What is the connection between genetic regulatory cascades and the observation that differentiation is a step-by-step process?

Which of the following provides the strongest evidence for the conservation of tool-kit genes?

a. Bicoid moved from one fly embryo into the posterior of another fly embryo causes the formation of two head regions.

b. Mutation of an unrelated gene in another species of fly has a similar effect to mutation of bicoid in Drosophila.

c. A mouse Hox gene can be used to take over the function of a mutated Drosophila Hox gene.

d. Sheep can be cloned by fusing a differentiated adult cell with an enucleated egg.

Some stickleback fish develop protective spines, and other stickleback fish are spineless. Spine development is controlled by the expression of a gene known as Pitx1. The spineless phenotype is due to a mutation in Pitx1 that results in no expression of Pitx1 during development in regions where spines would otherwise form. When scientists compared the Pitx1 coding sequence in spined and spineless fish, they found this sequence was the same in both types of fish. Propose plausible hypotheses for the location of this mutation and for how it alters spine development.

A tool-kit gene is               .

The following predictions ask you to consider how genetic regulatory cascades provide positional information. Select True or False for each statement.

T/F Mutation of a gene at one level of a regulatory cascade will affect the expression of genes at all levels of the cascade.

T/F Mutation of a gene that is expressed later in a regulatory cascade will affect a smaller region of the body than mutation of gene that is expressed early in the cascade.

T/F In the regulatory cascade used by Drosophila, a gene at one level of the cascade will be controlled only by genes at the level immediately above it.

T/F Genes that control the largest regions of the Drosophila embryo are not transcribed in the embryo.

A friend is interested in isolating genes that are expressed solely in liver cells but only has access to skin cells. She asks you for advice on whether to start her studies. What will you say?

Imagine a situation in which a morphogen has its source at the posterior end of a Drosophila embryo. Every 100 µm from the posterior pole, the morphogen concentration decreases by half. If a cell required 1/16th the amount of morphogen found at the posterior pole to form part of a leg, how far from the posterior pole would the leg form?

a. 100μm

b. 160μm

c. 400μm

d. 1600 μm