610 Chapter 18 • The Genetic Basis of Development C HALLENGING P ROBLEMS 30. a. When you remove the anterior 20 percent of the cytoplasm of a newly formed Drosophila embryo, you can cause a bicaudal phenotype, in which there is a mirror-image duplication of the abdominal seg-ments. In other words, from the anterior tip of the embryo to the posterior tip, the order of segments is A8-A7-A6-A5-A4-A4-A5-A6-A7-A8. Explain this phenotype in regard to the action of the anterior and posterior determinants and how they affect gap-gene expression. b. Females homozygous for the maternally acting mutation nanos (nos) produce embryos in which the abdominal segments are absent and in which the head and thoracic segments are broad. In regard to the action of the anterior and posterior determinants and gap-gene action, explain how nos produces this mutant phenotype. In your answer, explain why there is a loss of segments rather than a mirror-image duplication of anterior segments. 31. The three homeodomain proteins ABD-B, ABD-A, and UBX are encoded by genes within the BX-C of Drosophila. In wild-type embryos, the Abd-B gene is expressed in the posterior abdominal segments, Abd-A in the middle abdominal segments, and
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