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Segmentation genes Like all insects, the fruit fly has a segmented body plan. When the basic dorsal-ventral and anterior-posterior axes of the fruit-fly embryo have been established, segmentation genes control the differentiation of the embryo into individual segments. These genes affect the number and organization of the segments, and mutations in them usually disrupt whole sets of segments. The approximately 25 segmentation genes in Drosophila are transcribed after fertilization; so they don't exhibit a genetic maternal effect, and their expression is regulated by the Bicoid and Nanos protein gradients.

The segmentation genes fall into three groups as shown in Table 21.4 and I Figure 21.8. Gap genes define large sections of the embryo; mutations in these genes eliminate whole groups of adjacent segments. Mutations in the Krüppel gene, for example, cause the absence of several adjacent segments. Pair-rule genes define regional sections of the embryo and affect alternate segments. Mutations in the even-skipped gene cause the deletion of even-numbered segments, whereas mutations in the fushi tarazu gene cause the absence of odd-numbered segments. Segment-polarity genes affect the organization of segments. Mutations in

Normal larva

Normal larva

Head Thoracic segments

Abdominal segments

Head Thoracic segments

Abdominal segments

Mutant larva llll

Krüppel a

Mutation of Krüppel causes the elimination of anterior segments.

(b) Pair-rule genes

Deleted segments

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