(a) Differentiate between polytene chromosomes and normal chromosomes. 15 (b) Describe polygenic inheritance by giving suitable…

Differences Between Polytene and Normal Chromosomes

Normal chromosomes consist of single DNA duplexes per chromatid that undergo regular cycles of replication followed by mitotic or meiotic segregation, maintaining standard diploid or haploid ploidy. In contrast, polytene chromosomes are giant, multistranded structures formed through endoreduplication (endomitosis), in which repeated cycles of DNA replication occur without intervening nuclear or cell division. While normal mitotic chromosomes are transiently condensed, dynamic bodies that require artificial chemical staining (such as G-banding) to reveal structural patterns, polytene chromosomes remain permanently in an enlarged interphase state.

Polytene chromosomes exhibit characteristic, highly visible alternating transverse bands (chromomeres with high DNA density) and interbands (low DNA density). Furthermore, active gene transcription in polytene chromosomes manifests as localized decondensations termed chromosomal puffs or Balbiani rings, which do not form in normal condensed mitotic chromosomes. First observed by E.G. Balbiani in Chironomus and widely characterized in the salivary glands of Drosophila melanogaster, polytene chromosomes provide exceptional resolution for physical gene mapping and structural rearrangement studies, contrasting with the lower cytological resolution of standard metaphase chromosomes.

Polygenic Inheritance and Examples

Polygenic inheritance refers to the transmission of a quantitative trait controlled by two or more independent, non-allelic genes (polygenes), where each dominant allele exerts a cumulative, additive effect on the phenotype. Unlike monogenic Mendelian inheritance, which produces discrete, discontinuous phenotypic classes governed by complete dominance, polygenic traits produce continuous phenotypic variation that forms a bell-shaped Gaussian distribution curve and is susceptible to environmental modifications.

A definitive example is kernel colour in wheat (Triticum aestivum), demonstrated by H. Nilsson-Ehle. When dark-red kernel wheat (R₁ R₁ R₂ R₂) is crossed with white kernel wheat (r₁ r₁ r₂ r₂), the F₁ generation produces intermediate medium-red seeds (R₁ r₁ R₂ r₂). In the F₂ generation, additive allele segregation yields a continuous phenotypic distribution of 1 dark-red : 4 medium-dark red : 6 medium red : 4 light red : 1 white (1:4:6:4:1 ratio), dictated by the number of contributing dominant alleles. Other classic manifestations include human skin pigmentation (Davenport model) and corolla length in Nicotiana longiflora (East).

Problems in Plant Gene Transfer and Transgenic Research in India

Gene transfer in plants encounters several technical and biosafety bottlenecks: Position effects, where random illegitimate transgene insertion into heterochromatic or transcriptionally inactive chromosomal regions results in variable, silenced, or unstable transgene expression; Gene silencing, occurring via transcriptional gene silencing through promoter hypermethylation or post-transcriptional gene silencing via RNA interference triggered by multi-copy or inverted transgene arrays; Pleiotropic disruptions, wherein insertion disrupts endogenous open reading frames, leading to altered plant metabolism, somaclonal variation, or yield penalties; Biosafety and ecological hazards, including the persistence of selectable antibiotic/herbicide marker genes, potential allergenicity of expressed proteins, and horizontal pollen-mediated gene flow to sexually compatible wild relatives, creating herbicide-resistant weeds.

In India, transgenic crop research and deployment are governed under the Environment (Protection) Act, 1986 (Rules, 1989) through a regulatory framework comprising the Review Committee on Genetic Manipulation (RCGM) under the Department of Biotechnology (DBT) and the Genetic Engineering Appraisal Committee (GEAC) under the Ministry of Environment, Forest and Climate Change (MoEFCC). Transgenic Bt cotton (Cry1Ac and Cry2Ab), approved in 2002, remains India's sole commercialized genetically modified crop. A moratorium was placed on Bt brinjal in 2010 over ecological and food-safety concerns. More recently, the GEAC recommended the environmental release of transgenic hybrid mustard DMH-11 (utilizing the barnase/barstar system developed by the University of Delhi), though its commercial adoption remains subject to legal review. Ongoing public sector research at ICAR institutes focuses on stress resilience and biofortification in rice, chickpea, and pigeonpea, alongside an increasing shift toward SDN-1 and SDN-2 genome-editing approaches that bypass conventional transgenesis regulations.

Integrating cytogenetic insights derived from chromosome mapping with modern site-specific gene-editing techniques enables precise targeting into genomic "safe harbour" sites. This approach circumvents position effects and transgene silencing, offering a viable path forward for deploying safe and climate-resilient transgenic crops in Indian agriculture.