Question 1

Use of double mutants has been invaluable in studying developmental pathways. In C. elegans, sex is determined by the ratio of sets of autosomes to X chromosomes. A diploid male has one X, and diploid hermaphrodites have two X's. The loss- of- function gene her- 1 produces XO worms that are hermaphrodites, but XX worms are unaffected. Mutation of a second gene, tra, causes the XX to develop as male. Given this information, what would be the phenotype of a tra, her- 1 double mutant?

Accepted Answer

The tra mutation causes XX individuals to develop as males, and the her-1 mutation causes XO individuals to develop as hermaphrodites. However, in a double mutant, the tra mutation would override the her-1 effect in XX individuals, causing them to develop as males. Since her-1 primarily affects XO individuals (making them hermaphrodites instead of males), and tra mutation causes XX individuals to develop as males, the double mutant would result in both XX and XO individuals developing as males.

Question 2

In Drosophila, a developmental mutation called ssa (spineless aristapedia) results in flies with mini leg parts on the antennae. What kind of gene does this mutation affect?

Accepted Answer

The ssa (spineless aristapedia) mutation in Drosophila affects a homeotic gene, which is responsible for determining the identity of body parts. In this case, the mutation causes the transformation of antennae into leg-like structures, indicating a disruption in the normal function of a gene that specifies the identity of body segments.

Question 3

The abdominal segments of Drosophila have no appendages. Loss of function of bithorax genes results in appendages forming in these abdominal segments. Therefore,_________ .

Accepted Answer

Bithorax genes normally suppress appendage formation in the abdomen by activating a gene that negatively regulates appendages.

Question 4

Suppose that a homeotic gene from Drosophila is introduced into an Arabidopsis embryo that has a pre- existing mutation in one of its homeotic flower genes. What effect would you expect and why?

Accepted Answer

EHomeotic genes from Drosophila (which are Hox genes) and those in Arabidopsis (which are primarily MADS box genes) are fundamentally different in sequence and function, explaining why there would be no effect based on the difference in gene types (C). However, despite these differences, both types of genes play roles in specifying the identity of body parts. Therefore, introducing a Drosophila homeotic gene into an Arabidopsis embryo could potentially rescue the phenotype of the mutant by supplying a protein that can perform a similar function, even if not by the exact same mechanism (E). This is because both types of genes are involved in determining the identity of structures in the organism, albeit through different molecular pathways.

Question 5

Polycomb group (PcG) gene mutants cause ectopic expression of Antennapedia (Antp) and other homeotic genes. The PcG protein encoded by the PgG gene has a domain that binds to methylated histone H3. This finding supports which of the following hypotheses?

Accepted Answer

PcG proteins repress transcription by modifying chromatin structure, as indicated by their ability to bind to methylated histone H3, which is a marker for transcriptionally inactive chromatin.

Question 6

There is much controversy in the news-both scientific and popular-about the use of embryonic stem cells for research. These cells are grown as cell lines in culture, created by taking the inner cell mass from 5- day- old embryos. What is the center of the controversy?

Accepted Answer

The controversy centers on the ethical and moral implications of using human embryos for research, with differing opinions on whether these embryos should be considered as persons.

Question 7

Since mice embryos with loss- of- function Hox mutations result in homeotic alterations, then introduction of such a mutation might cause ___________.

Accepted Answer

Hox genes are crucial for determining the body plan and the identity of body parts during embryonic development. Loss-of-function mutations in these genes can lead to homeotic transformations, where one body part is transformed into another. For example, causing ribs to grow on normally rib-less vertebrae is a classic example of a homeotic transformation, which is directly related to the role of Hox genes in specifying the identity of different segments of the body.

Question 8

What is the name of the Drosophila multinucleated cell formed early in development?

Accepted Answer

The multinucleated cell formed early in Drosophila development is called a syncytium. This occurs when nuclei divide without the cytoplasm dividing, leading to a single cell with multiple nuclei.

Question 9

Mutations of regulatory elements of developmental genes may have either dominant or recessive effects. If a mutation deletes a silencer upstream of a HOX gene, which do you expect?

Accepted Answer

Deleting a silencer upstream of a HOX gene removes a regulatory element that normally suppresses gene expression. Therefore, the activity of the HOX gene located on the same chromosome (in cis) would be up-regulated. This effect would be dominant because the increased expression from one allele would be sufficient to alter the phenotype, regardless of the state of the other allele.

Question 10

A pair- rule gene in Drosophila called runt controls nervous system formation. In the mouse, runt controls blood cell formation and genital structures. In humans, the analog of runt, when mutated, causes children to lack collar bones, and the opening at the top of the skull fails to close. What kind of experiment could tell you whether these genes are conserved in function?

Accepted Answer

An in vivo experiment that replaces the coding sequence of one species' gene with that of another can directly test if the genes are functionally conserved across species by observing if the replaced gene can fulfill the same biological role.

Question 11

Many mutations in the bithorax gene are known, such as postbithorax, contrabithorax and bithoriaxoid. Each mutation has a different effect on development of the reduced wing- like T3 appendage, the haltere. None of these mutations occurs in the coding sequence of bithorax, instead, where are these mutations and how do they effect development?

Accepted Answer

These mutations affect enhancers near the bithorax gene, leading to misexpression of bithorax and resulting in patterning defects in the haltere. Enhancers are regulatory elements that control the spatial and temporal expression of genes, and mutations in these regions can disrupt normal development by altering gene expression patterns.

Question 12

Drosophila mutations such as bcd, if homozygous in the mother, produce offspring with lethal defects, no matter what the genotype of the offspring. For some other genes, the mutant phenotype can be "rescued" by introducing a wild- type allele into the embryo. What is the reason for these different outcomes?

Accepted Answer

The different outcomes are due to the nature of the mutations. bcd (bicoid) is a maternal effect mutation, meaning the phenotype of the offspring is determined by the genotype of the mother, not the genotype of the offspring itself. This is why introducing a wild-type allele into the embryo can't rescue the phenotype for maternal effect mutations like bcd, but can for other types of mutations where the offspring's genotype directly influences the phenotype.

Question 13

As a Drosophila larva molts and becomes a pupa, most of the larval tissues die and adult structures are formed from cells in clusters known as imaginal discs that have remained unchanged throughout the larval stages. Therefore, the disc cells are_________ .

Accepted Answer

The imaginal discs are embryonal cells because they are derived from the embryo and have remained unchanged throughout the larval stages.

Question 14

Human Hox genes are in four clusters: HOXA, HOXB, HOXC, and HOXD, for a total of 39 genes. Besides conserving the clustering of genes, evolution has also conserved________ .

Accepted Answer

The order of the genes within the clusters is conserved, reflecting their expression pattern along the anterior-posterior axis during development.

Question 15

In early vertebrate embryonic development, a number of pathways, such as Wnt, Hedgehog, and Notch, regulate differentiation and organ formation. They operate by___________ .

Accepted Answer

These pathways (Wnt, Hedgehog, Notch) are known for their roles in cell signaling, which is mediated by the binding of signaling molecules to their respective receptors on cells, leading to a cascade of events that regulate differentiation and organ formation.

Question 16

In an organism such as a sea urchin, administration of actinomycin D prevents all mRNA synthesis. However, early development up to gastrula formation can take place in the presence of actinomycin D but gastrulation cannot. This implies that _________.

Accepted Answer

Maternal effect genes provide the mRNA and proteins necessary for early development, including up to gastrulation, even when mRNA synthesis is inhibited. This is why development can proceed to a certain stage without new mRNA synthesis.

Question 17

An Antennapedia (Antp) mutant in Drosophila has legs in place of antennae. This is caused by______ .

Accepted Answer

The Antennapedia (Antp) mutant phenotype, where legs grow in place of antennae in Drosophila, is caused by the misexpression of the Antp gene in the head region, where antennae normally develop, instead of its usual expression domain in the thorax.

Question 18

Segmentation genes in Drosophila act in which of the following orders?

Accepted Answer

The correct order of action for segmentation genes in Drosophila is: gap genes, pair-rule genes, and then segment polarity genes. Gap genes define broad areas, pair-rule genes refine these areas into segments, and segment polarity genes further define the anterior-posterior axis within each segment.

Question 19

Aniridia is a human condition in which the eye has no iris. The protein encoded by the gene responsible for aniridia is very nearly identical to the protein product of the fly Eyeless protein. What experiment could provide evidence that the two genes are functionally equivalent?

Accepted Answer

Introducing the mouse aniridia gene into a fly model where the eyeless gene is mutated and observing rescue (normal eye development) would directly test the functional equivalence of the two genes. If the mouse gene can compensate for the loss of the fly gene, it suggests they have similar functions.

Question 20

Which of the following is not a mechanism of differentiation?

Accepted Answer

Apoptosis is a process of programmed cell death, not a mechanism of differentiation. Differentiation involves changes in a cell's size, shape, polarity, metabolic activity, and responsiveness to signals, which are driven by mechanisms such as induction, nuclear migration, inhibition, and morphogen secretion.

Quiz 18: Organelle Inheritance and the Evolution of Organelle Genomes