Chapter 13

12.  The primary pair-rule gene eve (even-skipped) would be expressed in seven stripes along the A–P axis of the late blastoderm.

15.  If you diagram these results, you will see that the deletion of a gene that functions posteriorly allows the next most anterior segments to extend in a posterior direction. Deletion of an anterior gene does not allow extension of the next most posterior segment in an anterior direction. The gap genes activate Ubx in both thoracic and abdominal segments, whereas the abd-A and Abd-B genes are activated only in the middle and posterior abdominal segments. The functioning of the abd-A and Abd-B genes in those segments somehow prevents Ubx expression. However, if the abd-A and Abd-B genes are deleted, Ubx can be expressed in these regions.

18. 

  1. A pair-rule gene.

  2. Look for expression of the mRNA from the candidate gene in a repeating pattern of seven stripes along the A–P axis of the developing embryo.

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  3. No. An embryo mutant for the gap gene Krüppel would be missing many anterior segments. This effect would be epistatic to the expression of a pair-rule gene.

21. 

  1. The homeodomain is a conserved protein domain containing 60 amino acids found in a significant number of transcription factors. Any protein that contains a functional homeodomain is almost certainly a sequence-specific DNA-binding transcription factor.

  2. The eyeless gene (named for its mutant phenotype) regulates eye development in Drosophila. You would expect that it is expressed only in those cells that will give rise to the eyes. To test this prediction, visualization of the location of eyeless mRNA expression by in situ hybridization and the Eyeless protein by immunological methods should be performed. Through genetic manipulation, the eyeless gene can be expressed in tissues in which it is not ordinarily expressed. For example, when eyeless is turned on in cells destined to form legs, eyes form on the legs.

  3. Transgenic experiments have shown that the mouse Small eye gene and the Drosophila eyeless gene are so similar that the mouse gene can substitute for eyeless when introduced into Drosophila. As in the answer to part b, when the mouse Small eye gene is expressed in Drosophila, even in cells destined to form legs, eyes form on the legs. (However, the “eyes” are not mouse eyes, because Small eye and eyeless act as master switches that turn on the entire cascade of genes needed to build the eye—in this case, the Drosophila set to build a Drosophila eye.)

25.  GLP-1 protein is localized to the two anterior cells of the fourcell C. elegans embryo by repression of its translation in the two posterior cells. The repression of GLP-1 translation requires the 3′ UTR spatial control region (SCR). Deletion of the SCR will allow glp-1 expression in both anterior and posterior cells. In both heterozygous and homozygous mutants, you would expect GLP-1 protein expression in all cells.