11.13.2 Pattern Formation in Drosophila, continued

Question 1

Pattern Formation in Drosophila, continued

Answer 1

• Egg-polarity genes (e.g. bicoid) are maternal-effect genes. The egg is given maternal mRNA in a gradient that produces protein and causes the anterior-posterior axis.
• Body-plan development in Drosophila is controlled by a cascade of genes each turning on the next step.
• After the egg-polarity genes act, the zygote begins to express its own genes and the segmentation genes determine the number and orientation of segments.
• The first set of segmentation genes are the gap genes. They affect broad areas of the embryo encompassing several adjacent segments. Mutants lose groups of segments in the embryo. The second set of segmentation genes are the pair-rule genes that affect sets of alternate segments. Mutants lose part or all of whole sets of alternating segments. The final set of
segmentation genes are the segment-polarity genes that determine the anterior-posterior orientation of each segment. Mutants have segments with two anterior ends (for example.

Question 2

note

Answer 2

• The bicoid gene is one example of a maternal-effect gene. In this mutant, the mother who is homozygous recessive (-/-) is not able to provision the egg with mRNA, which would normally produce a protein in the egg once it is fertilized. The mutation results in embryos with two posterior ends. The cause is a missing molecule, a morphogen that determines which end of the egg develops into the anterior part of the organism. The location of the morphogen is the anterior end of the egg.

Question 3

gap genes

Answer 3

• Gap genes set up the segmentation pattern of a larva.

Mutations in gap genes produce the loss of specific segments. Gap genes activate pair-rule genes.

Question 4

pair-rule genes

Answer 4

• responsible for whole sets of alternate segments or parts of those segments. Mutations in pair-rule genes cause the disappearance of whole sets of alternate
  segments or parts of those segments. The products of
  pair-rule genes turn on segment-polarity genes.

Question 5

Segment-polarity genes

Answer 5

• set up anterior/posterior polarity within each individual segment. Mutations in these genes cause the disappearance of the same half of every segment and its replacement by the mirror image of the remaining
  half.
• Segment polarity genes activate homeotic genes, which determine the developmental fate of groups of cells. When mutated, they cause one part of a body to develop as if it were another part of the body.

Question 6

Homeotic genes are activated by _______________ genes.

Answer 6

• segment-polarity

Question 7

Pair-rule genes activate _______________ genes.

Answer 7

• segment-polarity

Question 8

The bicoid gene, when mutated, causes a larva to have no anterior structures. It is an example of a

Answer 8

• maternal-effect gene

Question 9

Which type of gene sets up the initial segmentation pattern in fly larvae?

Answer 9

• gap gene

Question 10

Which of the following would be expected if a fly was homozygous for a pair-rule gene mutation?

Answer 10

• The resulting embryo would have only segments 2, 4, 6, and 8.

Question 11

The antennapedia gene is a _______________ gene.

Answer 11

• homeotic