1. Humans and pufferfish diverged from a common ancestor about 450 MYA, and
these two genomes have—
a. very few of the same genes in common
b. all the same genes
c. share about 75% of the genes in the genomes
d. no nucleotide divergence
The correct answer is c—
A. Answer a is incorrect. Although these species diverged long ago, many of the
genes found in one genome have counterparts in the other.
The correct answer is c—
B. Answer b is incorrect. They do not have exactly the same genes; some new genes
are unique to both species.
The correct answer is c—share about 75% of the genes in the genomes
C. Answer c is correct. Based on sequence analysis and genome mapping, this is the
correct answer.
The correct answer is c—
D. Answer d is incorrect. Even though many of the genes in one species have
counterparts in the other species, there is substantial divergence at the level of the
DNA.
2. Genome comparisons have suggested that mouse DNA has mutated about twice
as fast as human DNA. What is a possible explanation for this discrepancy?
a. Mice are much smaller than humans.
b. Mice live in much less sanitary conditions than humans and are therefore
exposed to a wider range of mutation-causing substances.
c. Mice have a smaller genome size.
d. Mice have a much shorter generation time.
The correct answer is d—
A. Answer a is incorrect. Although it is true that mice are much smaller than humans,
this difference does not account for the higher mutational rate in mice.
The correct answer is d—
B. Answer b is incorrect. Although living conditions of humans and mice can
influence mutations and natural selection, the difference in living conditions
between the two does not account for the higher mutational rate in mice.
The correct answer is d—
C. Answer c is incorrect. Although mice do have a smaller genome size, this fact
does not appear to affect the mutational rate in mice.
The correct answer is d—Mice have a much shorter generation time.
D. Answer d is correct. Mutations can occur at any given point in the lifespan of an
organism, but only those that accumulate in germ cells will be passed on to future
generations. Since mice have a much faster generation time than humans, it is
thought that this difference may allow mutations to accumulate more quickly in
mice than in humans.
3. Polyploidy in plants—
a. has only arisen once and therefore is very rare
b. only occurs naturally when there is a hybridization event between two species
c. is common, but never occurs in animals
d. is common, and does occur in some animals
The correct answer is d—
A. Answer a is incorrect. Polyploidy in plants has evolved independently many
times, and is ubiquitous.
The correct answer is d—
B. Answer b is incorrect. Polyploidy can also occur by autopolyploidy (within a
single species).
The correct answer is d—
C. Answer c is incorrect. Although polyploidy in plants is common, it also does
occur in animals. It is just not as widespread.
The correct answer is d—is common, and does occur in some animals
D. Answer d is correct. Not only is polyploidy in plants fairly common, it also occurs
in animals.
4. Homologous genes in distantly related organisms can often be easily located on
chromosomes due to—
a. horizontal gene transfer
b. conservation of synteny
c. gene inactivation
d. pseudogenes
The correct answer is b—
A. Answer a is incorrect. Horizontal gene transfer (HCT) is the process of gene
transfer between species. Such transfer can actually make it harder to find
homologous genes when the transfers are extensive.
The correct answer is b—conservation of synteny
B. Answer b is correct. Synteny refers to the relative order and placement of genes
within a species genome, and when that is conserved, maps from one species may
be useful in helping to locate sites in other species.
The correct answer is b—
C. Answer c is incorrect. Gene inactivation should have little effect on the position of
genes across different species, therefore, it would not be expected to contribute to
finding homologous genes across species
The correct answer is b—
D. Answer d is incorrect. Pseudogenes are intact genes that nevertheless do not
produce active products. As such they usually do not play a role in location of
homologous genes across different taxa.
5. All of the following are believed to contribute to genomic diversity among
various species, except—
a. gene duplication
b. gene transcription
c. lateral gene transfer
d. chromosomal rearrangements
The correct answer is b—
A. Answer a is incorrect. Gene duplication can, in fact, contribute to genomic
changes and diversity.
The correct answer is b—gene transcription
B. Answer b is correct. Gene transcription is the process by which genomic
information is converted to mRNA, which is then translated to form a protein, the
final gene product. Thus, gene transcription does not, in itself, contribute to
genomic diversity.
The correct answer is b—
C. Answer c is incorrect. Lateral gene transfer refers to the process by which DNA
from one species is transferred to another. This process does lead to increased
genomic diversity.
The correct answer is b—
D. Answer d is incorrect. Chromosomal rearrangements can contribute to increased
genomic diversity.
6. What is the fate of most duplicated genes?
a. Gene inactivation
b. Gain of a novel function through subsequent mutation
c. They are transferred to a new organism using lateral gene transfer
d. They become orthologues
The correct answer is a—Gene inactivation
A. Answer a is correct. When a gene duplicates it will most often eventually lose its
function through subsequent mutations. However, it can also gain a novel
function through subsequent mutation.
The correct answer is a—
B. Answer b is incorrect. Although a duplicated gene can gain a novel function
through subsequent mutation, more often it will eventually be inactivated through
subsequent mutation.
The correct answer is a—
C. Answer c is incorrect. Although lateral gene transfer can occur between
organisms, it is not the usual fate of most duplicated genes.
The correct answer is a—
D. Answer d is incorrect. Orthologues are related genes in different organisms.
However, duplicated genes can result in paralogues, which are closely related
genes within the same organism.
7. Chimp and human DNA is close to 99 % similar; morphological differences—
a. must be due largely to gene expression
b. must be due exclusively to environmental differences
c. cannot be explained with current genetic theory
d. are caused by random effects during development
The correct answer is a—must be due largely to gene expression
A. Answer a is correct. Gene expression is a major source of variation in phenotypes
and it is under regulatory control. Furthermore, recent exploration using
microarrays demonstrates that expression patterns vary substantially between
chimp and human DNA even though most of the genes are the same.
The correct answer is a—
B. Answer b is incorrect. Although environmental differences have a major influence
on phenotypes that emerge from gene expression during development, it is not as
though you could take a fertilized chimp egg, expose it to a different environment,
and have a human result.
The correct answer is a—
C. Answer c is incorrect. A full explanation of how the differences between human
and chimp DNA result in the observed morphological differences is not limited by
genetic theory; it is limited by lack of a complete description of the specific
mechanics of gene expression and development.
The correct answer is a—
D. Answer d is incorrect. Random effects would lead to chaos during development.
The genetic and gene expression differences responsible for observed
morphological variation between chimp and human is the result of highly
regulated processes.
8. The noncoding repetitive DNA common in both plants and animals—
a. is probably “junk” DNA
b. directly produces protein through mechanisms other than transcription
c. is still usually translated
d. may often produce RNA transcripts that themselves have regulatory function
The correct answer is d—
A. Answer a is incorrect. This was widely believed prior to new technologies,
allowing DNA sequencing and quantification of gene expression.
The correct answer is d—
B. Answer b is incorrect. Until a DNA transcript is produced, there is nothing to
specify an amino acid sequence to produce a protein.
The correct answer is d—
C. Answer c is incorrect. Although much of the repetitive DNA is transcribed, it is
unlikely to be translated into proteins.
The correct answer is d—may often produce RNA transcripts that themselves have
regulatory function
D. Answer d is correct. There is increasing evidence that many RNA transcripts are
produced that are never translated, and several studies have shown that many of
these RNA transcripts have regulatory functions.
9. In general, as genome size increases there is—
a. a proportional increase in number of genes
b. a proportional decrease in number of genes
c. an increase in the amount of DNA
d. a decrease in the amount of DNA
The correct answer is c—
A. Answer a is incorrect. Organisms with large genomes do not necessarily have a
large number of genes. In fact, there is often no correlation between genome size
and number of genes.
The correct answer is c—
B. Answer b is incorrect. Organisms with large genomes do not necessarily have a
small number of genes. In fact, there is often no correlation between genome size
and number of genes.
The correct answer is c—an increase in the amount of DNA
C. Answer c is correct. Since the genome is constructed of DNA, as its size
increases, so does the amount of DNA it contains.
The correct answer is c—
D. Answer d is incorrect. A larger genome requires more DNA, even if that DNA is
not in the form of functional genes.
10. An herbicide that targets the chloroplast might be effective against malaria
because—
a. Plasmodium needs a functional apicoplast
b. the main vector for malaria is a plant
c. mosquitoes require plant leaves for food
d. none of the above
The correct answer is a—Plasmodium needs a functional apicoplast
A. Answer a is correct. The apicoplast is derived from the chloroplast deep in the
phylogenetic history of Plasmodium. Targeting the chloroplast could lead to
development of an effective drug against Plasmodium that is also harmless to
human hosts.
The correct answer is a—
B. Answer b is incorrect. Plants are not part of the life cycle for Plasmodium.
The correct answer is a—
C. Answer c is incorrect. Adult mosquitoes do not eat plant leaves, they feed on
blood (females) and nectar.
The correct answer is a—
D. Answer d is incorrect. One of the answers is correct, answer a.
Challenge Questions
1. The loss of functional olfactory receptor (OR) genes in humans compared with
chimps may be explained by reliance on other sensory systems and therefore
weakened selection against individuals that lose olfactory capabilities. Could
natural selection work in any other way to decrease the number of human OR
genes without selecting for OR function in chimps?
Answer—Besides weakened selection for OR genes in humans, there might also be
selection for loss in humans if there is a nontrivial cost to maintaining the cellular
machinery associated with olfactory capability. For example, if receptor proteins are
expensive and turnover is high, there may be a significant energy cost to the olfactory
capabilities, such that animals that reduce that cost (because they can rely on other
sensory modalities) could use that energy in other ways to enhance their fitness.
2. One of the common misconceptions about sequencing projects (especially the
high-profile Human Genome Project) is that creating a complete roadmap of the
DNA will lead directly to cures for genetically based diseases. Given the
percentage similarity in DNA between humans and chimps, is this simplistic view
justified? Explain.
Answer—Human and chimp DNA is close to 99% similar, yet our phenotypes are
conspicuously different in many ways. This suggests that a catalogue of genes is just the
first step to identifying the mechanisms underlying genetically influenced diseases like
cancer or cystic fibrosis. Clearly, gene expression, which might involve the actions of
multiple noncoding segments of the DNA and other potentially complex regulatory
mechanics, are important sources of how phenotypes are formed, and it is likely that
many genetically determined diseases result from such complex underlying mechanisms,
making the gene identification of genomics just the first step; a necessary but not nearly
sufficient strategy. What complete genomes do offer is a starting point to correlating
sequence differences among humans with genetic disease, as well as the opportunity to
examine how multiple genes and regulatory sequences interact to cause disease.
3. How does horizontal gene transfer (HGT) complicate phylogenetic analysis?
Answer—Phylogenetic analysis usually assumes that most genetic and phenotypic
variation arises from descent with modification (vertical inheritance). If genetic and
phenotypic characteristics can be passed horizontally (that is, not vertically through
genetic lineages) then using patterns of shared character variation to infer genealogical
relationships will be subject to potentially significant error. We might expect that
organisms with higher rates of HGT will have phylogenetic hypotheses that are less
reliable or at least are not resolved as a neatly branching tree.
4. What are the consequences of polyploidy for rates of speciation? Explain.
Answer—In general, polyploidy, whether auto- or allopolyploidy, will increase the rate
of speciation if all else is equal. The reason is that the offspring produced are incapable
of back-crossing with the parental species due to inability of even pairing of
chromosomes among germ cells. Therefore, with the original doubling of chromosomes,
a lineage of reproductively isolated individuals is formed, leading to a new genetic
lineage that is capable of evolutionary divergence from parental stock.