#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 precise developmental timing and positional identity establishment during embryogenesis.

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 from antenna to leg, establishing principle of master regulatory genes and providing molecular basis for homeosis concept central to evolutionary developmental biology and appendage diversification mechanisms.

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 change rewires entire organ identity, maintaining segment count while altering appendage morphology and function essential for flight stability.

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.