Bithorax mutation

William Bateson first described in 1895. These mutations do not arrest development but transform one part of the body into another. Antennapedia mutations, for example, transform antennae into legs, which gives rise to an insect with two legs sprouting from its head, whereas bithorax mutations transform the third thorax into a second one, giving the insect an extra pair of wings. 

The e (postbithorax) mutation occupies the other end of the bithorax mutation series. With this mutation, the posterior metathorax is transformed into the posterior mesothorax (PMT PMS). Progeny with this mutation and the a mutation demonstrate a complete transformation of the metathorax into the mesothorax. There is a double mesothorax with four wings. 

FIGURE 28-39 Effects of mutations in homeotic genes in Drosophila, (a) Normal head, (b) Homeotic mutant (antennapedia) in which antennae are replaced by legs, (c) Normal body structure, (d) Homeotic mutant (bithorax) in which a segment has developed incorrectly to produce an extra set of wings. 

Abnormal phenotypes resulting from homeotic mutations. Antennapedia results in a pair of extra legs sprouting from the head in place of antennae. Bithorax results in an extra pair of wings appearing where halteres normally appear. 

Fig. 98. Genetic map, bithorax locus structure and phenotypes of D. melanogaster. Arrows indicate the transformation of loci as a result of mutations AMT, anterior metathorax PMT, posterior metathorax (gray) AMS, anterior mesothorax PMS, posterior mesotho-rax (white) ABi the first abdomenal segment (black). (After Swanson et al, 1969)...

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