DRAW IT Each blue circle in the figure below represents a cell in a cell lineage. Draw two modified versions of the cell lineage so that each version produces three cells. Use apoptosis in one of the versions, marking any dead cells with an X.

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

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

What is 'selected' during natural selection? Explain your answer.

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.

PROCESS OF SCIENCE Explain why researchers formulate a null hypothesis in addition to a hypothesis when designing an experimental study.

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. 100μmb. 160μmc. 400μmd. 1600 μm

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.

Some humans have genes that make them resistant to infection by HIV. Would human populations likely evolve differently in areas of the world where HIV infection rates are high? Explain your logic.