Nüsslein-Volhard's and Wieschaus' Discoveries

Most of the embryonic development is controlled by

zygotically active genes

(e.g. the egg's own genes). These genes act as follows: The gap-genes are busy at the start, specifying a rough body plan along the head-to-tail axis. The pair rule-genes govern formation of every second body segment. The segment polarity-genes refine the head-to-tail polarity of each individual segment, meaning that the head-end and the tail-end of a segment look different.
Loss of gap gene results in a reduced number of segments as shown in the embryo to the right. Loss of a pair rule-gene, e.g. even-skipped, allows only odd-numbered segments to develop. Loss of a segment polarity-gene leads to segments with similar head and tail ends.

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