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Myanmar Matriculation Biology study materials
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Myanmar Matriculation Biology practice questions

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Question 1Cell Biology and Cell Division

Which organelle is described as the site of aerobic respiration and ATP production in a eukaryotic cell?

  1. Lysosome
  2. Mitochondrion Correct answer.
  3. Ribosome
  4. Golgi body

Explanation

The mitochondrion carries out the reactions of aerobic respiration, including the Krebs cycle and the electron transport chain, and produces most of the cell's ATP. Its folded inner membrane, the cristae, provides a large surface for these reactions.

Question 2Cell Biology and Cell Division

A plant cell is placed in a solution whose water potential is higher than that of the cell sap. What happens to the cell?

  1. It bursts because the cell wall cannot resist pressure.
  2. It becomes turgid as water enters by osmosis. Correct answer.
  3. It stays unchanged because the cell wall is impermeable.
  4. It becomes plasmolysed as water leaves.

Explanation

Water moves by osmosis from a region of higher water potential to a region of lower water potential. Since the external solution has the higher water potential, water enters the vacuole, the protoplast swells and presses against the cell wall, and the cell becomes turgid.

Question 3Cell Biology and Cell Division

During which stage of mitosis do the chromosomes line up singly along the equator of the spindle?

  1. Anaphase
  2. Metaphase Correct answer.
  3. Telophase
  4. Prophase

Explanation

In metaphase the chromosomes become attached to spindle fibres at their centromeres and are arranged along the equatorial plane of the cell. This alignment ensures that one chromatid of each chromosome will pass to each daughter nucleus.

Question 4Cell Biology and Cell Division

Which structure controls the entry and exit of substances in all cells and is described as selectively permeable?

  1. Vacuole membrane
  2. Plasma membrane Correct answer.
  3. Cell wall
  4. Nuclear envelope

Explanation

The plasma membrane is a phospholipid bilayer with embedded proteins that allows some substances to pass while restricting others, making it selectively permeable. This property underlies osmosis, diffusion and active transport at the cell surface.

Question 5Cell Biology and Cell Division

Which molecule carries the genetic instructions from the nucleus to the ribosome for protein synthesis?

  1. Ribosomal RNA
  2. Deoxyribonucleic acid
  3. Transfer RNA
  4. Messenger RNA Correct answer.

Explanation

Messenger RNA is transcribed from a gene in the nucleus and carries the base sequence, as codons, to the ribosome. There the sequence is translated into the amino acid sequence of a polypeptide.

Question 6Cell Biology and Cell Division

Which pair of organelles is found in a typical plant cell but absent from an animal cell?

  1. Lysosome and centriole
  2. Mitochondrion and ribosome
  3. Nucleus and Golgi body
  4. Chloroplast and large permanent vacuole Correct answer.

Explanation

Plant cells typically contain chloroplasts for photosynthesis and a large permanent vacuole filled with cell sap that maintains turgor. Animal cells lack both structures and rely on small temporary vesicles instead.

Question 7Cell Biology and Cell Division

In a cell that has a diploid number of 24 chromosomes, how many chromosomes are present in each daughter cell at the end of meiosis II?

  1. 12 Correct answer.
  2. 48
  3. 24
  4. 6

Explanation

Meiosis halves the chromosome number, so the haploid daughter cells contain half the diploid number. Half of 24 is 12, and each of these chromosomes is a single chromatid after meiosis II.

Question 8Cell Biology and Cell Division

Which process allows a white blood cell to engulf a bacterium by surrounding it with extensions of the plasma membrane?

  1. Plasmolysis
  2. Phagocytosis Correct answer.
  3. Active transport
  4. Osmosis

Explanation

Phagocytosis is a form of endocytosis in which the plasma membrane extends around a solid particle and encloses it in a vesicle. The vesicle then fuses with lysosomes whose enzymes digest the bacterium.

Question 9Cell Biology and Cell Division

An enzyme is added to a reaction mixture and the rate of reaction increases. Which statement best explains this effect?

  1. The enzyme raises the temperature of the reaction mixture.
  2. The enzyme is used up and provides energy for the reaction.
  3. The enzyme lowers the activation energy needed for the reaction. Correct answer.
  4. The enzyme increases the activation energy of the reaction.

Explanation

Enzymes are biological catalysts that provide an alternative reaction pathway with a lower activation energy. This allows more substrate molecules to react per unit time at the same temperature.

Question 10Cell Biology and Cell Division

A cell is treated with a drug that prevents the formation of spindle fibres. Which stage of mitosis will be blocked most directly?

  1. Condensation of chromosomes
  2. Reformation of the nuclear envelope
  3. Replication of DNA
  4. Movement of chromatids to opposite poles Correct answer.

Explanation

Spindle fibres attach to centromeres and pull sister chromatids to opposite poles during anaphase. Without a spindle, this separation and movement cannot occur, so the cell arrests before completing anaphase.

Question 11Cell Biology and Cell Division

Which statement correctly distinguishes prokaryotic from eukaryotic cells?

  1. Prokaryotic cells lack a membrane-bound nucleus, while eukaryotic cells have one. Correct answer.
  2. Prokaryotic cells contain chloroplasts, while eukaryotic cells lack them.
  3. Prokaryotic cells contain mitochondria, while eukaryotic cells do not.
  4. Prokaryotic cells have a nuclear envelope, while eukaryotic cells lack one.

Explanation

The defining difference is that prokaryotic cells have no membrane-bound nucleus, so their DNA lies free in the cytoplasm, usually as a circular chromosome. Eukaryotic cells enclose their DNA within a nuclear envelope.

Question 12Cell Biology and Cell Division

In the light-dependent reactions of photosynthesis, what is the main role of water?

  1. It is used to reduce NADP directly without any light.
  2. It combines with carbon dioxide to form glucose.
  3. It is split to provide electrons and protons and releases oxygen. Correct answer.
  4. It is the final electron acceptor and becomes reduced.

Explanation

Photolysis of water in the thylakoid membranes supplies electrons to replace those lost by chlorophyll and provides protons for the proton gradient. Oxygen is released as a by-product of this splitting.

Question 13Cell Biology and Cell Division

A red blood cell is placed in distilled water. Which observation is expected?

  1. The cell remains unchanged because it is isotonic to water.
  2. The cell shrinks because water leaves by osmosis.
  3. The cell becomes turgid and pushes against a cell wall.
  4. The cell swells and bursts because it lacks a cell wall. Correct answer.

Explanation

Distilled water has a higher water potential than the cytoplasm, so water enters the red blood cell by osmosis. Because animal cells have no rigid cell wall to resist expansion, the membrane is stretched until it ruptures, a process called haemolysis.

Question 14Cell Biology and Cell Division

Which description best explains why the rate of enzyme-controlled reaction falls at temperatures above the optimum?

  1. The pH of the mixture falls sharply.
  2. Hydrogen bonds break and the active site loses its shape. Correct answer.
  3. The enzyme is converted into a substrate.
  4. The substrate molecules stop moving completely.

Explanation

Above the optimum temperature, increased vibration breaks the hydrogen and ionic bonds holding the enzyme's tertiary structure. The active site changes shape so the substrate no longer fits, and the enzyme is denatured.

Question 15Cell Biology and Cell Division

What is the significance of crossing over during prophase I of meiosis?

  1. It halves the chromosome number of the cell.
  2. It duplicates the DNA before division.
  3. It separates homologous chromosomes to opposite poles.
  4. It produces new combinations of alleles on the chromatids. Correct answer.

Explanation

Crossing over is the exchange of segments between non-sister chromatids of homologous chromosomes at chiasmata. This recombines alleles and increases genetic variation among the gametes produced.

Question 16Cell Biology and Cell Division

Which feature of the cell surface membrane is best explained by the fluid mosaic model?

  1. The membrane consists of a single layer of phospholipids.
  2. Proteins are scattered within or across a moving phospholipid bilayer. Correct answer.
  3. The membrane is impermeable to all small molecules.
  4. The membrane is a rigid layer of protein with no lipid.

Explanation

The fluid mosaic model describes the membrane as a fluid phospholipid bilayer in which proteins are embedded or partly exposed, like tiles in a mosaic. Both lipids and proteins can move laterally, which explains membrane flexibility and the distribution of transport proteins.

Question 17Cell Biology and Cell Division

A student observes a root tip squash and counts cells in different stages of mitosis. Why is a root tip suitable material for this study?

  1. Its cells are permanently arrested in interphase.
  2. It contains meristematic tissue where cells divide actively. Correct answer.
  3. It contains only cells in anaphase.
  4. Its cells are large enough to see chromosomes without a microscope.

Explanation

The root tip contains meristematic tissue, a region of actively dividing cells, so many cells can be found in prophase, metaphase, anaphase and telophase. This makes it easy to obtain a sample showing the different stages of mitosis.

Question 18Cell Biology and Cell Division

Which statement about the products of meiosis in a human male is correct?

  1. Four diploid cells are formed from one haploid cell.
  2. Four haploid cells are formed from one diploid cell. Correct answer.
  3. One haploid cell is formed from four diploid cells.
  4. Two diploid cells are formed from one diploid cell.

Explanation

Meiosis consists of two successive divisions after a single round of DNA replication, so one diploid cell gives rise to four haploid cells. In the male these become four sperm cells, each with 23 chromosomes.

Question 19Cell Biology and Cell Division

A cell has 20 chromosomes at the start of meiosis. How many chromatids are present in the cell after the completion of prophase I?

  1. 10
  2. 20
  3. 40 Correct answer.
  4. 80

Explanation

DNA replication in interphase doubles the chromatid number to 40 while the chromosome number remains 20. After prophase I, each chromosome still consists of two chromatids, so 40 chromatids are present.

Question 20Cell Biology and Cell Division

In the light-independent reactions of photosynthesis, how is carbon dioxide incorporated into carbohydrate?

  1. It is converted into glucose by the electron transport chain.
  2. It combines with a five-carbon acceptor and is reduced using ATP and NADPH. Correct answer.
  3. It is split by light to release oxygen and electrons.
  4. It is reduced directly by water without any energy input.

Explanation

In the Calvin cycle, carbon dioxide joins ribulose bisphosphate and the resulting compound is split and reduced to triose phosphate using ATP and NADPH from the light reactions. Two triose phosphate molecules are then used to build glucose.

Question 21Cell Biology and Cell Division

A cell is placed in a solution of a substance that inhibits ATP synthesis. Which process will be affected most directly?

  1. Diffusion of oxygen across the membrane
  2. Active transport of ions against a concentration gradient Correct answer.
  3. Movement of carbon dioxide out of the cell
  4. Osmosis of water into the cell

Explanation

Active transport uses carrier proteins and energy from ATP to move substances against their concentration gradient. If ATP synthesis is blocked, these carriers cannot function, whereas passive processes continue.

Question 22Cell Biology and Cell Division

A drug permits DNA replication but prevents the separation of sister chromatids at the centromere. What is the most likely consequence for the dividing cell?

  1. The nuclear envelope breaks down too early.
  2. Chromosomes condense prematurely and fragment.
  3. Chromatids remain joined and cannot pass to opposite poles. Correct answer.
  4. DNA is replicated twice within one cycle.

Explanation

Separation of sister chromatids depends on the splitting of centromeres at the start of anaphase. If this fails, the chromatids remain attached and cannot be drawn to opposite poles, so the daughter nuclei receive unequal or no chromatids.

Question 23Cell Biology and Cell Division

Which observation would provide the strongest evidence that a cell is in anaphase rather than telophase?

  1. A nuclear envelope is reforming around each chromosome set.
  2. The chromosomes are aligned at the equatorial plane.
  3. Two groups of chromatids are moving toward opposite poles. Correct answer.
  4. Chromosomes are visible and scattered in the cytoplasm.

Explanation

In anaphase the centromeres have divided and the chromatids are being pulled polewards, so two separating groups of chromatids are seen. Telophase follows, when the chromatids have reached the poles and nuclei are re-forming.

Question 24Cell Biology and Cell Division

A plant cell with a solute potential of -800 kPa and a pressure potential of 300 kPa is placed in pure water. What is its water potential, and will water enter?

  1. -500 kPa, so water leaves the cell
  2. -500 kPa, so water enters the cell Correct answer.
  3. +500 kPa, so water enters the cell
  4. -1100 kPa, so water leaves the cell

Explanation

Water potential equals solute potential plus pressure potential, so -800 + 300 = -500 kPa. Pure water has a water potential of 0 kPa, which is higher, so water enters the cell by osmosis.

Question 25Biomolecules and Enzymes

Which element is present in all proteins but absent from carbohydrates and lipids?

  1. Nitrogen Correct answer.
  2. Carbon
  3. Oxygen
  4. Hydrogen

Explanation

Proteins are polymers of amino acids, and every amino acid contains an amino group built around a nitrogen atom. Carbohydrates and most lipids are composed only of carbon, hydrogen and oxygen, so nitrogen is the element that distinguishes proteins from them.

Question 26Biomolecules and Enzymes

A condensation reaction joins two monosaccharides to form a disaccharide. What else is produced in this reaction?

  1. Water Correct answer.
  2. Ammonia
  3. Oxygen gas
  4. Carbon dioxide

Explanation

In condensation, a hydroxyl group of one monosaccharide reacts with the hydroxyl group of another, releasing one molecule of water as the glycosidic bond forms. Hydrolysis is the reverse process, in which water is added to break that bond.

Question 27Biomolecules and Enzymes

Which reagent gives a brick-red precipitate with a reducing sugar such as glucose after heating?

  1. Ethanol emulsion test
  2. Iodine solution
  3. Biuret reagent
  4. Benedict's solution Correct answer.

Explanation

Benedict's solution contains blue copper(II) ions that are reduced to red copper(I) oxide by a reducing sugar on heating, giving the brick-red precipitate. The colour change from blue through green and yellow to brick-red reflects the sugar concentration.

Question 28Biomolecules and Enzymes

Which bond links two amino acids together in a polypeptide chain?

  1. Ester bond
  2. Phosphodiester bond
  3. Glycosidic bond
  4. Peptide bond Correct answer.

Explanation

The carboxyl group of one amino acid condenses with the amino group of the next, forming a peptide bond with the loss of water. Repeated peptide bonds build the polypeptide backbone of a protein.

Question 29Biomolecules and Enzymes

Which property of water allows it to act as a good solvent for salts and sugars in the cytoplasm?

  1. It has a high boiling point
  2. It has a low specific heat capacity
  3. It is less dense as a solid
  4. Its molecules are polar Correct answer.

Explanation

Water molecules are polar because oxygen attracts electrons more strongly than hydrogen, giving partial charges. These charges attract and surround ions and polar molecules, keeping them dispersed in solution.

Question 30Biomolecules and Enzymes

Which statement best describes the induced fit model of enzyme action?

  1. The active site changes shape slightly to fit the substrate more closely Correct answer.
  2. The enzyme is permanently altered after each catalytic cycle
  3. The active site binds any molecule of similar size without specificity
  4. The substrate changes shape to match a rigid active site exactly

Explanation

In the induced fit model the active site is flexible and moulds itself around the substrate as binding occurs, improving the fit and straining bonds. This contrasts with the older lock-and-key idea of a rigid, preformed active site.

Question 31Biomolecules and Enzymes

An enzyme-catalysed reaction is carried out at pH 2, well below the optimum for the enzyme. Which change best explains the fall in rate?

  1. Ionic bonds and hydrogen bonds in the active site break, altering its shape Correct answer.
  2. The substrate molecules gain kinetic energy and move too fast
  3. The activation energy of the reaction increases permanently
  4. The enzyme is used up faster than it can be replaced

Explanation

Extreme pH alters the charges on amino acid side chains, disrupting the ionic and hydrogen bonds that hold the tertiary structure. The active site loses its complementary shape, so fewer enzyme-substrate complexes form and the rate falls.

Question 32Biomolecules and Enzymes

Which statement correctly describes the effect of a competitive inhibitor on an enzyme-catalysed reaction?

  1. It changes the shape of the active site by binding elsewhere
  2. It raises the substrate concentration needed to reach maximum rate Correct answer.
  3. It lowers the maximum rate achieved at high substrate concentration
  4. It binds permanently to the active site and cannot be displaced

Explanation

A competitive inhibitor resembles the substrate and competes for the same active site, so more substrate is needed to outcompete it and reach maximum velocity. Because inhibition can be overcome by excess substrate, the maximum rate itself is unchanged.

Question 33Biomolecules and Enzymes

A student tests a food sample with iodine solution and obtains a blue-black colour, but the sample gives no brick-red colour with Benedict's solution. What is the best conclusion?

  1. Neither starch nor reducing sugar is present
  2. Reducing sugar is present but starch is absent
  3. Both starch and reducing sugar are present
  4. Starch is present but reducing sugar is absent Correct answer.

Explanation

The blue-black colour with iodine indicates starch, while the absence of a brick-red precipitate with Benedict's solution shows that no reducing sugar was detected. The two tests are independent, so starch can be present without reducing sugar.

Question 34Biomolecules and Enzymes

Which feature of a phospholipid molecule makes it suitable for forming the bilayer of a cell membrane?

  1. Two hydrophilic heads joined by a hydrophobic glycerol backbone
  2. A hydrophilic phosphate head and two hydrophobic fatty acid tails Correct answer.
  3. A hydrophobic head and a hydrophilic tail containing phosphate
  4. A completely hydrophobic molecule with no charged region

Explanation

Phospholipids are amphipathic: the phosphate-containing head is polar and attracted to water, while the fatty acid tails are non-polar and repelled by water. In water the tails turn inwards and the heads face outwards, producing a stable bilayer.

Question 35Biomolecules and Enzymes

Which level of protein structure is stabilised mainly by hydrogen bonds between the C=O of one amino acid and the N-H of another nearby in the chain?

  1. Quaternary structure
  2. Secondary structure Correct answer.
  3. Primary structure
  4. Tertiary structure

Explanation

Secondary structure refers to regular local folding such as the alpha helix and beta pleated sheet, held together by hydrogen bonds along the polypeptide backbone. These bonds form between the carbonyl oxygen and the amide hydrogen of the peptide groups.

Question 36Biomolecules and Enzymes

A reaction mixture contains enzyme and excess substrate at optimum temperature and pH. The rate of product formation then levels off. What limits the rate at this plateau?

  1. The volume of the reaction vessel
  2. The number of active sites available per unit time Correct answer.
  3. The activation energy of the reaction
  4. The concentration of substrate still present

Explanation

Once every active site is occupied, the enzyme is working at maximum turnover and adding more substrate cannot increase the rate. The plateau therefore reflects enzyme concentration, since only so many substrate molecules can be converted per second.

Question 37Biomolecules and Enzymes

Which structural difference distinguishes cellulose from starch?

  1. Cellulose is a monosaccharide, while starch is a disaccharide
  2. Cellulose has beta-glucose units joined in straight chains, while starch has alpha-glucose units Correct answer.
  3. Cellulose contains nitrogen, while starch contains only carbon, hydrogen and oxygen
  4. Cellulose is made of fructose units, while starch is made of glucose units

Explanation

Both are glucose polymers, but cellulose is built from beta-glucose, which alternates in orientation to give straight, unbranched chains held by hydrogen bonds. Starch uses alpha-glucose, whose uniform orientation produces coiled chains such as amylose.

Question 38Biomolecules and Enzymes

Why does a small rise in temperature above the optimum sharply reduce the rate of an enzyme-catalysed reaction?

  1. Vibration breaks hydrogen and ionic bonds, so the active site loses its shape Correct answer.
  2. The pH of the medium falls and denatures the enzyme
  3. The enzyme immediately hydrolyses into its amino acids
  4. Substrate molecules lose kinetic energy and collide less often

Explanation

Above the optimum, increased molecular vibration disrupts the weak hydrogen and ionic bonds maintaining the tertiary structure. The active site is distorted so substrates can no longer bind, and denaturation is usually irreversible.

Question 39Biomolecules and Enzymes

Which statement about the role of coenzymes is correct?

  1. They are competitive inhibitors that slow the reaction
  2. They are non-protein organic molecules that assist enzyme action by transferring chemical groups Correct answer.
  3. They are protein molecules that permanently replace the enzyme active site
  4. They are inorganic ions that bind irreversibly to the substrate

Explanation

Coenzymes such as NAD and coenzyme A are small organic, non-protein molecules that associate loosely with enzymes and carry groups like hydrogen or acyl units between reactions. Many are derived from vitamins and must be supplied in the diet.

Question 40Biomolecules and Enzymes

A polypeptide of 60 amino acids is synthesised. Assuming no other bonds are counted, how many peptide bonds does it contain?

  1. 61
  2. 60
  3. 59 Correct answer.
  4. 120

Explanation

Each peptide bond joins one amino acid to the next, so a chain of n amino acids contains n minus one bonds. For 60 amino acids this gives 59 peptide bonds.

Question 41Biomolecules and Enzymes

Lysozyme catalyses the hydrolysis of bacterial cell wall peptidoglycan. Which statement best explains why it does not digest the proteins of the bacterial cytoplasm?

  1. It raises the activation energy for protein hydrolysis
  2. Its active site is complementary only to the peptidoglycan substrate Correct answer.
  3. Cytoplasmic proteins have no peptide bonds
  4. It is present only outside the bacterial cell

Explanation

Enzyme specificity arises from the shape and chemistry of the active site, which is complementary only to a particular substrate. Peptidoglycan fits the site, whereas cytoplasmic proteins have a different structure and cannot form a stable enzyme-substrate complex.

Question 42Biomolecules and Enzymes

In an experiment, the rate of an enzyme reaction is measured as substrate concentration increases while enzyme concentration is kept constant. Which description of the resulting curve is correct?

  1. Rate stays constant at all substrate concentrations
  2. Rate rises steeply at first and then plateaus as active sites become saturated Correct answer.
  3. Rate rises linearly without limit as substrate increases
  4. Rate falls steadily as substrate concentration increases

Explanation

At low substrate concentration many active sites are free, so rate increases almost proportionally with substrate. As sites become occupied, fewer are available and the curve flattens until all sites are saturated and the maximum rate is reached.

Question 43Biomolecules and Enzymes

A non-competitive inhibitor binds to a site on an enzyme away from the active site. Which outcome is expected when substrate concentration is increased greatly?

  1. The optimum pH of the enzyme shifts sharply upwards
  2. The maximum rate remains lower than without the inhibitor Correct answer.
  3. The substrate is converted without forming enzyme-substrate complexes
  4. The inhibition is completely reversed and the original maximum rate returns

Explanation

A non-competitive inhibitor distorts the enzyme's tertiary structure so that the active site is no longer complementary to the substrate. Adding more substrate cannot displace an inhibitor bound elsewhere, so the maximum rate stays depressed even at high substrate concentration.

Question 44Biomolecules and Enzymes

A sample contains a triglyceride. Which set of products is obtained after complete hydrolysis with boiling acid?

  1. Glucose and three fatty acids
  2. Glycerol and three amino acids
  3. Glycerol and three nucleotides
  4. Glycerol and three fatty acid molecules Correct answer.

Explanation

A triglyceride consists of glycerol esterified to three fatty acid chains. Hydrolysis breaks the three ester bonds, releasing one glycerol molecule and three fatty acid molecules.

Question 45Genetics and Heredity

In a monohybrid cross between two heterozygous tall pea plants (Tt × Tt), what is the expected ratio of tall to short offspring?

  1. 1 : 1
  2. 9 : 3 : 3 : 1
  3. 1 : 2 : 1
  4. 3 : 1 Correct answer.

Explanation

A Tt × Tt cross produces genotypes TT, Tt, Tt and tt in a 1:2:1 genotypic ratio. Since T is dominant, the three genotypes carrying T give tall plants and only tt is short, so the phenotypic ratio is 3 tall : 1 short.

Question 46Genetics and Heredity

Which term describes an organism that carries two identical alleles for a particular gene?

  1. Heterozygous
  2. Homozygous Correct answer.
  3. Hemizygous
  4. Hybrid

Explanation

Homozygous describes a genotype in which both alleles of a gene are the same, such as TT or tt. Such an individual breeds true for that trait because all its gametes carry the same allele.

Question 47Genetics and Heredity

A pure-breeding red-flowered plant is crossed with a pure-breeding white-flowered plant and all the F1 offspring have pink flowers. Which pattern of inheritance does this show?

  1. Codominance
  2. Complete dominance
  3. Sex-linked inheritance
  4. Incomplete dominance Correct answer.

Explanation

In incomplete dominance the heterozygote shows a phenotype intermediate between the two homozygotes, so red × white gives pink. The two alleles are both expressed but neither fully masks the other, producing a blended colour.

Question 48Genetics and Heredity

Which of the following is the correct base-pairing rule in a DNA double helix?

  1. Adenine pairs with uracil, and guanine pairs with cytosine
  2. Adenine pairs with cytosine, and guanine pairs with thymine
  3. Adenine pairs with thymine, and guanine pairs with cytosine Correct answer.
  4. Adenine pairs with guanine, and thymine pairs with cytosine

Explanation

In DNA, adenine forms two hydrogen bonds with thymine and guanine forms three hydrogen bonds with cytosine. This complementary pairing keeps the helix diameter constant and allows accurate replication.

Question 49Genetics and Heredity

In a dihybrid cross between two plants heterozygous for both genes (RrYy × RrYy), the genes assort independently. What fraction of the offspring is expected to be homozygous recessive for both traits (rryy)?

  1. 3/16
  2. 1/4
  3. 9/16
  4. 1/16 Correct answer.

Explanation

Each parent produces ry gametes with probability 1/4, and independent assortment means the joint probability is 1/4 × 1/4 = 1/16. This is the smallest class in the classic 9:3:3:1 F2 ratio.

Question 50Genetics and Heredity

A woman with genotype X^H X^h is a carrier of haemophilia. If she has children with a normal man (X^H Y), what is the probability that a son will be affected?

  1. 0
  2. 1/2 Correct answer.
  3. 1/4
  4. 1

Explanation

Sons receive the Y chromosome from the father and one of the mother's X chromosomes. Half the mother's X chromosomes carry h, so half of her sons receive X^h and are affected, while the other half receive X^H and are normal.