Mixed Deck — All USABO Topics Flashcards

Which of the following is most accurate regarding the anatomy and physiology of the vertebrate vascular system?

Answer: Elastic fibers made of the protein elastin in the tunica media allow arteries to accommodate fluctuating blood pressures. They are almost absent in the tunica media of veins.

Explanation: Elastic fibers in the tunica media of arteries allow them to stretch and recoil, accommodating the pulsatile flow of blood from the heart and helping maintain blood pressure. Veins, on the other hand, have thinner walls and do not require as much elasticity to handle blood flow, so elastic fibers are less abundant in their tunica media.

In Mendel's dihybrid cross experiments with pea plants (RrYy x RrYy), he observed that the traits for seed shape (Round/wrinkled) and seed color (Yellow/green) assorted independently. What is the probability of obtaining an F2 offspring that is heterozygous for both traits (RrYy)?

Answer: 1/4

In a dihybrid cross (RrYy x RrYy), we can consider each trait separately. The probability of getting Rr from a Rr x Rr cross is 1/2. Similarly, the probability of getting Yy from a Yy x Yy cross is 1/2. According to the law of independent assortment, the probability of both events occurring together is the product of their individual probabilities: (1/2) * (1/2) = 1/4.

A species of marine invertebrate releases millions of eggs into the water, of which very few survive to maturity. However, those that do survive the initial larval stages have a high probability of living for a long time. Which type of survivorship curve would this species most likely exhibit?

Answer: Type III

This life history strategy is characteristic of a Type III survivorship curve. [8, 9] This curve represents high mortality rates for the very young, followed by a much lower, constant death rate for the few individuals that survive to older age. [6, 14] This is common in species that produce a large number of offspring with little to no parental care. [8, 15]

In the context of the lac operon in E. coli, which of the following scenarios would result in the highest level of transcription of the structural genes (lacZ, lacY, lacA)?

Answer: Low glucose, high lactose

Maximum transcription of the lac operon requires two conditions to be met: 1) Lactose must be present to inactivate the lac repressor protein, which otherwise blocks transcription. Allolactose, an isomer of lactose, binds to the repressor, causing it to detach from the operator site. 2) Glucose levels must be low. Low glucose leads to high levels of cyclic AMP (cAMP), which binds to the Catabolite Activator Protein (CAP). The cAMP-CAP complex then binds to the promoter region and significantly enhances the binding of RNA polymerase, leading to a high rate of transcription.

Photorespiration reduces photosynthetic efficiency primarily because it:

Answer: Releases previously fixed CO2 via the glycolate pathway

When Rubisco oxygenates RuBP, the products enter the glycolate pathway in peroxisomes and mitochondria, releasing CO2 and consuming ATP without net carbon gain.

A plant cell has a solute potential (Ψs) of -0.8 MPa and a pressure potential (Ψp) of +0.5 MPa. The cell is placed in a beaker of pure water (Ψ = 0 MPa). What will be the initial net direction of water movement?

Answer: Into the cell, because its water potential is -0.3 MPa.

The total water potential (Ψ) of the cell is the sum of its solute potential (Ψs) and pressure potential (Ψp). In this case, Ψcell = Ψs + Ψp = -0.8 MPa + 0.5 MPa = -0.3 MPa. Pure water has a water potential of 0 MPa. Water always moves from an area of higher water potential to an area of lower water potential. Since the water potential of the surrounding pure water (0 MPa) is higher than the cell's water potential (-0.3 MPa), water will move into the cell.

In USA Biology Olympiad, why is immunology and disease knowledge important for professional certification?

Answer: It demonstrates competence and ensures practitioners meet established standards

Professional certification in specific knowledge areas demonstrates that practitioners have met established competency standards, ensuring quality of service and public protection.

An age-structure diagram for a developing country shows a classic pyramid shape, with a very broad base that narrows progressively toward the top. Which of the following is the most accurate long-term prediction for this population, assuming current trends continue?

Answer: The population will experience rapid growth due to a large proportion of individuals in pre-reproductive and reproductive age groups.

A pyramid-shaped age-structure diagram indicates that a large percentage of the population is young (pre-reproductive) and in their reproductive years. [23, 24] This large cohort of young individuals will mature and have children, leading to a significant increase in population size over time, characteristic of rapid population growth. [25]

A patient is diagnosed with a demyelinating disease, where the myelin sheaths surrounding axons in the central nervous system are progressively damaged. Which of the following is the most direct physiological consequence of this loss of myelin on nerve impulse transmission?

Answer: The speed of saltatory conduction is decreased, potentially leading to transmission failure.

Myelin acts as an electrical insulator, allowing the action potential to 'jump' from one node of Ranvier to the next in a process called saltatory conduction. This dramatically increases the speed of nerve impulse transmission. When myelin is lost, the electrical current dissipates through the now-exposed axonal membrane, slowing down or completely halting the propagation of the action potential as it may no longer be strong enough to depolarize the next node to threshold.

In USA Biology Olympiad, what role does continuing education play in immunology and disease?

Answer: To keep professionals current with evolving standards, technologies, and best practices

Continuing education ensures professionals stay current with new developments, evolving standards, and emerging best practices in their field, maintaining competence throughout their careers.

In the sliding filament model of muscle contraction, which of the following events directly triggers the 'power stroke' where the myosin head pivots and pulls the actin filament?

Answer: The release of inorganic phosphate (Pi) from the myosin head.

The cross-bridge cycle begins when the myosin head, already energized by ATP hydrolysis, binds to actin. The power stroke itself—the pivoting motion that pulls the actin filament—is directly initiated by the release of the inorganic phosphate (Pi) that was bound to the myosin head. Following the power stroke, ADP is released, and the binding of a new ATP molecule is required for the myosin head to detach from actin.

In non-cyclic photophosphorylation, which molecule serves as the ultimate electron donor?

Answer: Water (H2O)

Water is oxidized at the oxygen-evolving complex of PS II, donating electrons that flow through the entire photosynthetic electron transport chain to ultimately reduce NADP+.

In Labrador retrievers, coat color is determined by recessive epistasis. The B/b locus determines pigment color (B=black, b=brown), while the E/e locus determines pigment deposition (E=deposit, e=no deposit). A dog with genotype 'ee' will be yellow regardless of its B/b genotype. If two dogs with the genotype BbEe are crossed, what is the expected phenotypic ratio of their offspring?

Answer: 9 black : 3 brown : 4 yellow

A dihybrid cross of BbEe x BbEe produces offspring with genotypes in a 9:3:3:1 ratio (B_E_ : bbE_ : B_ee : bbee). The phenotypes are: B_E_ = Black; bbE_ = Brown; B_ee = Yellow; bbee = Yellow. Because the 'ee' genotype masks the B/b locus, both B_ee and bbee genotypes result in a yellow coat. Therefore, the phenotypic ratio is 9 Black : 3 Brown : (3+1) Yellow, which simplifies to 9:3:4.

During a period of intense exercise, the affinity of hemoglobin for oxygen changes to facilitate delivery to muscle tissues. Which of the following factors causes a 'right shift' in the oxygen-hemoglobin dissociation curve?

Answer: An increase in the partial pressure of CO2 and a decrease in blood pH.

A right shift in the oxygen-hemoglobin dissociation curve indicates a decreased affinity of hemoglobin for oxygen, which promotes oxygen unloading in tissues. During exercise, muscle metabolism increases the production of CO2 and lactic acid. CO2 increases the partial pressure of CO2 (PCO2) and also forms carbonic acid, both of which lower blood pH. This phenomenon, known as the Bohr effect, facilitates the release of oxygen where it is most needed.

A biologist classifies a group of organisms based on their shared evolutionary history. The traditional class "Reptilia" is often cited as an example of a paraphyletic group because it includes lizards, snakes, turtles, and crocodiles, but excludes birds. What is the defining characteristic of a paraphyletic group?

Answer: It includes the group's most recent common ancestor but not all of its descendants.

A paraphyletic group is one that contains the group's last common ancestor but fails to include all of that ancestor's descendants. The classic example is the traditional definition of Reptilia, which includes the common ancestor of reptiles and birds but excludes birds, which are a descendant lineage.

What is a best practice in USA Biology Olympiad immunology and disease?

Answer: A method or technique recognized as superior based on evidence and expert consensus

Best practices are methods, techniques, or approaches that are recognized through evidence, research, and expert consensus as producing superior results and are recommended for adoption.

When systematists construct phylogenetic trees, they often evaluate multiple possible trees and select the one that is considered the 'best' hypothesis. The principle of maximum parsimony is a common method for this. Which of the following statements best describes this principle?

Answer: The preferred tree is the one that requires the fewest evolutionary changes to explain the observed data.

The principle of maximum parsimony states that the best phylogenetic tree is the one that minimizes the total number of character-state changes or evolutionary steps. This is an application of Occam's razor, favoring the simplest explanation for the relationships among the organisms being studied.

The ratio of synonymous to nonsynonymous mutations that a protein-coding gene experiences in a given period of time is frequently used to infer the evolutionary pressures acting upon that gene. In particular, let ω = dN / dS, where dN is the number of nonsynonymous mutations per nonsynonymous site, and dS is the number of synonymous mutations per synonymous site. Which of the following statements about ω and protein evolution is NOT correct?

Answer: For most genes, ω = 1.

Explanation: The statement that is NOT correct is: For most genes, ω = 1. In reality, most genes don't evolve neutrally, so their ω values vary depending on the evolutionary pressures they face.

What is the primary function of the Casparian strip in the root endodermis?

Answer: Blocking apoplastic water movement and forcing symplastic passage

The hydrophobic suberin lamellae of the Casparian strip block the apoplastic pathway, forcing water and ions to cross the endodermal plasma membrane where selective transport can occur.

In cladistics, the presence of hair is a key character for defining the clade Mammalia, distinguishing it from other vertebrates. In contrast, the presence of a vertebral column, while common to all mammals, is not a defining character for this specific clade because it is also found in fish, amphibians, and reptiles. How are the vertebral column and hair best described, respectively, in the context of defining Mammalia?

Answer: Symplesiomorphy; Synapomorphy

A symplesiomorphy is a shared ancestral character. The vertebral column is ancestral to all vertebrates, so for mammals, it is a symplesiomorphy. A synapomorphy is a shared derived character that uniquely defines a particular clade. Hair is a derived trait that is shared by all mammals and distinguishes them as a group, making it a synapomorphy for the clade Mammalia.