The gene whose mutation leads to embryos with two telsons in Drosophila:
Bicoid defines anterior identity; loss inverts embryo polarity. Homozygous bicoid mutant mothers produce embryos lacking anterior and thoracic structures because anterior gap genes hunchback and orthodenticle fail to activate and caudal translation remains derepressed throughout. Consequently both ends adopt posterior program driven by unopposed Nanos and Caudal, producing duplicated telson structures at anterior and posterior, known as bicaudal phenotype. Posterior markers forked, tailless appear at both poles. This mirror duplication proves Bicoid necessary to suppress posterior fate anteriorly. Therefore mutation leading to two telsons identifies bicoid gene function.
Ref: NCBI Bookshelf, Developmental Biology: Bicoid mutants producing bicaudal two telson phenotype.