USABO OPEN Exam 2025 Test Bank 2 With 250

pg. 1 USABO OPEN Exam 2025 Test Bank 2 With 250 Prep Questions and Correct Answers (Questions Derived From Previous Tests) USABO OPEN Practice Exam Test Bank (Brand New!)

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. 314

B. 9/16

C. 56/256

• None
• Cannot be determined from the information given.

C. 56/256

The skin forming the flying surface in the wing of a bat and the feathers forming

the flying surface in the wing of a bird:

• evolved from a similar structure in a common ancestor.
• are different because of adaptive radiation.
• do not develop from the same sources in the embryos of bat and bird, and are

analogous.

• perform different functions in the bat and the bird.
• evolved in response to "felt need" in adult birds and bats.
• do not develop from the same sources in the embryos of bat and bird, and are

analogous. 1 / 4

pg. 2 You are trying to isolate glyoxysomes and peroxisomes from a mixture of cellular organelles using differential centrifugation. After a few centrifugation steps, you think you may have a relatively pure suspension. How might you determine that your suspension does indeed include these organelles?

• Perform at1 assay for the enzyme succinate dehydrogenase.
• Perform an assay for the presence of nucleic acids.
• Perform an assay for the enzyme catalase.
• Examine microscopically for double membrane bound organelles.
• This cannot be determined experimentally.
• Perform an assay for the enzyme catalase.

A large population of laboratory rats was allowed to breed randomly for a number of generations. In this population the A allele codes for agouti fur and a allele codes for black fur. At the beginning of the breeding cycles, 49% of the rats had black fur. At the end of the breeding cycles, this percentage was the same. The rest of the animals had agouti fur, with heterozygotes indistinguishable from

homozygous dominants. The estimated frequency of the a allele in these rats is:

A. 0.07

B. 0.3

C. 0.42

D. 0.49

E. 0.7

E. 0.7

What effect do the processes meiosis and fertilization have on allele frequencies in a population?

• They reduce the frequency of deleterious alleles.
• They increase the frequency of advantageous alleles.
• They tend to selectively combine favorable alleles into the same zygote without

changing overall allele frequencies.

• They tend to increase the frequency of new alleles and decrease the frequency

of old ones.

• They have no direct effect on allele frequencies.
• They have no direct effect on allele frequencies. 2 / 4

pg. 3 You find a chunk of petrified fossil wood and take it to a paleobotanist who identifies it as palm (monocot) wood. What is the basis of this identification?

• Only ground tissue and scattered vascular bundles occur in the specimen.
• There are numerous tiny growth rings.
• There is lots of heartwood and very little sapwood.
• There are almost equal amounts of spring wood and summer wood in each

year's growth.

• The vascular cambium only appears active within vascular bundles.
• Only ground tissue and scattered vascular bundles occur in the specimen.

The best index fossils that a paleontologist could use to assign relative ages to

different strata of sedimentary rock would be those of:

• dinosaurs.
• shelled aquatic/marine organisms.
• bacteria.
• flightless birds.
• tunicates.
• shelled aquatic/marine organisms.

The endosperm of a plant with "monosporic" development has 72 chromosomes.How many chromosomes does the egg cell have?

• 24
• 28
• 72
• 98

E. 120

• 24

In plant cells grown in the presence of a metabolic poison that specifically inhibits

mitochondria1 FlATP synthase, one would expect:

• the pH difference across the cristae to be greater than normal.
• the pH difference across the cristae to be less than normal.
• the electron transport chain to become inoperative. 3 / 4

pg. 4

• oxygen consumption to cease.
• water to move out of the mitochondria by osmosis.
• the pH difference across the cristae to be greater than normal.

Net primary productivity, in most ecosystems, is important because it represents

the:

• stored chemical energy that would be available to heterotrophs.
• total solar energy converted to chemical energy by producers.
• energy used in respiration by heterotrophs.
• energy available to producers.
• biomass of all producers.
• stored chemical energy that would be available to heterotrophs.

The presence of a vertebral column is _____ with respect to all vertebrates [first term], but _____ with respect to all chordates [second term].

• analogous; homologous
• analogous; derived
• derived; primitive
• homologous; analogous
• primitive; derived
• primitive; derived

Which two brain centers control mammalian respiration?

• Medulla oblongata and substantia nigra
• Substantia nigra and red nucleus
• Red nucleus and pons
• Medulla oblongata and red nucleus
• Medulla oblongata and pons
• Medulla oblongata and pons

The universal distribution of antibiotics to poultry at commercial farms may have

an adverse evolutionary effect because:

• Chickens prone to bacterial diseases may survive.
• / 4