USABO OPEN Exam 2025 With 250 Prep Questions

pg. 1 USABO OPEN Exam 2025 With 250 Prep Questions and Correct Answers From Previous Tests | USABO OPEN Practice Exam Test Bank 1 (Brand New!)

A population of iguanas with varying degrees of pigmentation inhabits an island. A newly introduced predator is better able to hunt lightly pigmented iguanas, allowing darker iguanas to breed more successfully. The population will exhibit

the effects of:

• Balancing Selection.
• Disruptive selection.
• Gene flow.
• Directional selection.
• Multifactorial traits.
• Directional selection.

If a yellow-coat colored mouse is crossed with a gray-coat colored mouse, what phenotypic frequency is expected among their progeny?

• 1/2 yellow, 1/2 gray
• 3/4 gray, 1/4 yellow
• 3/4 yellow, 1/4 gray
• 7/16 gray, 9/16 yellow
• 9/16 yellow, 7/16 gray
• 1/2 yellow, 1/2 gray

A pea plant X dihybrid for flower color and height is crossed with a pea plant Y true breeding for flower color and height. Pea plant X is tall and has purple flowers while pea plant Y is short and has white flowers. Given that the genes are on separate chromosomes, tall is dominant to short and purple is dominant to white, find the phenotypic ratios of the offspring.

• 1 tall purple: 1 short purple: 1 tall white: 1 short white.
• 9 tall purple: 3 short purple: 3 tall white: 1 short white.
• 9 tall purple: 3 short purple: 4 short white. 1 / 4

pg. 2

D. 9 tall purple: 7 short white.

E. 9 tall purple: 4 short white.

• 1 tall purple: 1 short purple: 1 tall white: 1 short white.

A homozygous black-eye, white-coat, short-hair mouse is mated to a homozygous brown-eye, brown-coat, long-hair mouse. One of the resulting F1 animals is mated to a homozygous brown-eye, browncoat, long-hair mouse. Given the following offspring from the second mating, what is the recombination frequency with respect to eye and coat color?243 black-eye, white-coat, short-hair 220 brown-eye, brown-coat, long-hair 200 brown-eye, brown-coat, short-hair 187 black-eye, white-coat, long-hair 49 brown-eye, white-coat, short-hair 42 black-eye, brown-coat, long-hair 36 brown-eye, white-coat, short-hair 23 black-eye, brown-coat, short-hair

A. 5%. B. 15% C. 25% D. 46% E. 75%

B. 15%

Which of the following do an ovum and sperm contribute equally?

• Cytoplasm.
• Mitochondria.
• Endoplasmic reticulum.
• Nuclear DNA.
• Nutrients.
• Nuclear DNA.

Boston is known for having aggressive geese along riverside paths, which frequently stand their ground rather than flee in response to rapidly approaching runners or bikers. City ordinance prohibits causing harm to these geese. At the individual level, what is the most likely explanation for this behavior?

• It is innate
• It is a consequence of classical conditioning 2 / 4

pg. 3

• It is a consequence of operant conditioning
• The geese did not observe other geese fleeing during a critical period in

development

• Runners and bikers do not present sign stimuli
• It is a consequence of operant conditioning

When one travels across time zones by airplane, one experiences being tired and disoriented. Decide which statement is true and which is false. Crossing time

zones:

• Causes a sudden inconsistency between the internal body clock and the outside

world.

• Synchronizes local zeitgebers (external or environmental cues that determine,

modify, or synchronize an organism's biological rhythms to the earth's 24-hour light/dark cycle and 12 month cycle).

• West to east does not affect our biological rhythms as much as flying from east

to west.

• Increased frequency and severity of piloting errors occur during aircraft

operations.

• True
• False
• False
• True

The large sea slug, Aplysia californicus, has an involuntary gill and siphon withdrawal reflex which causes the siphon and gill to retract when disturbed.

Through experimentation, researchers found:

• Sensitization was innate but habituation was genetically determined.
• Repeated stimulation exhausted the neurotransmitter being released and caused

desensitization.

• Repeated stimulation reduced the amount of the neurotransmitter being released

and induced the habituation.

• Repeated stimulation exhausted the neurotransmitter being released and caused

the sensitization.

• Shocking the interneurons caused habituation. 3 / 4

pg. 4

• Repeated stimulation reduced the amount of the neurotransmitter being released

and induced the habituation.Black and white mice live on an island and have allele frequencies of B=0.20, b=0.80. On the continent, there is a much large population of mice with allele frequencies B=0.80, b=0.20. Ships begin moving back and forth between the island and the continent. Occasionally, a mouse finds its way onto the ship and leaves the ship and breeds with the mice of the other population. Equal numbers of mice ride in each direction. If the ships move continuously between the island and the continent, will the allele frequencies ultimately stabilize? (Assume that no other forces are affecting allele frequencies.)

• Both populations will end up at approximately B=0.80, b=0.20.
• The island and continent populations will stabilize with an allele frequency of

B=0.50, b=0.50.

• Black mice will move to fixation in both populations, because the B allele is

dominant.

• Both populations will end up at B=0.20, b=0.80.
• The continent's allele frequencies will not change; the island will settle at an

equilibrium that is somewhere between the continent's and the island's original states.

• Both populations will end up at approximately B=0.80, b=0.20.

Chromosome segregation is an important step in both mitosis and meiosis.Determine if the following statements are true or false.

• Abnormalities in sex chromosome segregation in humans are always fatal.
• In most mammals and birds, males have XY sex chromosomes.
• Down's Syndrome is often caused by a chromosome segregation error.
• It is not uncommon for plants to have more than 2 copies of each chromosome.
• False
• False
• True
• True

Which of the following is a genetic variation of Klinefelter's Syndrome?

• XXX.
• / 4