, B-3, C-1, D-2 C. A-2, B-4, C-1, D-3 D. A-3, B-4, C-1, D-2

27. Match the scientist or collaborators to the contribution. (a) Hugo de Vries (b) Walter Sutton and Theodor Boveri (c) Rosalind Franklin and Raymond Gosling (d) Gregor Mendel. (i) Chromosome theory of inheritance (ii) Pea-plant inheritance laws (iii) Historical mutation theory (iv) DNA X-ray diffraction image Photo 51.

A. a-i, b-iii, c-iv, d-ii B. a-iii, b-i, c-ii, d-iv C. a-iv, b-i, c-iii, d-ii D. a-iii, b-i, c-iv, d-ii

28. Of the 23 pairs of chromosomes in a human somatic cell, how many pairs are autosomes?

A. 22 B. 23 C. 44 D. 21

29. In the common X-linked recessive model, why is red–green colour-vision deficiency more common among people with one X chromosome?

A. The trait is dominant and expressed more strongly in men B. Men have two X chromosomes and women have only one C. The gene is on the X chromosome, and men, with only one X, show the trait with a single recessive allele D. The gene is carried on the Y chromosome, which only men have

30. The chromosomal theory of inheritance, which linked Mendel's 'factors' to chromosomes, was proposed by:

A. Alfred Hershey and Martha Chase B. Matthias Schleiden and Theodor Schwann C. Walter Sutton and Theodor Boveri D. James Watson and Francis Crick

31. The mutation theory of evolution was proposed by:

A. Charles Darwin B. Jean-Baptiste Lamarck C. Hugo de Vries D. Gregor Mendel

Answers and explanations

1. C. A 28-day cycle is a common teaching average, not a fixed duration: normal cycle length varies among individuals and across life stages. Menstrual cycles begin around puberty and cease after menopause.

2. A. Watson and Crick published the double-helix model in 1953. Its development drew on X-ray diffraction work by Rosalind Franklin and Raymond Gosling and research by Maurice Wilkins; credit for the structural evidence extends beyond the two model builders.

3. A. RNA has uracil in place of thymine; its bases are A, U, G and C, whereas DNA has A, T, G and C. RNA also has ribose sugar and is usually single-stranded.

4. A. Among the four named structures the order is nephron → ureter → bladder → urethra. Between a nephron and the ureter, urine passes through collecting ducts, the renal calyces and the renal pelvis. The bladder stores urine before it passes out through the urethra.

5. D. A is false and R is true. Fertilisation normally happens in the fallopian tube (oviduct); the zygote then travels to the uterus, implants and the foetus develops there over about nine months.

6. A. The liver converts ammonia to urea in the urea cycle. The kidneys normally clear urea from blood into urine. A raised blood-urea result can have several causes and alone does not diagnose liver or kidney failure.

7. D. The scrotum keeps the testes cooler for sperm production (a-iv), fertilisation normally occurs in the fallopian tube (b-iii), the foetus develops in the uterus (c-ii), and seminal vesicles contribute fluid to semen (d-i).

8. C. A commonly taught approximation is about one million nephrons per kidney (10 lakh). Actual counts vary substantially between people; the question asks for an approximate value.

9. B. Birds excrete nitrogen mainly as uric acid, which conserves water. Many terrestrial reptiles and insects also use uric acid, though patterns vary by species and habitat. Humans mainly excrete urea and many aquatic bony fish mainly excrete ammonia.

10. B. The glomerulus is a knot of capillaries where blood pressure forces water, salts, glucose and urea out of the blood into Bowman's capsule. Reabsorption happens later in the tubule, including the loop of Henle and collecting duct.

11. A. Calcium stones are the commonest kidney stones, and calcium oxalate is more common than calcium phosphate. A stone may obstruct urine flow and cause severe pain. Chemical composition is tested here; the listed alternatives are not the usual common stone type.

12. A. There are two ureters, one from each kidney, carrying urine to the urinary bladder. The single urethra carries urine from the bladder to the outside, the vas deferens carries sperm, and the renal vein carries filtered blood away from the kidney.

13. B. Vasopressin (ADH), released from the posterior pituitary, increases water reabsorption in the kidney collecting ducts. More water returns to blood, leaving a smaller volume of more concentrated urine when other factors are comparable.

14. C. Humans and other mammals are ureotelic, excreting urea made from ammonia in the liver. Ammonotelic animals such as most bony fish excrete ammonia, uricotelic animals such as birds excrete uric acid, and aminotelic animals excrete amino acids.

15. C. In the usual XX/XY model, an egg contributes X and a sperm may contribute X or Y, giving the common XX or XY patterns. Human sex development is more complex than chromosomes alone, and neither parent controls which sperm fertilises an egg.

16. A. The BUN test measures urea nitrogen in blood. A raised result may suggest reduced kidney clearance but is interpreted with other findings. Uraemia refers to a clinical syndrome from retention of multiple waste substances in advanced kidney dysfunction; it is not simply a synonym for one high urea value.

17. C. Bowman's capsule is the double-walled cup around the glomerulus that receives the filtered fluid. The loop of Henle is part of the renal tubule, the collecting duct drains urine, and the renal pelvis funnels urine into the ureter.

18. A. Only Statement I is correct. The liver converts highly toxic ammonia into much less toxic urea through the urea (ornithine) cycle; the kidneys then remove urea from the blood. Ammonia is far more toxic than urea, not less.

19. C. Terrestrial lizards generally conserve water by excreting nitrogen chiefly as uric acid rather than urea. Humans chiefly excrete urea; insects commonly use uric acid; many aquatic bony fishes release ammonia. Species and habitat can change the pattern, so the qualifiers matter.

20. B. Removing undigested food as faeces is egestion, not excretion, because that material never entered body cells as metabolic waste. Excretion removes metabolic wastes: urea by the kidneys, CO2 by the lungs and salts with water in sweat.

21. C. The testes produce testosterone, an androgen that helps develop traits such as deeper voice and facial hair during typical male puberty. Oestrogens and progesterone are also present in all sexes at varying levels; prolactin is a pituitary hormone associated with milk production.

22. D. Mendel performed pea-plant work in the mid-1800s; Bateson introduced the word “genetics” in the early 1900s; Watson and Crick presented their model in 1953; and Watson, Crick and Wilkins received the 1962 Nobel Prize in Physiology or Medicine. Thus B, C, A, D. The 1953 work also depended on diffraction evidence from Franklin and others.

23. B. Queen Victoria was a carrier of the X-linked recessive haemophilia allele and passed it to several European royal houses, which is why haemophilia is nicknamed the royal disease.

24. B. Tt x Tt gives 1 TT : 2 Tt : 1 tt, so 3 tall : 1 dwarf. Dwarf plants (tt) form 1/4 of the offspring, so 400 x 1/4 = 100 dwarf plants and 300 tall plants.

25. D. Tt x Tt gives TT, Tt, Tt and tt, i.e. a genotypic ratio of 1:2:1. Because T is dominant, TT and Tt both look tall, which is why the phenotypic ratio becomes 3 tall : 1 dwarf.

26. D. The common associations are Down syndrome → trisomy 21; Turner syndrome → 45,X; Klinefelter syndrome → 47,XXY; haemophilia A → X-linked recessive. Hence A-3, B-4, C-1, D-2. Mosaic and other genetic forms exist, so these are characteristic patterns rather than exhaustive definitions.

27. D. De Vries is linked with the historical mutation theory (iii); Sutton and Boveri with chromosome theory (i); Franklin and her student Raymond Gosling produced Photo 51 (iv); Mendel with pea-plant inheritance patterns (ii). Thus a-iii, b-i, c-iv, d-ii.

28. A. In the usual human karyotype, 46 chromosomes are arranged in 23 pairs. Twenty-two pairs are autosomes; the remaining pair is the sex-chromosome pair. The count 44 refers to individual autosomes, not pairs. Other chromosome patterns also occur.

29. C. Most inherited red–green colour-vision deficiency follows an X-linked recessive pattern. A person with one X can express the trait when that X carries the variant; a person with two X chromosomes usually needs variants on both. Other genetic patterns and biological variation exist, so this explains the common pattern rather than every case.

30. C. Sutton and Boveri independently connected Mendelian inheritance with chromosome behaviour around 1902–03. Watson and Crick proposed the double-helix model, Hershey and Chase provided strong evidence in bacteriophages that DNA carries genetic information, and Schleiden and Schwann developed cell theory.

31. C. Hugo de Vries advanced a historical mutation theory after work with evening primrose. He argued that sudden heritable changes could produce new forms. Modern evolutionary biology combines mutation with selection, genetic drift and other processes; his historical theory is not a complete current explanation of evolution.