Lewis' model

Question 1

When did Lewis’ model occur?

Answer 1

1970

Question 2

What did he do?

Answer 2

Undertook detailed genetic analysis of the BX-C by studying dominant and recessive mutations. Before PS were defined so still observed results in terms of segments initially.

Question 3

What did dominant mutations in BX-C cause?

Answer 3

Caused segment to change to that of the next more posterior.

Question 4

What did recessive mutations cause?

Answer 4

Segment identity to change to that of the next more anterior.

Question 5

Give examples of recessive mutations.

Answer 5

• Mutation of bithoraxoid (bxd) caused first abdominal segment to take on partial characteristics of third thoraic segment.
• Mutation of bithorax convert anterior half of T3 to anterior half of T2. In terms of PS this is PS5 –> PS4

Question 6

What happened when the whole of BX-C was removed?

Answer 6

Everything became PS4 suggesting PS4 is the ground state when no genes are active, and the fates more posterior require activation of one or more BX-C genes.

Question 7

So what did Lewis’ propose?

Answer 7

Each additional BX-C gene activated in each more posterior segment.

None in PS4, Bx in PS5, bx + pxd in PS6, followed by iab2-8

Draw diagram.

Also saw co-linearity. The earlier the gene on the chromosome, the earlier it’s expressed.

Question 8

How did he propose this stepwise activation was achieved?

Answer 8

By a gradient of repressor molecule highest in PS4, and each gene having a different affinity for the repressor.