#gene mutation

C-Elegans Development-l

Which phenotype results from a loss-of-function mutation in SKN-1?

SKN-1 specifies mesendodermal progenitors central to nematode gastrulation and gut formation. Maternal SKN-1 accumulates in EMS nucleus where it directly binds promoters of med-1,2 GATA factors, which activate end-1,3 and tbx-35 to segregate endoderm versus mesoderm lineages differentially. Null skn-1 alleles lack pharynx derived from MS, body wall muscle from MS, intestine derived from E, causing embryonic lethality with excess skin, neurons but no gut markers. Rescue shows autonomous requirement in EMS not P2. Thus loss mirrors mesendoderm deletion rather than vulval defects highlighting master regulator role.

Ref: Bowerman et al., Cell 1992: SKN-1 mutants lack mesoderm and endoderm derivatives from EMS lineage.

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Eye Lens Induction -l

What happens in Pax6 heterozygous mutants?

Pax6 exhibits haploinsufficiency. Heterozygous Small eye mice carry one null allele, reducing transcription factor dosage to 50 percent. Eye field still forms but retinal progenitor pool proliferation, lens induction and iris development are attenuated. Resulting eyes are significantly smaller, termed microphthalmia, with cataracts, corneal opacification and underdeveloped retina. Dosage threshold differs among targets; lens placode markers require higher Pax6 levels than optic vesicle evagination. Human PAX6 heterozygotes show aniridia, similar dosage effect. Complete absence requires homozygous null, whereas heterozygous produces small eye phenotype reflecting quantitative gene regulation.

Ref: Glaser et al. Science 1990; Gilbert Chapter 7: Pax6 dosage - heterozygous small eye formation.

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Limb Development -l

Which gene mutation is associated with polydactyly?

Polydactyly reflecting supernumerary digits often arises from dysregulation of SHH-Gli3 and distal Hox networks controlling digit periodicity. Mutations in HOXD13, particularly polyalanine tract expansions, cause synpolydactyly with fused extra digits due to altered transcriptional regulation of autopod patterning and ectopic activation of anterior Gli3 processing leading to broadened SHH targets. Tbx4 dictates hindlimb identity, Pax6 eye development, FGF10 limb initiation. HOXD13 normally limits digit number controlling interdigital proliferation and termination timing, thus mutation disrupts fine-tuning leading to polydactylous phenotype in mammals.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 19: HOXD13 mutations and synpolydactyly – digit number regulation.

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Limb Development -ll

Which gene mutation is associated with clubfoot phenotype?

Clubfoot talipes equinovarus involves abnormal hindlimb tendon, muscle and skeletal patterning linked to reduced Pitx1 pathway dosage. Human genome-wide association and mouse models show Pitx1 haploinsufficiency or cis-regulatory enhancer mutations produce shortened hindlimbs, abnormal calf muscle morphology, altered Achilles tendon insertion and foot malrotation resembling clubfoot deformity with dorsal flexion. TBX5 causes Holt-Oram forelimb defects, SHH affects anterior-posterior digit number, PAX6 governs eye development. PITX1 regulation of Tbx4 and hindlimb-specific effectors including Tbx2 makes it key gene associated with clubfoot and hindlimb malformations across species.

Ref: Alvarado et al., NEJM 2011, Gilbert Chapter 20: PITX1 mutations and clubfoot phenotype.

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Sex determination-l

XX individuals with mutated RSPO1 gene develop:

XX humans homozygous for loss-of-function RSPO1 mutations present with complete female-to-male sex reversal clinically. Without Rspo1 amplification of Wnt4/β-catenin cascade, ovarian program collapses and Sox9 becomes derepressed, causing supporting cells to transdifferentiate into Sertoli-like cells organizing testis tubules and synthesizing androgens. Phenotype includes testes or ovotestes, masculinized external genitalia, palmoplantar hyperkeratosis and predisposition to squamous cell carcinoma. Mouse knockout mirrors masculinized gonads with ectopic Sox9, male vasculature and steroidogenesis, proving indispensability of Rspo1 for ovarian maintenance and suppression of testis program.

Ref: Nature, RSPO1 mutations cause XX sex reversal; Gilbert Chapter 6: RSPO1 required for ovary.

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Drosophila -l

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.

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Drosophila -ll

Mutations in cactus gene lead to:

Cactus negatively regulates Dorsal nuclear import by sequestration. Loss of Cactus permits constitutive nuclear Dorsal throughout axis, expanding twist and snail dorsally while repressing decapentaplegic and zerknüllt. Cuticle shows loss of dorsal hairs and amnioserosa, expanded ventral denticle belts producing ventralized embryos that fail to gastrulate properly. Dorsalized embryos result from Toll or Spatzle loss. Posteriorized phenotype involves torso pathway. Hence cactus mutations epitomize inhibitor removal causing hyperventralization, confirming I-kappa B-like antagonistic role maintaining graded Dorsal distribution necessary for DV pattern refinement and mesoderm specification.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: cactus loss-of-function ventralized embryos - ubiquitous Dorsal nuclear entry.

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Dictyostelium

Mutation in ACA gene results in:

ACA establishes positive feedback loop of cAMP signaling that organizes collective movement. Mutants lacking functional ACA fail to synthesize pulses, therefore no extracellular cAMP wave propagates, cells do not polarize, stream, or form mounds, and developmental program arrests early. Intracellular cAMP remains low, preventing induction of early genes like discoidin and contact site A. Phenotype includes lack of aggregation territory, no slug formation despite starvation. Normal aggregation patterns require ACA-mediated relay; excessive aggregation would require increased cyclase activity, while normal culmination occurs only if aggregation succeeded, demonstrating ACA indispensability at initiation of multicellular development.

Ref: Science, ACA null phenotype - failure to aggregate, cAMP pulse rescue and developmental arrest.

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plant Development -ll

PLT mutants lack:

PLETHORA genes are master regulators specifying embryonic root fate downstream of auxin accumulation. Loss-of-function plt1 plt2 double and higher order plt mutants produce seedlings completely lacking embryonic root pole despite presence of auxin maximum and basal polarity. Quiescent center marker WOX5 never induced, columella stem cells differentiate prematurely, basal pole converts to hypocotyl-like tissue showing reduced PIN expression and ectopic shoot reporters. Since PLTs also promote PIN transcription, their absence destabilizes auxin transport further reinforcing rootless phenotype, demonstrating absolute requirement of auxin-induced PLT effectors.

Ref: Aida et al., Cell 2004; NCBI Plant Development: PLT1/PLT2 double mutants lack embryonic root and QC specification.

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plant Development -ll

SCR mutation results in:

In wild-type root meristem, cortex/endodermis initial stemming from stem cell niche undergoes formative asymmetric periclinal division producing distinct cortex and endodermis layers strictly controlled by SHR-SCR regulatory module. SCARECROW is essential to promote that division and to impose endodermal fate while repressing cortical program through downstream transcriptional activation. scr mutants fail to execute periclinal division, resulting single ground tissue layer surrounding stele. Molecular profiling reveals this solitary layer co-expresses cortical marker Co2 alongside endodermal markers CASP1 and ectopic lignin, indicating mixed identity and demonstrating necessity of SCR for both division and fate separation.

Ref: Di Laurenzio et al., Cell 1996; Taiz Physiol: SCR mutation yields single mixed cortex-endodermis layer in Arabidopsis root.

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plant Development -ll

FACKEL mutants exhibit:

FACKEL encodes sterol C-14 reductase integral to phytosterol and brassinosteroid biosynthetic pathway converting 4α-methylsterol intermediates like foliasterol into functional bulk sterols sitosterol, campesterol, and brassinolide. Sterols govern membrane fluidity, lipid raft organization, and polar targeting of PIN auxin carriers and cellulose synthases. fackel-J79 dwarf mutants resemble brassinosteroid-deficient mutants yet additionally exhibit severe embryonic patterning defects: enlarged, fused, or supernumerary cotyledons, ectopic shoot meristems, and stunted roots. This demonstrates sterol composition profoundly influences embryonic patterning and meristem programming beyond mere brassinosteroid hormone supply alone.

Ref: Jang et al., Genes Dev 2000; Schrick et al., Plant Cell 2004: FACKEL encodes sterol C-14 reductase, mutants show cotyledon defects.

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plant Development -ll

MONOPTEROS (MP) mutation results in:

MONOPTEROS encodes ARF5 auxin response factor activated when high auxin promotes TIR1/AFB-mediated ubiquitination and degradation of BODENLOS/IAA12 repressor in nucleus. Liberated MP directly induces transcription of PIN1, TMO5, ATHB8, and other vascular precursor genes in basal embryo and provascular strands. In monopteros null mutants hypophysis specification fails, basal lineage divides irregularly, seedlings germinate lacking embryonic root and hypocotyl while retaining cotyledons and functional shoot meristem. This reveals essential role of ARF-mediated transcriptional activation downstream of auxin perception for establishing lower embryonic tier and root stem cell niche.

Ref: Aida et al., Cell 2004; Taiz Plant Physiology, Chap 19: MONOPTEROS/ARF5 regulates hypophysis and root-hypocotyl formation via auxin.

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