Exam # 2:

Please note: I have bolded correct answers in the body of the exam below.

Exam Instructions

You have 90 minutes to complete this exam, unless your SAS accommodation affords you additional time. You may refer to any course materials (lecture slides, outline, textbook, your own notes), but may not use a search engine (e.g. Google) or any AI tool (e.g. ChatGPT). You may not collaborate with anyone, including other students in this class, while taking this exam.

Please do not share this exam, or give hints about the exam contents, with students in any other section of this course.

For all fill-in-the blank responses, please use ONLY lowercase letters and do not add extra words or spaces in your answer. Misspelled words will not receive credit.

All written answers must be your own individual, unique work, written in complete, grammatically-correct sentences. Make sure to answer the prompts thoroughly, but without including information that does not directly answer the question.

If you have any technical difficulties, please contact me immediately at wstjohn@marin.edu. I will be more than happy to help you troubleshoot (including rescheduling the exam), but you must contact me before the exam has closed.

Question 1 (3 pts)

A mutation changes one base in DNA, but the protein’s amino acid sequence is unchanged. What best explains this outcome?

Mutations in somatic cells have no effect on proteins.
The mutation happened in RNA, not DNA.
Multiple codons can code for the same amino acid, so this was a silent mutation.
Mutations in DNA are always corrected before translation.

Question 2 (3 pts)

Which scenario best represents how loss of cell cycle regulation leads to cancer?

Cells with mutations fail to respond to “stop” signals and divide uncontrollably.
Cells replicate DNA too slowly.
Cells only divide when anchored to the substrate.
Cells stop dividing when crowded and enter G₀ phase.

Question 3 (3 pts)

Identify the correct phase of cell division in the image pictured below:

Prophase of Meiosis II
Anaphase of Meiosis II
Telophase of Mitosis
Cytokinesis of Meiosis I
Anaphase of Mitosis

Question 4 (3 pts)

Why are hydrogen bonds crucial to DNA replication and transcription?

They’re weak enough to be broken for strand separation, allowing enzymes to copy or transcribe DNA.
They provide energy for polymerase.
They permanently lock DNA strands together.
They form between amino acids in proteins.

Question 5 (3 pts)

Influenza viruses frequently change their surface proteins (H and N). What evolutionary principle does this demonstrate?

Viral evolution stops once a vaccine is developed.
Environmental change triggers identical mutations in all viruses.
Mutations are caused by human immune systems.
Random mutation and selection lead to new variants that escape immune detection.

Question 6 (3 pts)

Two pink dragons mate, both of whom are heterozygotes for the gene that determines color (R). If the inheritance of this trait is an example of incomplete dominance, which of the following is true?

Each offspring will be red and white at the same time.
Half of the offspring will be red; half will be white (1:1 ratio)
A 1:2:1 genotypic ratio applies, but phenotypes differ because Rr is intermediate.
100% of offspring will be pink because pink is dominant.

Question 7 (3 pts)

In a pedigree, a trait appears mostly in males, and females can be carriers without showing symptoms. Which explanation integrates chromosomal and Mendelian inheritance?

The trait is polygenic.
The trait is X-linked recessive.
The trait is autosomal dominant.
The trait is autosomal recessive.

Question 8 (3 pts)

Sickle-cell heterozygotes (carriers) have some normal and some sickled red blood cells, yet resist malaria. This example simultaneously demonstrates:

A dominant lethal mutation
Environmental plasticity
Codominance and an evolutionary advantage for heterozygotes
Incomplete dominance and environmental influence

Question 9 (3 pts)

A calico cat has patches of orange and black fur because:

Each fur cell randomly inactivates one X chromosome, expressing different alleles in different areas.
Only male cats carry coat-color genes.
Calico coat color depends solely on environmental factors.
Orange is codominant with black in both sexes.

Question 10 (3 pts)

A population of frogs lives in a pond where water levels fluctuate seasonally. Some frogs lay eggs in shallow water early in the season; others wait for deeper pools to form. Both strategies persist over time. What does this suggest about evolutionary “fitness”?

Only one of these strategies can be considered truly fit.
Fitness is measured by physical strength, not behavior.
Multiple strategies can be “fit enough” if each leads to successful reproduction.
Evolution always eliminates one alternative strategy.

Question 11 (3 pts)

If predators in the beach habitat were replaced by nocturnal owls that hunt mainly by sound, what would likely happen over time to mouse coat color?

Color variation would no longer affect survival, so color would not change due to selection.
Mice would evolve even lighter fur to avoid detection.
Darker mice would suddenly become more common due to heat absorption.
Natural selection would remove all color variation.

Question 12 (4 pts)

You find an unknown organism while out on a hike. It appears to have tiny roots, and when you look at it under the microscope, the cells have nuclei. Experiments show that the organism lives on the bark of trees and cannot photosynthesize. Which of the following are true (select all that apply):

The organism is eukaryotic
The organism is an alga
The organism is prokaryotic
The organism is a bacterium
The organism is a fungus
The organism is autotrophic
The organism is heterotrophic
The organism is a plant

Question 13 (3 pts)

Consider the statement: “Because the desert got hotter, lizards evolved thicker scales to avoid dehydration.” Which revision best reflects Darwinian reasoning?

Lizards needed thicker scales, so they developed them.
The desert selected for lizards that could learn to stay in the shade.
Lizards with thicker scales survived and reproduced more often in hot conditions.
Heat caused mutations for thicker scales.

Question 14 (3 pts)

A clade is:

A group of related organisms that only includes extant species
A phylogenetic tree based on geographic location rather than evolutionary history
A group of related species and their common ancestor
An unrelated group of organisms grouped together based on ecological or morphological similarity

Question 15 (3 pts)

What are the most abundant living organisms on the planet?

Fungi
Eukaryotes
Humans
Algae
Bacteria

Question 16 (3 pts)

Infectious diseases are caused by . . .

pathogens that spread from one organism to another.
genetic disorders.
All of these are true.
environmental pollutants.

Question 17 (10 pts)

Identify the genotypes of the numbered individuals in this pedigree chart showing an autosomal recessive condition.

1 Aa AA or Aa aa AA

2 Aa AA or Aa aa AA

3 Aa AA or Aa aa AA

4 Aa AA or Aa aa AA

5 Aa AA or Aa aa AA

Fill-in-the-Blank

Question 18 (2 pts)

The base pairing rules for DNA state that adenine pairs with thymine and cytosine pairs with [guanine].

Question 19 (2 pts)

The process by which ribosomes read an mRNA sequence to build a protein is known as [translation].

Question 20 (3 pts)

The process when cells organize themselves into an embryo with multiple germ layers during gestation is called [gastrulation].

Question 21 (2 pts)

A trait influenced by multiple genes and showing continuous variation is called a [polygenic] trait.

Question 22 (2 pts)

A mutation introduces new [alleles] into a population, providing raw material for evolution.

Question 23 (2 pts)

The measure of how successful an organism is at reproducing is known as [fitness] .

Short Essay Questions

Please note: I have included a variety of student responses, and indicated how many points each response was worth.

Question 24 (6 pts)

In the context of what we learned in lecture about the adaptive immune system, explain why Trypanosoma is such a successful pathogen.

“Trypanosoma is such a successful pathogen because it can evade the host’s adaptive immune system through antigenic variation. It frequently changes the antigens on its surface. These antibodies and the memory cells produced by the immune system can no longer recognize it. Changes its surface glycoproteins (VSGs) regularly.This avoids immune detection and prevents the immune system from ‘remembering’ the pathogen Leads to recurrent waves of infection”(Full-credit answer)

“Trypanosoma is so successful because it will enter the body, and then it will resemble something our immune system perceives as non-threatening and neutral, and disguises itself. And then trypanosoma will attack the body out of nowhere and infect our immune system with the deadly pathogen if untreated.” (2-point answer, too general, with no pathogen-specific details included).

Question 25 (6 pts)

Explain why vaccines can provide immunity from infectious disease, and the process by which this happens.

“Vaccines can provide immunity because they expose the body to a safe form of a pathogen, which trains the immune system to recognize it. This causes the body to make antibodies to fight the pathogen, and memory cells to remember it, so if the real pathogen appears later, the immune system is able to act quickly and prevent the body from illness.” (Full-credit response)

“Vaccines protect our bodies by giving us a suit of armor at the microscopic level. We need new flu shots every year because the viruses also are adapting to our vaccines. Vaccines give us antibodies that are our little soldiers that fight disease” (2 point answer; not enough specific details; no description of antibodies, memory cells, or the process by which a vaccine is delivered).

Question 26 (10 pts)

A population of asexually reproducing fish suddenly experiences rapid climate change. After several generations, most die off. A related sexually reproducing species survives. Using what you know about meiosis and genetic diversity, explain why the sexual population was more resilient. Be sure to connect genetic mechanism to ecological outcome.

“Organisms that reproduce sexually have more genetic diversity due to the crossing over of chromosomes during prophase 1 of meiosis. One of the sexually reproducing organisms may have inherited a trait that made them more fit to withstand rapid climate change. This genetic diversity allowed them to survive and reproduce. Organisms that reproduce asexually all have the same genes so one factor could wipe the whole population out.” (9-point response; would have liked mention of independent assortment and random fertilization)

“The sexually reproducing species provides more genetic diversity because it uses the process of meiosis for cell division. During meiosis, the egg and sperm provide one set of chromosomes each, creating a new and unique cell and therefore, genetically diverse offspring.” (4 point answer; no mention of the asexual species limitations, and not enough details about what generates diversity in meiosis).

Question 27 (6 pts)

Explain how the domestic dog came to have such a wide variety of breeds/forms.

“The significant variation in dog breeds has essentially resulted from artificial selection, which is human made rather than natural. It began when puppies of wolves came to humans for food and humans started to take care of them. For thousands of years, humans have bred dogs for certain traits, including size, coat color, temperament, or behavior, to improve certain traits which humans preferred or needed for a task, such as hunting, herding, guarding, or companionship. By choosing which dogs could reproduce for generations, humans made certain traits more of a constant within the dogs that resulted in the creation of hundreds of different breeds that share a common ancestor, yet contain a lot of diversity in appearance and purpose.” (Full-credit response)

“The domestic dog came to have a large variety of breeds and forms because of species diversity, evolution, and genetic factors.” (Response with no specific details; worth 0 points)

Question 28 (extra credit)

For 5 points of extra credit: Explain the difference between Darwin’s concept of natural selection and Lamarck’s idea of evolution.

“Darwin and Lamarck had similar evolution theories, but they did differ slightly in terms of how traits were gained and passed on. Darwin believed in natural selection and inheritance of genes of organisms that survived and thrived in their environment. Meanwhile, Lamarck believed that traits gained during an offsprings life would be passed on, for example, his theory suggests that giraffes stretched their necks to reach the leaves, and as they got longer, that was passed onto their offspring.” (Full-credit response)

“While both scientists believed that life evolves, Jean Baptiste Lamarck suggested that frequently used traits were passed down to offspring. For example, a giraffe evolved to have a long neck because it was constantly stretching its neck to reach high tree folliage. Charles Darwin agreed that life evolves, but that organisms with certain inherited traits are more likely to survive and reproduce than individuals of other traits. Individuals with the most favorable variations survive and reproduce. While those with unfavorable traits, are eliminated via natural selection.” (Full-credit response)

“The diffenrce is that Darwins belived that some genetic traits were passed on because it helps the survival and production rate while Lamarack belived that characteristic were inherited and the traits developed during a lifetime where passed on to offspring.” (2-point response; partially correct, but not clearly stated, and many typos)