USABO Open Exam 2025 All Possible Questions

USABO Open Exam 2025 All Possible Questions Latest Update 2025-2026 Exam 285 Questions with 100% Verified Correct Answers Guaranteed A+

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% - CORRECT ANSWER: B. 15%

A short-day plant has a critical night length of 6 hours. Which of the following 24-hour light/dark cycles would not initiate flowering?

• 4 hours light/8 hours dark, 4 hours light/8 hours dark
• 6 hour light/6 hour dark, 6 hour light/6 hour dark
• 6.5 hour light/5.5 hour dark, 6.5 hour light/5.5 hour dark
• 7 hour light/5 hour dark, 7 hour light/5 hour dark
• 8 hours light/16 hour dark - CORRECT ANSWER: A. 4 hours light/8 hours dark, 4

hours light/8 hours dark 1 / 4

An inbred strain of plants has a mean height of 24 cm. A second strain of the same species also has a mean height of 24 cm. When these plants are crossed, the F1 are also 24 cm in height. However, when the F1 plants are crossed, the F2 plants show a wide range of heights; the greatest number are similar to the P1 and F1, but approximately 4 of 1000 are only 12 cm tall and 4 of 1000 are 40 cm tall. What fraction of the F2 plants will be approximately 28 cm in height? [Assume that each of the alleles involved in determining increase in plant height contributes the same amount.]

A. 3/4

B. 9/16

C. 56/256

• None

E. Cannot be determined from the information given - CORRECT ANSWER: C. 56/256

Arrange the following five events in the order that explains the bulk flow of substances in the phloem.

• Sugar moves down the stem.
• Leaf cells produce sugar by photosynthesis.
• Sugar is transported from cell to cell via the apoplast and/or symplast.
• Solutes are actively transported into sieve elements.
• Water diffuses into the sieve tube elements.

A. 1,2,3,4,5

B. 2,1,4,3,4

C. 2,3,4,1,5

D. 2,4,1,3,5

E. 4,2,1,3,5 - CORRECT ANSWER: C. 2,3,4,1,5

Arthropods are the most numerous of all animal groups. This diversity in numbers may be the consequence of the presence of ancient regulating genes present prior to the 2 / 4

division of the protostome-deuterostome group. The evolution of increased

morphological change in the basic body plan of arthropods is attributed to the:

• Differential success of alternate developmental variations.
• Flexible modular body plan.
• Genetic peculiarity among arthropods.
• Plasticity of regulatory genes.

E. All of above. - CORRECT ANSWER: D. Plasticity of regulatory genes.

Blood entering the kidney is filtered by the Bowman's capsule. In what order does the filtrate pass through the following structures of the nephron?(1) Ascending limb of the Loop of Henle (2) Distal convoluted tubule (3) Descending limb of the Loop of Henle (4) Proximal convoluted tubule (5) Collecting duct

A. 1, 3, 4, 2, 5

B. 2, 1, 3, 4

C. 2, 3, 1, 4, 5

D. 4, 1, 3, 5

E. 4, 3, 1, 2, 5 - CORRECT ANSWER: E. 4, 3, 1, 2, 5

Both amino acids and nucleotides form complex chains. Select all the answers below that are accurate.

• The chains formed by both have polar phosphate groups.
• Both contain peptide bonds.
• Both contain nitrogen. 3 / 4

• The chains formed by both can be lysed by trypsin.
• The sequence of chains both form from preexisting templates. - CORRECT

ANSWER: C. Both contain nitrogen.

• The sequence of chains both form from preexisting templates.

During the late Carboniferous period, terrestrial arthropods such as Hexapoda, Myriapoda, and Arachnida showed great diversity in form and size. An example is the giant dragonfly, Meganeura, with a wing span of over 70 cm. Our modern day dragonflies and other arthropods are much smaller than their Carboniferous ancestors

because:

• The amount of nutrients is less today than in the Carboniferous.
• The amount of free carbon is greater today.
• The competition among organisms for existing habitats is greater today and small

size provides a competitive edge.

• The thickness of the exoskeleton required in large Carboniferous insects limited the

ability of the arthropods to successfully molt and subsequently to compete with smaller forms.

• The volume of oxygen in the atmosphere is much lower today than in the

Carboniferous. - CORRECT ANSWER: E. The volume of oxygen in the atmosphere is

much lower today than in the Carboniferous.

Genetic Drift is best described as:

• A random change in allele frequencies that benefits the population.
• Changes in the proportion of homozygous recessive individuals due to random

mating

• Evolutionary change due to natural selection.
• Evolutionary change due to non-random events.

E. Evolutionary change due to random events. - CORRECT ANSWER: E. Evolutionary

change due to random events.

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