#gene

Limb Development -ll

Which gene is critical for forelimb specification in limb development?

Forelimb identity is encoded by TBX5 restricted to rostral lateral plate mesoderm downstream of Hoxb4 Hoxb5 and retinoic acid signaling. TBX5 activates a forelimb-specific enhancer of FGF10 and Wnt2b, triggering epithelial-mesenchymal signaling cascade leading to bud outgrowth and AER induction via Wnt3a. Loss of Tbx5 abolishes forelimb formation entirely, while ectopic Tbx5 expression in hindlimb field can induce forelimb-like features including wing-type morphology. TBX4 performs analogous hindlimb function, Pitx1 reinforces hindlimb character, Islet1 contributes early to both fields. Thus TBX5 is master forelimb specifier.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 20: TBX5 as critical forelimb specification gene.

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Dictyostelium

Gene controlling aggregate size:

Aggregate size control utilizes secreted counting factor complex comprising proteins countin, cf45, cf50, cf60, and regulatory small proteins SmlA and SslA. SmlA encodes small hydrophobic protein that negatively regulates counting factor secretion. As aggregate gathers more cells, counting factor concentration rises informing cells about group size through inhibition of adhesion and motility. Loss of counting factor produces excessively large aggregates, while mutations in small regulators like smlA alter complex abundance. Adenylyl cyclase genes govern cAMP signaling rather than quorum sensing. Therefore smlA participates in community size sensing mechanism analogous to bacterial quorum regulation but mediated by secreted proteins.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 22: Counting factor and smlA regulation of aggregate size.

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plant Development -ll

GNOM gene encodes:

Arabidopsis GNOM, also known as EMB30 or VAN7, encodes brefeldin A-sensitive ARF-GEF, guanine nucleotide exchange factor for ADP-ribosylation factor small GTPases. It localizes to recycling endosomes and trans-Golgi network regulating vesicle budding required for polar cargo delivery. Principal cargo includes PIN1 and related PIN auxin efflux carriers whose polar plasma membrane localization depends on GNOM-mediated endosomal recycling. Disruption impairs directional auxin transport causing embryonic and post-embryonic defects resembling auxin transport inhibitor treatments, revealing link between membrane trafficking machinery and hormonal patterning controlling embryogenesis and lateral root development.

Ref: Geldner et al., Cell 2003; NCBI Bookshelf, Plant ARF-GEF: GNOM encodes BFA-sensitive ARF-GEF regulating PIN recycling.

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Basics of Inheritance

A gene is defined as

A gene is defined as a functional segment of genetic material, DNA in most organisms or RNA in some viruses, carrying information for a product or regulatory role influencing phenotype. It encompasses protein-coding open reading frames, RNAs like tRNA and rRNA and essential cis regulatory elements controlling transcription. Modern cistron concept views gene as unit of function, mutation and complementation. Isolated protein, whole chromosome or trait alone are incomplete descriptors, functional segment captures hereditary, biochemical and evolutionary significance across linkage and molecular biology. This principle is routinely tested in NEET, CBSE 11-12, CUET, CSIR-NET, GATE

Ref: Hartl & Ruvolo, Genetics, 9th ed., Chapter 10: Gene Structure and Function

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Basics of Inheritance

The term gene was coined by

The term gene was introduced by Danish botanist Wilhelm Johannsen in 1909 to describe the Mendelian unit of heredity. Mendel referred to factors, but Johannsen clarified genotype as inherited constitution and phenotype as observable expression. He emphasised that genes are discrete entities at loci, with allelic variants influencing characters, segregating intact across generations. This terminology replaced vague concepts like gemmules and pangenes, providing precise vocabulary for linkage, mutation, complementation and molecular definitions still central to modern genetics and medical literature. Conceptual clarity supports solving numerical problems involving segregation ratios, recombination frequencies and probability calculations in crosses.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 1: History of Genetics, Johannsen

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