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Armenia Biology Unified Exam study materials
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Armenia Biology Unified Exam practice questions

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Question 1Cell Biology and Cytology

Which organelle is the principal site of ATP synthesis by oxidative phosphorylation in a eukaryotic cell?

  1. Mitochondrion Correct answer.
  2. Golgi apparatus
  3. Lysosome
  4. Smooth endoplasmic reticulum

Explanation

The inner mitochondrial membrane carries the electron transport chain and ATP synthase, which together generate most cellular ATP through oxidative phosphorylation. Other organelles listed perform synthesis, modification, or degradation tasks rather than bulk ATP production.

Question 2Cell Biology and Cytology

A cell is placed in a hypertonic solution. What is the immediate osmotic consequence for the cell?

  1. Water enters the cell and it swells
  2. Water leaves the cell and it shrinks Correct answer.
  3. The cell wall expands to resist turgor
  4. Solute moves into the cell down its gradient

Explanation

In a hypertonic environment the external solute concentration exceeds that inside the cell, so water moves outward by osmosis across the membrane. Loss of water reduces cell volume, causing shrinkage.

Question 3Cell Biology and Cytology

Which molecule serves as the universal short-term energy currency of the cell?

  1. ATP Correct answer.
  2. Pyruvate
  3. Glucose
  4. NADH

Explanation

ATP couples exergonic reactions to endergonic ones by transferring a terminal phosphate group, making it the cell's immediate energy currency. Glucose and NADH store energy but must be processed to yield ATP.

Question 4Cell Biology and Cytology

Which structural feature distinguishes prokaryotic cells from eukaryotic cells?

  1. Absence of a membrane-bound nucleus Correct answer.
  2. Presence of a plasma membrane
  3. Presence of DNA
  4. Presence of ribosomes

Explanation

Prokaryotes lack a nuclear envelope, so their genetic material lies in the nucleoid region rather than inside a membrane-bound nucleus. Plasma membranes, ribosomes, and DNA are shared by both cell types.

Question 5Cell Biology and Cytology

During which phase of mitosis do sister chromatids separate and move toward opposite poles?

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

Explanation

In anaphase the cohesin holding sister chromatids is cleaved and the chromatids are pulled to opposite poles by spindle fibers. Prophase, metaphase, and telophase involve condensation, alignment, and nuclear reformation respectively.

Question 6Cell Biology and Cytology

Which part of the phospholipid bilayer faces the aqueous exterior of the cell?

  1. The integral membrane proteins
  2. The hydrophilic phosphate heads Correct answer.
  3. The cholesterol molecules
  4. The hydrophobic fatty acid tails

Explanation

Phospholipids are amphipathic: their polar phosphate heads interact with water and orient outward, while the nonpolar tails face inward. This arrangement produces the bilayer that separates aqueous compartments.

Question 7Cell Biology and Cytology

Which enzyme unwinds the DNA double helix ahead of the replication fork?

  1. Primase
  2. Ligase
  3. Helicase Correct answer.
  4. DNA polymerase

Explanation

Helicase breaks the hydrogen bonds between complementary bases to separate the strands, creating the replication fork. The other enzymes synthesize DNA, join fragments, or lay down primers.

Question 8Cell Biology and Cytology

A cell that normally transports large amounts of protein for secretion would be expected to have an especially abundant amount of which structure?

  1. Peroxisomes
  2. Rough endoplasmic reticulum Correct answer.
  3. Lysosomes
  4. Centrioles

Explanation

Secretory proteins are synthesized on ribosomes bound to the rough endoplasmic reticulum, so cells specialized for protein secretion show an expanded rough ER. Lysosomes, peroxisomes, and centrioles serve degradation, oxidative reactions, and cell division respectively.

Question 9Cell Biology and Cytology

In the fluid mosaic model, which property best describes the movement of membrane phospholipids?

  1. Fixed positions determined by proteins
  2. Lateral diffusion within the same leaflet Correct answer.
  3. Rapid flip-flop between leaflets without enzymes
  4. Directed transport toward the nucleus

Explanation

Phospholipids diffuse laterally within their own leaflet, giving the membrane fluid character. Transverse flip-flop is rare without the aid of flippase enzymes because it requires moving a polar head through the hydrophobic core.

Question 10Cell Biology and Cytology

Which statement correctly describes the net yield of glycolysis per molecule of glucose?

  1. 2 ATP and 4 NADH
  2. 2 ATP and 2 NADH Correct answer.
  3. 4 ATP and 2 NADH
  4. 4 ATP and 4 NADH

Explanation

Glycolysis consumes 2 ATP and produces 4 ATP, giving a net gain of 2 ATP, while 2 NADH are generated by the oxidation of glyceraldehyde-3-phosphate. The gross ATP figure overstates the net yield.

Question 11Cell Biology and Cytology

A mutation prevents the formation of the mitotic spindle. Which stage of mitosis will be most directly blocked?

  1. Re-formation of the nuclear envelope
  2. Chromosome alignment at the metaphase plate Correct answer.
  3. Condensation of chromatin
  4. Replication of DNA

Explanation

Spindle microtubules attach to kinetochores and move chromosomes to the equator, so without a spindle chromosomes cannot align at the metaphase plate. Condensation and nuclear envelope re-formation do not depend on spindle assembly.

Question 12Cell Biology and Cytology

Which process produces the greatest number of ATP molecules per glucose molecule in aerobic respiration?

  1. Oxidative phosphorylation Correct answer.
  2. Pyruvate oxidation
  3. The citric acid cycle
  4. Glycolysis

Explanation

Oxidative phosphorylation generates most of the ATP through the proton gradient and ATP synthase. Glycolysis, pyruvate oxidation, and the citric acid cycle contribute mainly reduced coenzymes rather than direct ATP.

Question 13Cell Biology and Cytology

Which feature is common to both facilitated diffusion and active transport?

  1. Movement of solute against its concentration gradient
  2. Movement of solute down its concentration gradient
  3. Involvement of membrane transport proteins Correct answer.
  4. Direct hydrolysis of ATP

Explanation

Both facilitated diffusion and active transport use membrane proteins to move solutes across the bilayer. They differ in energetics: facilitated diffusion is passive and down-gradient, whereas active transport moves solutes against the gradient and requires energy.

Question 14Cell Biology and Cytology

Which organelle is primarily responsible for modifying and sorting newly synthesized proteins for delivery to their destinations?

  1. Smooth endoplasmic reticulum
  2. Mitochondrion
  3. Golgi apparatus Correct answer.
  4. Nucleolus

Explanation

The Golgi apparatus receives proteins from the ER, chemically modifies them, and packages them into vesicles addressed to specific destinations. The nucleolus builds ribosomes, mitochondria produce ATP, and smooth ER synthesizes lipids.

Question 15Cell Biology and Cytology

In a cell, a protein is synthesized on a free cytosolic ribosome. Which destination is most consistent with this location of synthesis?

  1. The extracellular space
  2. The cytosol itself Correct answer.
  3. The lysosomal lumen
  4. The interior of the endoplasmic reticulum

Explanation

Proteins made on free ribosomes remain in the cytosol or are targeted post-translationally to organelles such as the nucleus or mitochondria. Proteins destined for the secretory pathway, lysosomes, or the cell exterior are synthesized on ribosomes bound to the rough ER.

Question 16Cell Biology and Cytology

Which statement best describes the relationship between the light reactions and the Calvin cycle in photosynthesis?

  1. The Calvin cycle generates oxygen as a by-product
  2. The light reactions fix carbon dioxide into sugars directly
  3. The light reactions supply ATP and NADPH used by the Calvin cycle Correct answer.
  4. The Calvin cycle supplies ATP and NADPH to the light reactions

Explanation

The light reactions convert light energy into chemical energy as ATP and NADPH, which the Calvin cycle then uses to reduce carbon dioxide into sugar. Oxygen is released by the light reactions, not the Calvin cycle.

Question 17Cell Biology and Cytology

A cell is treated with a drug that blocks the function of the sodium-potassium pump. What immediate effect is expected on the resting membrane potential?

  1. It reverses polarity instantly due to calcium influx
  2. It becomes more negative because potassium influx increases
  3. It remains unchanged because the pump does not affect the membrane
  4. It gradually depolarizes because the ion gradients dissipate Correct answer.

Explanation

The sodium-potassium pump maintains the high extracellular sodium and high intracellular potassium gradients that underlie the resting potential. When the pump is inhibited, these gradients slowly run down and the membrane potential drifts toward zero, i.e., depolarizes gradually.

Question 18Cell Biology and Cytology

Which observation would best support the conclusion that a molecule crosses a membrane by simple diffusion rather than by facilitated diffusion?

  1. Transport is blocked by a structural analog of the molecule
  2. The rate of transport increases linearly with concentration and is not saturable Correct answer.
  3. The rate of transport levels off at high concentration
  4. Transport stops when ATP synthesis is inhibited

Explanation

Simple diffusion depends only on the concentration gradient, so its rate rises with concentration without reaching a saturation plateau. Facilitated diffusion is protein-mediated and therefore shows saturation kinetics and sensitivity to specific inhibitors.

Question 19Cell Biology and Cytology

A cell cycle checkpoint detects unrepaired DNA damage during G2. Which outcome is most consistent with the normal function of this checkpoint?

  1. Immediate entry into mitosis despite the damage
  2. Skipping S phase to avoid further replication
  3. Direct activation of the apoptotic pathway without any repair attempt
  4. Arrest of the cycle before mitosis until repair is complete Correct answer.

Explanation

The G2 checkpoint halts progression into mitosis so that DNA lesions can be repaired before chromosomes segregate. If repair fails, apoptosis may follow, but the checkpoint's primary role is a temporary arrest.

Question 20Cell Biology and Cytology

In an experiment, isolated mitochondria are supplied with an electron transport chain inhibitor that blocks proton pumping. Which result is most expected?

  1. ATP synthesis rises because oxygen consumption increases
  2. ATP synthesis is unaffected because substrate-level phosphorylation continues
  3. ATP synthesis falls because the proton gradient collapses Correct answer.
  4. ATP synthesis rises because the inner membrane becomes permeable to protons

Explanation

Chemiosmotic ATP synthesis depends on the proton gradient built by the electron transport chain. If proton pumping is blocked, the gradient dissipates and ATP synthase has no driving force, so ATP output falls.

Question 21Cell Biology and Cytology

A researcher finds that a cell membrane contains unusually high levels of cholesterol. Which functional consequence is most likely?

  1. Complete loss of selective permeability
  2. Increased permeability to large polar molecules
  3. Loss of all integral membrane proteins
  4. Reduced membrane fluidity at high temperatures Correct answer.

Explanation

Cholesterol packs between phospholipid tails and restricts their movement, decreasing fluidity at high temperatures while preventing rigid packing at low temperatures. It does not abolish selective permeability or remove membrane proteins.

Question 22Cell Biology and Cytology

During protein synthesis, a ribosome reads the sequence of an mRNA molecule. Which statement correctly describes this process?

  1. Ribosomal RNA carries amino acids to the ribosome
  2. Messenger RNA transfers amino acids into the polypeptide chain
  3. Transfer RNA delivers amino acids that match mRNA codons Correct answer.
  4. DNA polymerase assembles the polypeptide chain

Explanation

Each tRNA carries a specific amino acid and has an anticodon that base-pairs with the complementary mRNA codon, ensuring the correct amino acid is added. rRNA forms the ribosome's structural and catalytic core, and DNA polymerase functions in replication, not translation.

Question 23Cell Biology and Cytology

Which structural adaptation of the small intestine epithelium most directly increases the surface area available for absorption?

  1. Smooth muscle in the intestinal wall
  2. Microvilli on the apical surface of epithelial cells Correct answer.
  3. Numerous goblet cells secreting mucus
  4. A thick layer of connective tissue beneath the epithelium

Explanation

Microvilli are fine membrane projections that greatly expand the apical surface area of absorptive epithelial cells. Connective tissue, mucus secretion, and muscle contraction serve support, protection, and motility rather than increasing absorptive area.

Question 24Cell Biology and Cytology

A cell is observed to have a large, prominent nucleolus. Which activity is most likely elevated in this cell?

  1. Lysosomal degradation
  2. Lipid synthesis in the smooth ER
  3. ATP production in mitochondria
  4. Ribosome subunit synthesis Correct answer.

Explanation

The nucleolus is the site where ribosomal RNA is transcribed and assembled with proteins into ribosomal subunits, so a prominent nucleolus reflects high ribosome production. Lipid synthesis, ATP production, and degradation occur elsewhere in the cell.

Question 25Metabolism and Energy Transformations

In eukaryotic cells, which stage of aerobic cellular respiration produces the largest amount of ATP?

  1. Pyruvate oxidation
  2. Oxidative phosphorylation Correct answer.
  3. Glycolysis
  4. The Krebs cycle

Explanation

Most ATP in aerobic respiration is generated by ATP synthase during oxidative phosphorylation, driven by the proton gradient built across the inner mitochondrial membrane. Glycolysis and the Krebs cycle yield only small amounts of direct ATP.

Question 26Metabolism and Energy Transformations

Which molecule is the final electron acceptor at the end of the mitochondrial electron transport chain?

  1. Water
  2. Oxygen Correct answer.
  3. NAD+
  4. Carbon dioxide

Explanation

Oxygen accepts electrons at the end of the chain and combines with protons to form water. This keeps electrons flowing through the complexes so the proton gradient can be maintained.

Question 27Metabolism and Energy Transformations

Where in a eukaryotic cell does glycolysis take place?

  1. In the mitochondrial matrix
  2. In the nucleus
  3. On the inner mitochondrial membrane
  4. In the cytosol Correct answer.

Explanation

Glycolysis is a cytoplasmic pathway that does not require organelles, which is why it occurs in the cytosol of both prokaryotes and eukaryotes. The later stages of aerobic respiration occur inside the mitochondrion.

Question 28Metabolism and Energy Transformations

Which of the following is the primary role of NADH in cellular respiration?

  1. It carries electrons to the electron transport chain Correct answer.
  2. It directly phosphorylates ADP
  3. It splits glucose into two pyruvate molecules
  4. It transports oxygen into the mitochondrion

Explanation

NADH is a reduced coenzyme that shuttles high-energy electrons from glycolysis and the Krebs cycle to the electron transport chain. There the electrons drive proton pumping and ultimately ATP synthesis.

Question 29Metabolism and Energy Transformations

In photosynthesis, which pigment absorbs light energy and transfers it to the reaction centers of the photosystems?

  1. Hemoglobin
  2. ATP synthase
  3. Chlorophyll and accessory pigments Correct answer.
  4. Cytochrome c

Explanation

Chlorophyll a, chlorophyll b, and accessory pigments such as carotenoids form antenna complexes that absorb photons and funnel excitation energy to reaction-center chlorophyll. This initiates the light reactions.

Question 30Metabolism and Energy Transformations

Which process converts light energy into chemical energy stored in organic molecules?

  1. Cellular respiration
  2. Photorespiration
  3. Fermentation
  4. Photosynthesis Correct answer.

Explanation

Photosynthesis uses light energy to fix carbon dioxide into carbohydrates, storing energy in chemical bonds. Respiration and fermentation instead release energy from organic molecules.

Question 31Metabolism and Energy Transformations

A sprinter performing intense exercise with limited oxygen produces lactate. Which pathway is responsible for this?

  1. Lactic acid fermentation Correct answer.
  2. The Krebs cycle
  3. Aerobic respiration
  4. The Calvin cycle

Explanation

When oxygen is scarce, cells regenerate NAD+ by reducing pyruvate to lactate, allowing glycolysis to continue producing ATP. This pathway is lactic acid fermentation.

Question 32Metabolism and Energy Transformations

A cell is treated with a drug that blocks ATP synthase. Which immediate consequence is expected during oxidative phosphorylation?

  1. The proton gradient across the inner membrane would collapse Correct answer.
  2. The Krebs cycle would produce more NADH
  3. Glycolysis would stop immediately
  4. Oxygen consumption would increase sharply

Explanation

ATP synthase lets protons flow back into the matrix, dissipating the gradient. If it is blocked, protons accumulate and the gradient cannot be discharged, which also slows electron transport.

Question 33Metabolism and Energy Transformations

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

  1. It supplies electrons to photosystem II and releases oxygen Correct answer.
  2. It acts as the terminal electron acceptor
  3. It fixes carbon dioxide into sugars
  4. It directly phosphorylates ADP

Explanation

Photolysis of water at photosystem II replaces the electrons lost by chlorophyll and releases molecular oxygen as a by-product. The electrons then flow through the chain toward NADP+.

Question 34Metabolism and Energy Transformations

Which statement best describes the relationship between the light reactions and the Calvin cycle?

  1. The Calvin cycle supplies ATP and NADPH to the light reactions
  2. Both processes occur only in the dark
  3. The light reactions supply ATP and NADPH that the Calvin cycle uses Correct answer.
  4. The light reactions fix carbon dioxide directly

Explanation

The light reactions generate ATP and NADPH, which are consumed by the Calvin cycle to reduce carbon dioxide into carbohydrate. The two stages are linked by these energy carriers.

Question 35Metabolism and Energy Transformations

A mutation reduces the activity of an enzyme in the Krebs cycle, slowing the production of NADH. Which effect on oxidative phosphorylation is most likely?

  1. Increased lactate production in the cytoplasm of all cells
  2. Less proton pumping and reduced ATP synthesis Correct answer.
  3. No effect because the electron transport chain does not use NADH
  4. More ATP synthesis because oxygen is conserved

Explanation

NADH from the Krebs cycle supplies electrons to the electron transport chain, which powers proton pumping. Less NADH means fewer electrons, weaker proton gradient, and less ATP synthesis.

Question 36Metabolism and Energy Transformations

During strenuous exercise, muscle cells switch to anaerobic respiration. What is the main purpose of converting pyruvate to lactate in this situation?

  1. To produce large amounts of ATP directly
  2. To increase oxygen delivery to muscle
  3. To store energy for later use in the Krebs cycle
  4. To regenerate NAD+ so glycolysis can continue Correct answer.

Explanation

Glycolysis requires a steady supply of NAD+ to keep oxidizing glucose. Reducing pyruvate to lactate regenerates NAD+, sustaining ATP production when oxygen is limiting.

Question 37Metabolism and Energy Transformations

Which statement correctly compares the net ATP yield of aerobic respiration with that of lactic acid fermentation per glucose molecule?

  1. Fermentation yields no ATP at all
  2. Fermentation yields more ATP than aerobic respiration
  3. Both yield the same amount of ATP
  4. Aerobic respiration yields far more ATP than fermentation Correct answer.

Explanation

Complete aerobic oxidation of glucose yields roughly 30-32 ATP per molecule, whereas fermentation yields only the net 2 ATP from glycolysis. The difference reflects the full oxidation of glucose in aerobic respiration.

Question 38Metabolism and Energy Transformations

Which of the following best explains why the Krebs cycle is described as a cycle?

  1. Oxaloacetate is regenerated at the end so it can accept another acetyl group Correct answer.
  2. ATP is continuously converted back into glucose
  3. Glucose is repeatedly split into pyruvate
  4. Electrons cycle between oxygen and water

Explanation

The Krebs cycle begins when acetyl-CoA joins oxaloacetate to form citrate, and a series of reactions regenerates oxaloacetate. Because the starting compound is restored, the process can repeat.

Question 39Metabolism and Energy Transformations

In a chloroplast, ATP synthase is located on which membrane and uses what energy source?

  1. The thylakoid membrane, driven by a proton gradient Correct answer.
  2. The nuclear envelope, driven by light directly
  3. The inner mitochondrial membrane, driven by NADH
  4. The outer chloroplast membrane, driven by ATP hydrolysis

Explanation

During the light reactions, protons accumulate in the thylakoid lumen, creating a gradient. ATP synthase in the thylakoid membrane uses the flow of protons back to the stroma to synthesize ATP.

Question 40Metabolism and Energy Transformations

A plant is placed in a sealed chamber in the dark. What happens to the net gas exchange of the plant?

  1. It takes up both oxygen and carbon dioxide
  2. It releases carbon dioxide and takes up oxygen Correct answer.
  3. It releases both oxygen and carbon dioxide
  4. It releases oxygen and takes up carbon dioxide

Explanation

In darkness, photosynthesis stops, so no oxygen is produced and no carbon dioxide is fixed. Cellular respiration continues, consuming oxygen and releasing carbon dioxide.

Question 41Metabolism and Energy Transformations

If a plant cell is supplied with a compound that blocks the reduction of NADP+ in the light reactions, which outcome is most likely?

  1. Photosystem II begins to reduce water
  2. The Calvin cycle accelerates because NADPH accumulates
  3. Oxygen production stops but ATP synthesis continues at full rate
  4. Electron flow through the photosystems is inhibited and ATP synthesis decreases Correct answer.

Explanation

NADP+ is the terminal electron acceptor of the light reactions. If it cannot be reduced, electrons back up in the transport chain, slowing electron flow and the proton pumping that drives ATP synthesis.

Question 42Metabolism and Energy Transformations

An organism lacks functional mitochondria but can survive in anaerobic conditions. Which metabolic strategy best explains its survival?

  1. Performing oxidative phosphorylation on the plasma membrane
  2. Carrying out the Calvin cycle to generate ATP
  3. Using the Krebs cycle in the cytoplasm to produce ATP
  4. Relying on glycolysis with fermentation to regenerate NAD+ Correct answer.

Explanation

Without mitochondria, oxidative phosphorylation and the Krebs cycle cannot operate normally, so ATP production depends on glycolysis. Fermentation regenerates NAD+ to keep glycolysis running under anaerobic conditions.

Question 43Metabolism and Energy Transformations

A researcher measures a higher rate of oxygen consumption in a tissue sample after adding a compound that increases the permeability of the inner mitochondrial membrane to protons. What is the best interpretation?

  1. The proton gradient is dissipated, so electron transport speeds up to compensate Correct answer.
  2. The Krebs cycle is inhibited by the added protons
  3. The electron transport chain stops completely
  4. ATP synthesis increases because protons enter more easily

Explanation

Uncoupling agents let protons leak across the membrane without passing through ATP synthase, weakening the gradient. Electron transport accelerates to restore it, raising oxygen consumption while ATP output falls.

Question 44Metabolism and Energy Transformations

A biochemist measures the energy yield of one molecule of glucose oxidized completely to CO2 and water in aerobic respiration. Approximately what share of the total ATP produced comes from substrate-level phosphorylation rather than from oxidative phosphorylation?

  1. About one-half, since glycolysis and the Krebs cycle together match the electron transport chain
  2. Essentially all of it, since ATP synthase is not involved in aerobic ATP production
  3. About three-quarters, since most ATP is generated while glucose is split into pyruvate
  4. About one-tenth, since only 4 of roughly 30–32 ATP are made directly by substrate-level phosphorylation Correct answer.

Explanation

Substrate-level phosphorylation occurs at only two steps of glycolysis and one step of the Krebs cycle, giving 2 ATP per glucose from glycolysis and 2 ATP per glucose from the Krebs cycle (about 4 total). The remaining ATP comes from oxidative phosphorylation driven by NADH and FADH2 at the electron transport chain, which contributes the large majority of the roughly 30–32 ATP.

Question 45Genetics and Heredity

In peas, tall stem (T) is dominant over short stem (t). A heterozygous tall plant is crossed with a short plant. What proportion of the offspring is expected to be short?

  1. One half Correct answer.
  2. Three quarters
  3. Zero
  4. One quarter

Explanation

A cross of Tt × tt produces gametes T and t from the heterozygote and only t from the short parent, giving Tt and tt in equal numbers. Thus half the progeny are expected to be short.

Question 46Genetics and Heredity

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

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

Explanation

Homozygous means both homologous chromosomes carry the same allele at a locus. Heterozygous means the alleles differ, and hemizygous refers to a single copy of a gene.

Question 47Genetics and Heredity

According to Mendel's law of independent assortment, which statement holds for two genes located on different chromosome pairs?

  1. Alleles of one gene segregate independently of alleles of the other gene Correct answer.
  2. Alleles of both genes always stay together during gamete formation
  3. Alleles of one gene are always dominant over the other gene
  4. Alleles of both genes recombine only in somatic cells

Explanation

Independent assortment states that alleles of genes on nonhomologous chromosomes are distributed to gametes independently of one another. This produces the 9:3:3:1 ratio in a dihybrid cross.

Question 48Genetics and Heredity

A cross between two heterozygous individuals (Aa × Aa) produces offspring in what genotypic ratio?

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

Explanation

Each heterozygote produces A and a gametes in equal frequency, and the Punnett square gives AA, Aa, Aa, aa. Counting genotypes yields 1 AA : 2 Aa : 1 aa.

Question 49Genetics and Heredity

Which process during meiosis is the main source of new combinations of alleles on a single chromosome?

  1. Formation of the spindle apparatus
  2. Cytokinesis after telophase
  3. Crossing over between homologous chromosomes Correct answer.
  4. Separation of sister chromatids

Explanation

Crossing over exchanges segments between nonsister chromatids of homologous chromosomes in prophase I, creating new allele combinations along a chromosome. Independent assortment shuffles whole chromosomes but does not recombine alleles within one chromosome.

Question 50Genetics and Heredity

In humans, the sex of a child is determined by which combination of sex chromosomes?

  1. The sperm contributes either X or Y to the egg's X Correct answer.
  2. Both parents contribute Y to produce a female
  3. The egg contributes either X or Y to the sperm's X
  4. The mother's X chromosome alone determines sex

Explanation

The mother provides an X chromosome in every egg, while the father produces X-bearing and Y-bearing sperm. Fertilization by a Y sperm gives XY (male) and by an X sperm gives XX (female).