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?
A. 4 hours light/8 hours dark, 4 hours light/8 hours dark
B. 6 hour light/6 hour dark, 6 hour light/6 hour dark
C. 6.5 hour light/5.5 hour dark, 6.5 hour light/5.5 hour dark
D. 7 hour light/5 hour dark, 7 hour light/5 hour dark
E. 8 hours light/16 hour dark - CORRECT ANSWER: A. 4 hours light/8 hours dark, 4
hours light/8 hours dark
,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
D. 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.
1. Sugar moves down the stem.
2. Leaf cells produce sugar by photosynthesis.
3. Sugar is transported from cell to cell via the apoplast and/or symplast.
4. Solutes are actively transported into sieve elements.
5. 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
,division of the protostome-deuterostome group. The evolution of increased
morphological change in the basic body plan of arthropods is attributed to the:
A. Differential success of alternate developmental variations.
B. Flexible modular body plan.
C. Genetic peculiarity among arthropods.
D. 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.
A. The chains formed by both have polar phosphate groups.
B. Both contain peptide bonds.
C. Both contain nitrogen.
, D. The chains formed by both can be lysed by trypsin.
E. The sequence of chains both form from preexisting templates. - CORRECT
ANSWER: C. Both contain nitrogen.
E. 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:
A. The amount of nutrients is less today than in the Carboniferous.
B. The amount of free carbon is greater today.
C. The competition among organisms for existing habitats is greater today and small
size provides a competitive edge.
D. 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.
E. 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. A random change in allele frequencies that benefits the population.
B. Changes in the proportion of homozygous recessive individuals due to random
mating
C. Evolutionary change due to natural selection.
D. Evolutionary change due to non-random events.
E. Evolutionary change due to random events. - CORRECT ANSWER: E. Evolutionary
change due to random events.
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?
A. 4 hours light/8 hours dark, 4 hours light/8 hours dark
B. 6 hour light/6 hour dark, 6 hour light/6 hour dark
C. 6.5 hour light/5.5 hour dark, 6.5 hour light/5.5 hour dark
D. 7 hour light/5 hour dark, 7 hour light/5 hour dark
E. 8 hours light/16 hour dark - CORRECT ANSWER: A. 4 hours light/8 hours dark, 4
hours light/8 hours dark
,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
D. 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.
1. Sugar moves down the stem.
2. Leaf cells produce sugar by photosynthesis.
3. Sugar is transported from cell to cell via the apoplast and/or symplast.
4. Solutes are actively transported into sieve elements.
5. 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
,division of the protostome-deuterostome group. The evolution of increased
morphological change in the basic body plan of arthropods is attributed to the:
A. Differential success of alternate developmental variations.
B. Flexible modular body plan.
C. Genetic peculiarity among arthropods.
D. 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.
A. The chains formed by both have polar phosphate groups.
B. Both contain peptide bonds.
C. Both contain nitrogen.
, D. The chains formed by both can be lysed by trypsin.
E. The sequence of chains both form from preexisting templates. - CORRECT
ANSWER: C. Both contain nitrogen.
E. 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:
A. The amount of nutrients is less today than in the Carboniferous.
B. The amount of free carbon is greater today.
C. The competition among organisms for existing habitats is greater today and small
size provides a competitive edge.
D. 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.
E. 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. A random change in allele frequencies that benefits the population.
B. Changes in the proportion of homozygous recessive individuals due to random
mating
C. Evolutionary change due to natural selection.
D. Evolutionary change due to non-random events.
E. Evolutionary change due to random events. - CORRECT ANSWER: E. Evolutionary
change due to random events.
