(a) Polytene chromosome (10 marks) (b) Difference between male heterogamety and female heterogamety with examples (10 marks) (c)…

(a) Polytene Chromosome

Polytene chromosomes are giant, multistranded chromosomes formed by repeated cycles of endoreduplication (DNA replication without cell division or separation of sister chromatids). First described by E. G. Balbiani (1881) in Chironomus, they are characteristically observed in the larval salivary glands, gut, and Malpighian tubules of Dipteran insects such as Drosophila melanogaster. They exhibit a distinct pattern of transverse alternating dark bands (dense, transcriptionally inactive chromatin) and light interbands (less dense DNA). Homologues remain somatically paired, and the arms radiate from a chromocentre formed by fused centromeric heterochromatin; in Drosophila about ten rounds of replication place roughly 1,000 chromatids side by side.

During specific developmental stages, certain bands decondense and loosen into localized swellings called chromosomal puffs, which serve as sites of active gene transcription and RNA synthesis. Prominent, exceptionally large puffs are termed Balbiani rings, and the sequence of early and late puffs induced by the moulting hormone ecdysone (Ashburner) showed directly how hormones switch genes on. Because their reproducible banding patterns allow precise physical identification of loci, polytene chromosomes have historically provided an indispensable cytogenetic tool for gene mapping, analyzing structural rearrangements like inversions and deletions, and visualizing gene expression in situ.

(b) Male Heterogamety versus Female Heterogamety

Heterogamety refers to the condition where individuals of a particular sex produce two cytogenetically distinct types of gametes with respect to sex chromosomes, thereby determining the sex of the offspring at fertilization.

Male heterogamety occurs when the male produces two distinct types of gametes, while the female is homogametic and produces a single type:

• XX-XY System: Females possess two identical X chromosomes (XX) and produce only X-bearing eggs, whereas males possess one X and one Y chromosome (XY) and produce 50% X-bearing and 50% Y-bearing sperms (e.g., humans, mammals, and Drosophila melanogaster).
• XX-XO System: Males possess a single X chromosome (XO) and produce gametes with or without an X chromosome (e.g., grasshoppers).

Female heterogamety occurs when the female produces two distinct types of gametes, while the male is homogametic:

• ZZ-ZW System: Males are homogametic (ZZ) producing identical Z-bearing sperm, whereas females are heterogametic (ZW) producing Z-bearing and W-bearing ova. This mechanism is standard in birds (Gallus gallus), reptiles, and lepidopteran insects such as the mulberry silkworm (Bombyx mori).
• ZZ-ZO System: Females possess a single Z chromosome (ZO) and males possess two (ZZ) (e.g., certain primitive moths and caddisflies). The mechanism differs even within a system: in Drosophila, sex depends on the X:autosome ratio and the Y is needed only for male fertility, whereas in mammals the SRY gene on the Y chromosome makes the embryo male.

(c) Types of Chromosomal Aberrations

Chromosomal aberrations represent modifications in standard chromosome structure or number, classified into structural and numerical aberrations.

Structural Aberrations involve changes in gene arrangement or content:

• Deletion (Deficiency): Loss of a chromosome segment. In humans, terminal deletion of the short arm of chromosome 5 (5p-) causes Cri-du-chat syndrome.
• Duplication: Presence of an extra chromosomal segment, increasing gene dosage (e.g., the Bar eye duplication in Drosophila).
• Inversion: A 180-degree rotation of an internal segment. It is termed paracentric if it excludes the centromere and pericentric if the centromere is included.
• Translocation: Exchange of segments between non-homologous chromosomes, such as reciprocal translocations or centric fusions (Robertsonian translocations).

Numerical Aberrations involve variations in chromosome count:

• Aneuploidy: Gain or loss of one or more individual chromosomes due to non-disjunction during meiosis. Examples include monosomy (2n−1, as in Turner syndrome, 45,X) and trisomy (2n+1, as in Down syndrome, trisomy 21, and Klinefelter syndrome, 47,XXY).
• Euploidy (Polyploidy): Variation involving complete haploid chromosome sets (e.g., 3n triploidy, 4n tetraploidy), categorized as autopolyploidy or allopolyploidy.

(d) Geographic versus Reproductive Isolation

Isolation mechanisms halt gene flow between populations, serving as the prerequisite for divergent evolution and speciation.

Geographic Isolation represents an extrinsic, spatial separation of populations by physical environmental barriers such as mountain ranges, rivers, deserts, or marine expanses. By preventing panmixia, it allows separated populations to diverge genetically through differential natural selection and genetic drift, driving allopatric speciation. An Indian example is the sholakili (shortwing) birds of the Western Ghats sky islands: populations separated by the Palghat Gap have diverged into two species, Sholicola major to the north and S. albiventris to the south.

Reproductive Isolation comprises intrinsic biological properties of organisms that prevent interbreeding between co-occurring populations:

• Prezygotic barriers prevent mating or fertilization (ecological, temporal, ethological/behavioral, mechanical, and gametic isolation).
• Postzygotic barriers reduce the viability or fertility of hybrid offspring (hybrid inviability, hybrid sterility as in the sterile mule produced from horse and donkey, and hybrid breakdown).

Geographic isolation typically acts as the initial physical precursor that allows reproductive isolation to evolve secondarily. The two differ in nature: geographic isolation is extrinsic and reversible (if the barrier disappears, gene flow resumes unless reproductive isolation has evolved), whereas reproductive isolation is an intrinsic property of the organisms and can arise even without geographic separation, as in sympatric speciation by polyploidy or by sexual selection in African cichlids.

(e) Main Types of Molecular Mutations and Their Effect on Phenotype

Molecular (point) mutations are alterations in the specific nucleotide sequence of DNA, classified by their structural nature and their functional consequences on the encoded polypeptide.

Base Substitutions:

• Silent (Synonymous) Mutations: A base change that, owing to the degeneracy of the genetic code, alters a codon to another coding for the same amino acid. These generally exert no phenotypic effect.
• Missense Mutations: A nucleotide replacement that alters a codon to encode a different amino acid. The phenotypic consequence ranges from neutral to severe; for example, a single transversion in the β-globin gene (GAG to GTG) substitutes glutamic acid with valine at position 6, altering protein conformation and causing Sickle Cell Anemia.
• Nonsense Mutations: A substitution that converts an amino acid codon into a premature translation termination codon (UAA, UAG, or UGA), resulting in truncated, non-functional polypeptides.

Frameshift Mutations:

• Insertions and Deletions (Indels): The addition or loss of nucleotide numbers not divisible by three shifts the triplet reading frame downstream of the mutation site. This radically alters the entire subsequent amino acid sequence and frequently generates premature stop codons, typically yielding a completely non-functional or degraded protein. Indels in multiples of three keep the frame (for example the ΔF508 deletion in CFTR causing cystic fibrosis), and trinucleotide-repeat expansions such as CAG in Huntington's disease are dynamic mutations. By effect, mutations may be loss-of-function (usually recessive), gain-of-function or dominant-negative.