(a) Explain the following: (10×2=20 marks) (i) Haemophilia (ii) Red-green colour blindness (b) Describe various stages of…

X-Linked Recessive Genetic Disorders

Haemophilia is an X-linked recessive bleeding disorder caused by mutations disrupting the intrinsic blood coagulation cascade, preventing effective fibrin clot formation. Haemophilia A arises from a deficiency of clotting Factor VIII (FVIII), whereas Haemophilia B (Christmas disease) is caused by a deficiency of Factor IX (FIX). Because males possess a single X chromosome (hemizygous, X^hY), a single defective allele manifests the disease, while heterozygous females (X^H X^h) remain largely asymptomatic carriers. The classic inheritance pattern is historically exemplified by the European Royal pedigree descending from Queen Victoria, passing the mutant allele through carrier daughters to male descendants across generations.

Red-green colour blindness is an X-linked condition characterized by an inability to discriminate red and green hues. The photopigment genes encoding long-wavelength-sensitive (L-opsin/red) and medium-wavelength-sensitive (M-opsin/green) cone opsins are clustered on chromosome locus Xq28, having evolved via tandem gene duplication. Unequal homologous recombination between these tandem arrays causes gene deletions or chimeric gene fusion. Complete loss of function leads to dichromacy, classified as protanopia (red-blindness) or deuteranopia (green-blindness), whereas spectral shifts in mutant opsins cause anomalous trichromacy, namely protanomaly or deuteranomaly. Clinical diagnosis relies on pseudoisochromatic Ishihara plates.

Stages of Prophase-I in Meiosis-I

Prophase-I is the extended reductional phase of meiosis wherein homologous chromosomes pair and undergo genetic exchange.

Leptotene: Chromatin begins progressive condensation into distinct, thread-like chromosomes. Telomeres orient and attach to the inner nuclear membrane adjacent to the centrosome, forming the polarized "bouquet stage."

Zygotene: Homologous chromosomes recognize each other and initiate intimate lengthwise pairing (synapsis). This association is stabilized by the assembly of a tripartite protein scaffold, the synaptonemal complex (SC), forming bivalents.

Pachytene: Bivalents fully condense to reveal four chromatids (tetrads). Recombination nodules assemble along the SC, facilitating enzyme-mediated crossing over between non-sister chromatids via double-strand breaks and strand exchange, generating novel allelic combinations.

Diplotene: The synaptonemal complex dissolves, allowing homologous chromosomes to desynapse and pull apart, remaining physically held together only at the sites of crossing over, visible as X-shaped chiasmata. In many oocytes, this stage enters prolonged arrest (dictyotene).

Diakinesis: Chromosomes reach maximum condensation. Chiasmata undergo terminalization, shifting progressively toward the chromosome ends. The nucleolus and nuclear envelope disassemble, and spindle microtubules attach to kinetochores to orient bivalents for metaphase-I.

Natural Selection: Mechanistic Basis and Examples

Natural selection is the differential survival and reproduction of genotypes mediated by environmental selective pressures, driving directional or balancing shifts in gene frequencies.

**Industrial Melanism in Peppered Moth (Biston betularia):** Documented by H.B.D. Kettlewell, pre-industrial England possessed mostly pale, speckled typica morphs, which were camouflaged against light, lichen-covered tree trunks against avian predators. Industrial soot deposition killed lichens and blackened tree bark, exerting directional selection that favored the rare, melanic carbonaria morph carrying a dominant transposon-derived mutation. The frequency of carbonaria rose dramatically in industrial areas due to cryptic advantage and subsequently declined following clean air legislation, proving rapid evolutionary adaptation to changing selective regimes.

Sickle Cell Anaemia and Balancing Selection: Caused by a point mutation (GAG → GTG; glutamic acid replaced by valine at codon 6) in the β-globin gene (HBB), the mutant allele (HbS) causes deoxygenated hemoglobin to polymerize, deforming erythrocytes into rigid sickled shapes. While homozygous individuals (HbSS) suffer lethal sickle cell anaemia, heterozygous individuals (HbAS) exhibit mild sickling that restricts the intra-erythrocytic reproduction of Plasmodium falciparum. This heterozygote advantage (overdominance) confers resistance to severe malaria, maintaining the deleterious HbS allele at high frequencies across malaria-endemic regions of sub-Saharan Africa and tribal belts of central and southern India (e.g., Nilgiris, Bastar).

Meiotic recombination in prophase-I produces novel phenotypic variation, providing the raw substrate upon which natural selection acts to drive adaptation or maintain balanced polymorphisms, while sex-linked traits persist within populations through the reservoir of heterozygous female carriers.