Homeoboxes and Development

Question 1

What does “homeosis” mean?

Who coined this term?

Answer 1

Coined by William Bateson, 1894

• The transformation of one segment into the likeness of another

Question 2

Drosophila is obviously segmented.

Which segment makes the legs?

Answer 2

Thoracic

Question 3

Which segment in Drosophila shows homeosis?

Answer 3

The abdominal segments; they are serially identical

Question 4

The thoracic segment of Drosophila is divided 3 times. What is the purpose of each of these segments?

Answer 4

• The first thoracic segment (T1) makes the legs
• The second thoracic segment (T2) makes a wing
• The third thoracic segment (T3) makes a haltere (a balance organ)

T2 & T3 show HETEROSIS

Question 5

Who is Thomas Hunt Morgan? What did he discover?

Answer 5

Thomas Hunt Morgan identified heritable homeosis defects.

- In 1923 he discovered the loci responsible lay on the fly’s 3rd chromosome

Question 6

What is the term used for a mutant fly with two sets of wings?

Answer 6

Ultrabithorax; a homeotic mutant

Question 7

Which parasegments is the gene Ubx expressed in?

Answer 7

Parasegments 5-12

Question 8

Abd-A is expressed in which parasegments?

Answer 8

Parasegments 7-13

Question 9

Which gene is expressed in parasegments 10-14

Answer 9

Abd-B

Question 10

If there is an absence of Ubx, Abd-A & Abd-B, what does this mean for the abdominal segment of a fly?

Answer 10

• All but the most anterior and posterior ends adopt the T2 segment identity

Question 11

If Ubx is the only gene expressed, what does this mean for the abdominal segment of a fly?

What conclusions can we draw from this?

Answer 11

• T3 and A1 segment identity is restored

Conclusion:
- Ubx is needed for normal T3 and A1 segment identity

Question 12

If Ubx and Abd-A are expressed, what does this mean for the abdominal segment of a fly?

What conclusions can we draw from this?

Answer 12

• Only segments A5-A8 are not properly specified

Conclusion:
- Abd-A needed for segments A2 - A4

Question 13

If Abd-A and Abd-B are expressed, what does this mean for the abdominal segment of a fly?

What conclusions can we draw from this?

Answer 13

• Segments T3 and A1 are missing

Conclusion:
- Ubx is needed for normal T3 and A1 segment identity

Question 14

Describe “Hox genes”?

Answer 14

Each animal has many homeobox genes, but those that cluster together along the chromosome are special and appear to control head to tail development in all animals.

The members of these clusters are called Hox Genes.

Question 15

How many “Hox clusters” do mammals have?

Answer 15

4

Question 16

What is a “Genetic Redundancy”?

Answer 16

Four copies of the “same” gene

Question 17

What are the two advantages of genetic redundancy?

Answer 17

1. Protection from errors

2. Development of innovations

Question 18

Vertebrates show evidence of segmentation and heterosis. Name, in order, the different segments in vertebrates

Answer 18

Cervical
Thoracic
Lumbar
Sacral
Caudal

Question 19

In mice, deletion of which Hox gene results in a transformation of the 1st lumbar vertebra into the 13th thoracic vertebra

Answer 19

Hoxc8

Question 20

Hox gene expression boundaries correlate with what?

Answer 20

Morphological boundaries