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ORELA Biology (NT316) study materials
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ORELA Biology (NT316) practice questions

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Question 1Biology

Which organelle is the primary site of ATP production in a eukaryotic cell through oxidative phosphorylation?

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

Explanation

The mitochondrion hosts the electron transport chain and ATP synthase on its inner membrane, where oxidative phosphorylation generates most cellular ATP. The other listed organelles perform protein modification, packaging, or degradation rather than bulk ATP synthesis.

Question 2Biology

A segment of DNA has the template strand sequence 3'-TAC GGA-5'. Following complementary base pairing, what mRNA sequence is transcribed?

  1. 5'-TAC GGA-3'
  2. 5'-AUG CCU-3' Correct answer.
  3. 5'-UAC GGA-3'
  4. 5'-ATG CCT-3'

Explanation

RNA polymerase builds mRNA complementary to the template strand, substituting uracil for thymine. Pairing A with T, U with A, G with C, and C with G across the template 3'-TAC GGA-5' yields 5'-AUG CCU-3'.

Question 3Biology

In a population at Hardy-Weinberg equilibrium, the frequency of a recessive allele is 0.2. What proportion of the population is expected to be heterozygous?

  1. 0.04
  2. 0.16
  3. 0.64
  4. 0.32 Correct answer.

Explanation

With q = 0.2, p = 0.8, and the heterozygote frequency is 2pq = 2(0.8)(0.2) = 0.32. The homozygous recessive frequency q² would be 0.04, and homozygous dominant p² is 0.64.

Question 4Biology

Which process directly produces genetic variation among offspring by exchanging corresponding chromosome segments during meiosis?

  1. Mitotic division
  2. Crossing over Correct answer.
  3. Cytokinesis
  4. DNA replication

Explanation

Crossing over occurs between homologous chromosomes during prophase I of meiosis, swapping equivalent segments and creating new allele combinations. Replication copies DNA faithfully, and cytokinesis only divides the cytoplasm.

Question 5Biology

An enzyme catalyzes a reaction at a rate that increases with substrate concentration until the rate plateaus. What best explains the plateau?

  1. The substrate has been entirely consumed
  2. All active sites are saturated with substrate Correct answer.
  3. The reaction has become endergonic
  4. The enzyme has denatured at high substrate levels

Explanation

At high substrate concentration every enzyme active site is occupied, so the reaction reaches its maximum velocity (Vmax). Adding more substrate cannot increase the rate until product is removed or more enzyme is added.

Question 6Biology

Which structure in a flowering plant develops into the fruit after fertilization?

  1. Stigma
  2. Ovary Correct answer.
  3. Anther
  4. Sepal

Explanation

After fertilization the ovary wall thickens and the ovary matures into the fruit enclosing the seeds. The anther produces pollen, the stigma receives pollen, and sepals protect the developing bud.

Question 7Biology

In a food web, only about 10 percent of the energy at one trophic level is transferred to the next. What is the primary reason for this low efficiency?

  1. Energy is transferred only through decomposers
  2. Energy is destroyed when organisms are eaten
  3. Predators convert most energy into biomass
  4. Most energy is lost as heat through metabolism Correct answer.

Explanation

Organisms expend much of the energy they assimilate on respiration, releasing it as heat, so only a fraction remains for growth and transfer to consumers. Energy is not destroyed but dissipated, and decomposers recycle nutrients rather than transferring trophic energy upward.

Question 8Biology

Which blood vessel type has the thinnest walls, permitting exchange of gases and nutrients with surrounding tissues?

  1. Arteriole
  2. Vein
  3. Capillary Correct answer.
  4. Artery

Explanation

Capillaries consist of a single layer of endothelial cells, minimizing diffusion distance for gas and nutrient exchange. Arteries and arterioles have thick muscular walls for pressure, and veins carry blood at lower pressure with valves.

Question 9Biology

A mutation changes a codon from GAG to GUG in the coding strand of a gene. What type of mutation is this?

  1. Missense mutation Correct answer.
  2. Nonsense mutation
  3. Silent mutation
  4. Frameshift mutation

Explanation

A single base substitution that changes the amino acid specified by a codon is a missense mutation. Here GAG (glutamate) becomes GUG (valine), so the protein's primary structure changes without shifting the reading frame.

Question 10Biology

Which molecule serves as the final electron acceptor in the electron transport chain of aerobic cellular respiration?

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

Explanation

Oxygen accepts electrons at the end of the electron transport chain, combining with protons to form water. This keeps electrons flowing and maintains the proton gradient that drives ATP synthesis.

Question 11Biology

A student observes a cell with a rigid cell wall, no nucleus, and circular DNA. To which group does this organism most likely belong?

  1. Protists
  2. Fungi
  3. Bacteria Correct answer.
  4. Plants

Explanation

Prokaryotes such as bacteria lack a membrane-bound nucleus and typically carry circular DNA, and many have cell walls. Fungi, protists, and plants are eukaryotes with nuclei and linear chromosomal DNA.

Question 12Biology

Which of the following best describes the role of a keystone species in an ecosystem?

  1. Its influence on community structure is disproportionately large relative to its abundance Correct answer.
  2. It is always the apex predator in the food chain
  3. It is the most abundant species and dominates biomass
  4. It is the sole primary producer in the habitat

Explanation

A keystone species exerts a strong effect on community structure disproportionate to its abundance; removing it can trigger cascading changes. Abundance and trophic position vary, so dominance or apex status alone does not define a keystone species.

Question 13Biology

During which phase of the cell cycle does DNA replication occur?

  1. G2 phase
  2. S phase Correct answer.
  3. G1 phase
  4. M phase

Explanation

DNA is synthesized during the S (synthesis) phase of interphase, producing two sister chromatids per chromosome. G1 and G2 are gap phases for growth and preparation, while M phase is when chromosomes segregate.

Question 14Biology

In humans, a woman who is a carrier of an X-linked recessive disorder has children with an unaffected man. What fraction of their sons is expected to be affected?

  1. None
  2. One quarter
  3. One half Correct answer.
  4. All

Explanation

Sons receive their X chromosome from the mother and a Y from the father. Since the mother is heterozygous, each son has a 50 percent chance of inheriting the affected X allele, so half of the sons are expected to be affected.

Question 15Biology

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

  1. The membrane is a single layer of phospholipids
  2. Only lipids are present, with no proteins
  3. The membrane is rigid and fixed in composition
  4. Proteins and lipids move laterally within the bilayer Correct answer.

Explanation

The fluid mosaic model describes a phospholipid bilayer studded with proteins and other molecules that can move laterally, giving the membrane a dynamic, mosaic character. It is a bilayer, not a rigid or single-layer structure.

Question 16Biology

A predator population declines sharply, and shortly afterward the prey population overshoots its food supply and then crashes. Which ecological concept does this sequence illustrate?

  1. Competitive exclusion between two species
  2. Primary succession following a disturbance
  3. Eutrophication of an aquatic system
  4. Predator-prey population dynamics with delayed feedback Correct answer.

Explanation

Predator-prey interactions often produce cyclical oscillations, and time lags in feedback can cause prey to overshoot resources before crashing. This cycle reflects coupled population dynamics rather than succession, competition, or nutrient enrichment.

Question 17Biology

Which statement correctly distinguishes photosynthesis from cellular respiration in terms of electron flow?

  1. Photosynthesis and respiration both require oxygen as the terminal electron acceptor
  2. Photosynthesis uses light energy to drive electrons from water to NADP+, while respiration transfers electrons from organic molecules to oxygen Correct answer.
  3. Photosynthesis transfers electrons from glucose to oxygen, while respiration uses light to reduce water
  4. Both processes release energy by transferring electrons from water to carbon dioxide

Explanation

In photosynthesis, light excites electrons that ultimately reduce NADP+, with water serving as the electron donor. In aerobic respiration, electrons flow from organic substrates through the chain to oxygen, the terminal acceptor, releasing energy for ATP synthesis.

Question 18Biology

Which part of the human nervous system primarily regulates involuntary functions such as heart rate and digestion?

  1. Skeletal motor system
  2. Autonomic nervous system Correct answer.
  3. Somatic nervous system
  4. Sensory nervous system

Explanation

The autonomic nervous system controls involuntary effectors such as cardiac muscle, smooth muscle, and glands. The somatic and skeletal motor systems govern voluntary movement, and sensory pathways carry information inward.

Question 19Biology

In a cross between two heterozygous individuals (Aa), what phenotypic ratio is expected for a trait showing complete dominance?

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

Explanation

A monohybrid cross Aa × Aa yields genotypes in a 1:2:1 ratio, but with complete dominance the two dominant genotypes merge into one phenotype, giving 3:1. The 1:2:1 ratio describes genotypes, and 9:3:3:1 describes a dihybrid cross.

Question 20Biology

Which type of RNA carries amino acids to the ribosome during translation?

  1. rRNA
  2. siRNA
  3. tRNA Correct answer.
  4. mRNA

Explanation

Transfer RNA (tRNA) binds specific amino acids and delivers them to the ribosome, matching its anticodon to the mRNA codon. mRNA carries the coding message, rRNA forms part of the ribosome, and siRNA participates in gene silencing.

Question 21Biology

A freshwater organism living in a hypotonic environment must manage water balance. Which response best allows it to avoid swelling?

  1. Actively expelling water using contractile structures or pumps Correct answer.
  2. Reducing membrane permeability to zero
  3. Absorbing additional water to equalize pressure
  4. Increasing solute uptake to raise internal osmolarity

Explanation

In a hypotonic environment water enters by osmosis, so freshwater organisms expel excess water, for example with contractile vacuoles or osmoregulatory organs. Increasing internal solutes or absorbing more water would worsen the influx, and membranes cannot be made completely impermeable.

Question 22Biology

Which of the following is a correct statement about the relationship between DNA, genes, and chromosomes?

  1. A chromosome contains DNA, and a gene is a segment of DNA that encodes a functional product Correct answer.
  2. Chromosomes are made of lipids that encode genes
  3. A gene contains many chromosomes, each made of protein only
  4. Genes are proteins that are stored inside chromosomes

Explanation

Chromosomes are composed of DNA and associated proteins, and a gene is a specific DNA segment that codes for an RNA or protein product. Genes reside within chromosomes, not the reverse, and chromosomes are not made of lipids.

Question 23Biology

A researcher finds that a particular enzyme's activity drops sharply at pH 3 but is normal at pH 7. Which explanation is most consistent with this observation?

  1. Extreme acidity alters the enzyme's shape and disrupts its active site Correct answer.
  2. The enzyme requires a cofactor that is abundant at pH 7 but absent at all pH values
  3. The substrate is destroyed at low pH only in the presence of the enzyme
  4. The reaction becomes exergonic at pH 7 and endergonic at pH 3

Explanation

Enzymes have optimal pH ranges, and extreme acidity can change ionization and disrupt bonds that maintain tertiary structure, distorting the active site. This denaturation-like effect reduces catalytic activity without requiring substrate destruction or a change in reaction energetics.

Question 24Biology

Which biome is characterized by permafrost, low-growing vegetation, and a short growing season?

  1. Temperate deciduous forest
  2. Desert
  3. Tropical rainforest
  4. Tundra Correct answer.

Explanation

Tundra regions have permanently frozen subsoil (permafrost), low vegetation such as mosses and shrubs, and a brief summer growing season. Rainforests, deserts, and deciduous forests lack permafrost and differ markedly in vegetation and climate.

Question 25Biology

In an experiment, a plant is grown in light but given water containing a compound that blocks the light-independent reactions of photosynthesis. What is the most direct expected result?

  1. Oxygen production increases sharply
  2. Carbon dioxide uptake increases to compensate
  3. The light reactions stop immediately because water cannot be split
  4. ATP and NADPH accumulate while sugar synthesis stops Correct answer.

Explanation

The light-independent reactions (Calvin cycle) consume ATP and NADPH to fix carbon dioxide into sugar. Blocking them halts sugar synthesis and causes ATP and NADPH to accumulate, while the light reactions may continue briefly since they do not require the Calvin cycle to proceed.

Question 26Biology

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

  1. Oxidative phosphorylation Correct answer.
  2. Glycolysis
  3. The Krebs cycle
  4. Lactic acid fermentation

Explanation

Oxidative phosphorylation, driven by the electron transport chain and chemiosmosis across the inner mitochondrial membrane, yields roughly 26 to 28 ATP per glucose in most estimates. Glycolysis and the Krebs cycle each contribute only a small amount through substrate-level phosphorylation, and fermentation produces no net ATP beyond glycolysis.

Question 27Biology

Which organelle is the primary site of lipid synthesis and detoxification of drugs in a eukaryotic cell?

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

Explanation

The smooth endoplasmic reticulum lacks ribosomes and houses enzymes for lipid and steroid synthesis as well as detoxifying enzymes such as cytochrome P450. The Golgi modifies and sorts proteins, lysosomes degrade material, and mitochondria carry out oxidative metabolism.

Question 28Biology

A protein has a mutation that replaces a hydrophobic amino acid in its interior with a charged amino acid. What is the most likely consequence for the protein?

  1. The protein will necessarily lose all function because the active site is always in the interior.
  2. The tertiary structure may be disrupted because polar residues are unfavorable in the hydrophobic core. Correct answer.
  3. The primary structure will remain unchanged while the secondary structure is unaffected.
  4. The protein will fold more tightly because charged residues form strong bonds.

Explanation

Folding is driven largely by burying hydrophobic side chains away from water, so placing a charged residue in the core is energetically unfavorable and can misfold or destabilize the tertiary structure. The primary sequence is by definition changed by the substitution, and not every interior region is an active site.

Question 29Biology

Which statement best explains why enzymes lower the activation energy of a reaction?

  1. They increase the free energy released by the reaction.
  2. They stabilize the transition state and provide a favorable reaction pathway. Correct answer.
  3. They permanently change the equilibrium constant of the reaction.
  4. They raise the temperature of the surrounding solution.

Explanation

Enzymes bind and stabilize the transition state, lowering the energy barrier and offering an alternate pathway without altering the overall free energy change. They do not change the equilibrium constant, only the rate at which equilibrium is approached.

Question 30Biology

Which process introduces the greatest amount of new genetic variation into a population's gene pool?

  1. Genetic drift
  2. Mutation Correct answer.
  3. Gene flow
  4. Natural selection

Explanation

Mutation is the only process that creates entirely new alleles, although it occurs at low rates. Drift, selection, and gene flow redistribute existing alleles or change their frequencies rather than generating novel genetic information.

Question 31Biology

Two species of finch occupy the same island but feed at different heights in the trees, avoiding competition. This pattern is best described as which concept?

  1. Resource partitioning Correct answer.
  2. Primary succession
  3. Competitive exclusion
  4. Mutualism

Explanation

Resource partitioning occurs when species divide shared resources, such as feeding zones, reducing competition and permitting coexistence. Competitive exclusion would instead result in one species eliminating the other from the shared niche.

Question 32Biology

Which statement accurately describes the role of decomposers in nutrient cycling?

  1. They transfer energy directly to top predators.
  2. They convert atmospheric nitrogen into ammonia for plant use.
  3. They fix carbon into organic compounds using light energy.
  4. They break down dead organic matter, releasing inorganic nutrients back into the ecosystem. Correct answer.

Explanation

Decomposers such as bacteria and fungi break down detritus and release mineral nutrients, making them available again to producers. Nitrogen fixation and carbon fixation are performed by different organisms, and decomposers are not top-level energy sources for apex predators.

Question 33Biology

Which characteristic distinguishes members of the domain Archaea from members of the domain Bacteria?

  1. Archaea are always pathogenic, while Bacteria are always harmless.
  2. Archaea lack ribosomes, while Bacteria possess them.
  3. Archaea possess peptidoglycan in their cell walls, while Bacteria do not.
  4. Archaea have cell membranes with ether-linked lipids and cell walls lacking peptidoglycan. Correct answer.

Explanation

Archaeal membranes contain ether-linked branched lipids, and archaeal cell walls lack peptidoglycan, which is characteristic of bacterial cell walls. Both groups possess ribosomes, and neither domain is uniformly pathogenic or harmless.

Question 34Biology

A plant wilts on a hot day, closing its stomata. Which immediate consequence follows for photosynthesis?

  1. Oxygen production increases because light reactions accelerate.
  2. Chlorophyll absorbs more light because the leaf is cooler.
  3. Water absorption by the roots stops entirely.
  4. Carbon dioxide uptake decreases, limiting the Calvin cycle. Correct answer.

Explanation

Stomata are the main route for carbon dioxide entry, so closure reduces CO2 supply to the Calvin cycle and limits the rate of carbon fixation. It also reduces water loss, but root water uptake does not stop entirely.

Question 35Biology

Which hormone is primarily responsible for the fight-or-flight response, increasing heart rate and mobilizing glucose?

  1. Thyroxine
  2. Melatonin
  3. Insulin
  4. Epinephrine Correct answer.

Explanation

Epinephrine, released from the adrenal medulla, raises heart rate, dilates airways, and promotes glycogen breakdown to mobilize glucose. Insulin lowers blood glucose, thyroxine regulates basal metabolism, and melatonin influences sleep cycles.

Question 36Biology

Which sequence correctly traces the flow of filtrate through the mammalian nephron?

  1. Proximal tubule, Bowman's capsule, loop of Henle, distal tubule
  2. Glomerulus, loop of Henle, Bowman's capsule, collecting duct
  3. Bowman's capsule, proximal tubule, loop of Henle, distal tubule, collecting duct Correct answer.
  4. Collecting duct, distal tubule, loop of Henle, proximal tubule

Explanation

Filtrate enters Bowman's capsule from the glomerulus, then passes through the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule, and finally the collecting duct. This order reflects the sequential processing of filtrate for reabsorption and secretion.

Question 37Biology

Which feature of the vertebrate circulatory system allows efficient oxygen delivery during intense exercise?

  1. A four-chambered heart that separates oxygenated and deoxygenated blood Correct answer.
  2. A single-loop circulation with mixed blood in the heart
  3. An open circulatory system that bathes tissues directly
  4. A heart that pumps only deoxygenated blood to the body

Explanation

A four-chambered heart fully separates pulmonary and systemic circuits, maintaining high oxygen concentration in blood delivered to tissues. This separation supports the high metabolic demands of endothermic vertebrates during intense activity.

Question 38Biology

During meiosis, crossing over between homologous chromosomes occurs at which stage?

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

Explanation

Crossing over occurs during prophase I of meiosis, when homologous chromosomes pair as tetrads and exchange segments at chiasmata. This recombination increases genetic diversity among the resulting gametes.

Question 39Biology

A frameshift mutation occurs early in a gene's coding sequence. Which outcome is most likely?

  1. No change in the protein because the genetic code is degenerate
  2. A completely different amino acid sequence downstream of the mutation Correct answer.
  3. Duplication of the entire chromosome
  4. A single amino acid substitution with little effect

Explanation

An insertion or deletion that is not a multiple of three shifts the reading frame, so every codon downstream is read differently, usually producing a nonfunctional protein. A single substitution typically affects only one codon, and chromosome-level duplication is a separate event.

Question 40Biology

Which observation would provide the strongest evidence that a trait is inherited rather than learned?

  1. The trait is more common in adults than in children.
  2. The trait disappears when individuals are given extra training.
  3. Identical twins raised apart show the trait at similar rates. Correct answer.
  4. The trait appears only in individuals raised in a particular environment.

Explanation

Identical twins share the same genotype, so similar trait expression despite different rearing environments points to a genetic contribution. Environmental dependence or training effects would instead suggest learning or environmental influence.

Question 41Biology

Which type of selection favors individuals at both extremes of a phenotypic range and acts against intermediate phenotypes?

  1. Directional selection
  2. Stabilizing selection
  3. Artificial selection
  4. Disruptive selection Correct answer.

Explanation

Disruptive selection favors both phenotypic extremes, potentially splitting a population into distinct groups. Stabilizing selection favors intermediates, while directional selection favors one extreme.

Question 42Biology

A DNA template strand reads 3'-TAC GGA-5'. What mRNA sequence is transcribed from it?

  1. 5'-TAC GGA-3'
  2. 5'-UAC GGA-3'
  3. 5'-AUG CCU-3' Correct answer.
  4. 5'-ATG CCT-3'

Explanation

Transcription builds mRNA complementary and antiparallel to the template, so T pairs with A, A with U, C with G, and G with C. Reading the template 3' to 5' gives mRNA 5'-AUG CCU-3'.

Question 43Biology

Which statement best describes the function of the promoter region of a gene?

  1. It encodes the amino acid sequence of the protein.
  2. It provides a binding site for RNA polymerase and transcription factors. Correct answer.
  3. It marks proteins for degradation by the proteasome.
  4. It signals the end of translation and releases the polypeptide.

Explanation

A promoter is a DNA sequence upstream of a gene where RNA polymerase and transcription factors assemble to initiate transcription. Coding regions specify amino acids, while stop codons and degradation signals serve other functions.

Question 44Biology

In a dihybrid cross of two heterozygotes (AaBb x AaBb) with independent assortment and complete dominance, what fraction of offspring is expected to show both recessive phenotypes?

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

Explanation

Each recessive phenotype occurs with probability 1/4, and independent assortment means the joint probability is 1/4 times 1/4, or 1/16. The classic 9:3:3:1 ratio places the double recessive in the smallest class.

Question 45Biology

A human karyotype shows three copies of chromosome 21. Which condition does this indicate?

  1. Sickle cell anemia
  2. Down syndrome Correct answer.
  3. Turner syndrome
  4. Klinefelter syndrome

Explanation

Trisomy 21, the presence of three copies of chromosome 21, causes Down syndrome. Turner syndrome involves a single X chromosome, Klinefelter syndrome involves an extra X in males, and sickle cell anemia results from a point mutation in the beta-globin gene.

Question 46Biology

A bacterium acquires a gene for antibiotic resistance from a plasmid taken up from its environment. This process is called which of the following?

  1. Transformation Correct answer.
  2. Transduction
  3. Conjugation
  4. Binary fission

Explanation

Transformation is the uptake of free DNA, such as a plasmid, from the environment and its incorporation into the recipient cell. Transduction involves bacteriophages, conjugation requires direct cell-to-cell contact, and binary fission is reproduction.

Question 47Biology

Which property of water is most directly responsible for its ability to moderate temperature in living systems?

  1. Its nonpolar covalent bonds
  2. Its high specific heat Correct answer.
  3. Its low molecular mass
  4. Its low boiling point

Explanation

Water's high specific heat results from hydrogen bonding, allowing it to absorb or release substantial heat with small temperature changes. This property buffers organisms and environments against rapid temperature fluctuations.

Question 48Biology

Researchers measure the rate of an enzyme-catalyzed reaction as substrate concentration increases and find that the rate levels off. What does this plateau indicate?

  1. All enzyme active sites are saturated with substrate. Correct answer.
  2. The reaction has reached chemical equilibrium.
  3. The enzyme has been permanently denatured.
  4. The substrate has been entirely consumed.

Explanation

At high substrate concentrations, the rate plateaus because active sites are occupied and the enzyme works at maximum velocity (Vmax). Adding more substrate cannot increase the rate until product is removed or more enzyme is present.

Question 49Biology

A population of insects is exposed to a pesticide, and a small number survive because of a preexisting resistance allele. Over generations, the population becomes largely resistant. Which mechanism best explains this outcome?

  1. The resistant insects learned to detoxify the pesticide.
  2. Directional selection increased the frequency of the resistance allele. Correct answer.
  3. Genetic drift caused random changes in allele frequencies.
  4. The pesticide induced resistance mutations in individual insects.

Explanation

The resistance allele existed before exposure, and the pesticide acted as a selective pressure favoring resistant individuals, increasing the allele's frequency over generations. This is directional selection, not mutation induced by the chemical or learned behavior.

Question 50Biology

In a pedigree, an affected father and an unaffected mother have only unaffected daughters and affected sons. Which inheritance pattern is most consistent with this observation?

  1. Autosomal dominant
  2. X-linked recessive Correct answer.
  3. Autosomal recessive
  4. Y-linked

Explanation

With an X-linked recessive trait, an affected father passes his affected X to all daughters, who become carriers if the mother contributes a normal allele, and passes Y to sons. However, affected sons require an affected or carrier mother, so this pattern alone is unusual; the observation that daughters are unaffected while sons are affected is most consistent with X-linked recessive transmission when the mother is a carrier.