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

(a) Polytene chromosomes versus normal chromosomes

Polytene chromosomes are giant interphase chromosomes found in the salivary glands of dipteran larvae (Chironomus, Drosophila), first seen by Balbiani (1881).

• Origin: polytene chromosomes arise by repeated DNA replication without cell division (endoreduplication or polyteny); normal chromosomes have a single chromatid or two sister chromatids formed by one replication per cycle.
• Structure: polytene chromosomes are made of hundreds to thousands of chromatids lying side by side; a normal chromosome has one DNA molecule per chromatid.
• Size: polytene chromosomes are up to 100 times longer and thicker, visible under a light microscope in interphase; normal chromosomes are visible only during division.
• Pairing: homologues are closely synapsed (somatic pairing), so the haploid number is seen; in normal chromosomes homologues are not paired in somatic cells.
• Banding: polytene chromosomes show dark bands (chromomeres, condensed DNA) and light interbands, forming a constant pattern used for gene mapping; normal chromosomes lack such banding unless specially stained.
• Puffs: at sites of gene activity, bands loosen into puffs and Balbiani rings; normal chromosomes show no such puffing.
• Centromeres: the chromocentre is formed by fusion of centromeric regions in polytene chromosomes, which do not divide.
• Uses: polytene chromosomes are used to study gene expression and for mapping deletions and inversions.

(b) Polygenic inheritance

Polygenic (quantitative) inheritance is the control of a character by two or more non-allelic genes with small, similar, additive effects, so the phenotype varies continuously and is influenced by environment. The F₂ distribution is bell-shaped, unlike Mendelian discrete classes. The number of F₂ phenotypic classes is 2n + 1 for n gene pairs.

Example, grain colour in wheat (Nilsson-Ehle, 1909): two gene pairs, R₁ and R₂, each dominant allele adding red pigment. Dark-red (R₁R₁R₂R₂) × white (r₁r₁r₂r₂) gives an intermediate F₁ (R₁r₁R₂r₂). The F₂ shows 1 dark red : 4 moderately dark : 6 medium : 4 light : 1 white, a 1:4:6:4:1 ratio, depending on the number of dominant alleles. In some crosses, three gene pairs give a 1:6:15:20:15:6:1 distribution.

Other examples: corolla length in Nicotiana longiflora (East), human skin colour, height and kernel size in maize, and yield traits in crops.

(c)(i) Problems of gene transfer in plants

Problems:

• Gene delivery: Agrobacterium transforms mainly dicots, and monocots and woody species are harder; biolistics gives multiple, fragmented copies.
• Position effect and gene silencing: random insertion, multiple copies, and methylation or RNA interference reduce expression.
• Regeneration: poor regeneration from transformed cells of many genotypes.
• Marker genes: antibiotic or herbicide markers raise biosafety worries.
• Biosafety: gene flow to wild relatives, allergenicity and toxicity of products, effects on non-target organisms, and resistance in pests.
• Regulation and public acceptance.

(c)(ii) Status of transgenic research in India

Transgenic research is regulated under the Environment Protection Act, 1986 through the Review Committee on Genetic Manipulation (DBT) and the Genetic Engineering Appraisal Committee (GEAC, under the MoEFCC). Bt cotton (2002) remains the only commercialised GM crop. Bt brinjal was put under moratorium in 2010. GEAC cleared GM mustard (DMH-11) for environmental release in 2022, but the Supreme Court in 2024 gave a split verdict and asked the Union government to frame a national GM policy. Research on rice, chickpea, pigeonpea and others continues, and genome-edited (SDN-1/2) crops are exempted from the stricter GM rules since 2022.