#homeotic genes

Drosophila -ll

Homeotic genes located on chromosome:

Homeotic selector genes clustered in HOM-C complex on right arm chromosome 3 covering Antennapedia complex at 84A-84B and Bithorax at 89E, separated by ten megabases but linked via shared Polycomb response elements. This location contrasts with segmentation genes on X and second chromosomes. Clustering permits colinear expression where 3' genes express anteriorly and 5' posteriorly, maintained by chromatin domains and insulators like Fab-7 and Mcp. Disruption via translocations misregulates Hox expression causing homeotic transformations confirming importance of chromosomal organization for pr

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 20: Homeotic genes located on chromosome 3R - ANT-C and BX-C cluster.

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Drosophila -ll

Antennapedia mutation forms:

Antennapedia is archetypal homeotic mutant discovered by Balkaschina. Dominant chromosome inversion places Antennapedia gene under eye-antennal enhancers causing ectopic expression in antennal disc. Antennapedia protein promotes thoracic leg programs including expression of homothorax and represses antennal determinants like Distal-less. Resultant antenna-to-leg transformation yields mesothoracic legs complete with claws emerging from head socket where antennae normally reside. Phenotype demonstrates selector gene instructive sufficiency: ectopic expression alone redirects imaginal disc fate f

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 20: Antennapedia mutation antenna-to-leg transformation - ectopic Hox expression.

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Drosophila -ll

Ubx mutation transforms:

Ultrabithorax protein, part of Bithorax complex, normally represses wing-promoting genes and promotes haltere developmental program in third thoracic segment. Haltere discs require Ubx to suppress wing margin and activate haltere-specific genes. Loss-of-function mutation derepresses wingless, vestigial, and other wing genes in T3 imaginal discs, converting halters into second pair of wings creating four-winged flies observed by Bateson. Transformation does not affect legs or antennae, indicating segment-specific target repertoire. Ubx illustrates Hox principle where relatively small regulatory

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 20: Ubx mutation transforms halteres into wings - Bithorax complex function.

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ABC MODEL+PYQS

If all five homeotic genes are mutated, flower develops into:

Leaf-like structures (C) is correct here. This is core ABC MODEL: once you know the definition or pathway step, Leaf-like structures is the clear fit. The wrong ones are A) Extra petals; B) Stamens only; D) Inflorescence. If the topic is about gradients or potentials, water/solutes move from higher to lower of the relevant quantity.