CHAPTER 8
Experimental Questions
E1.
With regard to the analysis of chromosome structure, explain the experimental
advantage that polytene chromosomes offer. Discuss why changes in chromosome
structure are more easily detected in polytene chromosomes compared to ordinary
(nonpolytene) chromosomes.
Answer: Polytene chromosomes can be viewed in greater detail under the microscope
because they are much larger. This makes it much easier to detect very small changes in
chromosome structure. Polytene chromosomes are produced from the sequential
replication and alignment of chromosomes. As an example, suppose a toothpick has fine
lines written on it. You would probably have trouble seeing the lines. However, if you
took 1,000 toothpicks with the same lines and stacked them up in a parallel manner, the
lines would be much easier to see. Similarly, small changes in chromosome structure are
hard to see in a single chromosome but much easier to detect in a polytene chromosome.
E2.
Describe how colchicine can be used to alter chromosome number.
Answer: Colchicine interferes with the spindle apparatus and thereby causes
nondisjunction. At high concentrations, it can cause complete nondisjunction and produce
polyploid cells.
E3
Female fruit flies homozygous for the X-linked white-eye allele are crossed to
males with red eyes. On very rare occasions, an offspring is a male with red eyes.
Assuming that these rare offspring are not due to a new mutation in one of the mother’s
X chromosomes that converted the white-eye allele into a red-eye allele, explain how this
red-eyed male arose.
Answer: The male offspring is the result of nondisjunction during oogenesis. The female
produced an egg without any sex chromosomes. The male parent transmitted a single X
chromosome carrying the red allele. This produces an X0 male offspring
E4.
A cytogeneticist has collected tissue samples from members of the same butterfly
species. Some of the butterflies were located in Canada, while others were found in
Mexico. Upon karyotyping, the cytogeneticist discovered that chromosome 5 of the
Canadian butterflies had a large inversion compared to the Mexican butterflies. The
Canadian butterflies were inversion homozygotes, whereas the Mexican butterflies had
two normal copies of chromosome 5.
A.
Explain whether a mating between the Canadian and Mexican butterflies
would produce phenotypically normal offspring.
B.
Explain whether the offspring of a cross between Canadian and Mexican
butterflies would be fertile.
Answer:
A.
The F1 offspring would probably be phenotypically normal, because they would
carry the correct number of genes.
B.
The F1 offspring would have lowered fertility because they are inversion
heterozygotes. Because this is a large inversion, crossing over is fairly likely in the
inverted region. When this occurs, it will produce deletions and duplications that will
probably be lethal in the resulting F2 offspring.
E5.
While conducting field studies on a chain of islands, you decide to karyotype two
phenotypically identical groups of turtles, which are found on different islands. The
turtles on one island have 24 chromosomes, while the turtles on another island have 48
chromosomes. How would you explain this observation? How do you think the turtles
with 48 chromosomes came into being? If you mated the two types of turtles together,
would you expect their offspring to be phenotypically normal? Would you expect them to
be fertile? Explain.
Answer: The turtles are two distinct species that appear phenotypically identical. The
turtles with 48 chromosomes are polyploid relatives (i.e., tetraploids) of the species with
24 chromosomes. In animals, it is somewhat hard to imagine how this could occur
because animals cannot self-fertilize, so there had to be two animals (i.e., one male and
one female) that became tetraploids. It is easy to imagine how one animal could become a
tetraploid; complete nondisjunction could occur during the first cell division of a
fertilized egg, thereby creating a tetraploid cell that continued to develop into a tetraploid
animal. This would have to happen independently (i.e., in two individuals of opposite
sex) to create a tetraploid species. If you mated a tetraploid turtle with a diploid turtle, the
offspring would be triploid and probably phenotypically normal. However, the triploid
offspring would be sterile because they would make highly aneuploid gametes.
E6.
It is an exciting time to be a plant breeder because so many options are available
for the development of new types of agriculturally useful plants. Let’s suppose you wish
to develop a seedless tomato that could grow in a very hot climate and is resistant to a
viral pathogen that commonly infects tomato plants. At your disposal, you have a seedbearing tomato strain that is heat resistant and produces great-tasting tomatoes. You also
have a wild strain of tomato plants (which have lousy-tasting tomatoes) that is resistant to
the viral pathogen. Suggest a series of steps that you might follow to produce a greattasting, seedless tomato that is resistant to heat and the viral pathogen.
Answer: First, you would cross the two strains together. It is difficult to predict the
phenotype of the offspring. Nevertheless, you would keep crossing offspring to each
other and backcrossing them to the parental strains until you obtained a great-tasting
tomato strain that was resistant to heat and the viral pathogen. You could then make this
strain tetraploid by treatment with colchicine. If you crossed the tetraploid strain with
your great-tasting diploid strain that was resistant to heat and the viral pathogen, you may
get a triploid that had these characteristics. This triploid would probably be seedless.
E7.
What is a G band? Discuss how G bands are useful in the analysis of chromosome
structure.
Answer: A G band is a dark band on a chromosome that has been stained with Giemsa.
The pattern of G bands on chromosomes can be used to distinguish chromosomes from
each other even if their sizes and centromere locations are very similar. Banding patterns
are also used to detect changes in chromosome structure.
E8.
A female fruit fly contains one normal X chromosome and one X chromosome
with a deficiency. The deficiency is in the middle of the X chromosome and is about 10%
of the entire length of the X chromosome. If you stained and observed the chromosomes
of this female fly in salivary gland cells, draw what the polytene arm of the X
chromosome would look like. Explain your drawing.
Answer: A polytene chromosome is formed when a chromosome replicates many times,
and the chromatids lie side by side, as shown in Figure 8.18. The homologous
chromosomes also lie side by side. Therefore, if there is a deletion, there will be a loop.
The loop is the segment that is not deleted from one of the two homologs.
E9.
Describe two different experimental strategies to create an allotetraploid from two
different diploid species of plants.
Answer: You could cross the two species together to first create an allodiploid. The
allodiploid could be treated with colchicine to make a segment of the plant an
allotetraploid. As described in Figure 8.27, a cutting of the allotetraploid segment of the
plant could be rooted to create a new allotetraploid plant. Alternatively, one could first
use colchicine to make two different tetraploids (autopolyploidy), which would make
diploid gametes. These two autopolyploids could be crossed to each other to make an
allotetraploid.