(a) What is chromosome mutation ? Describe various types of polyploidy with suitable examples. Add a note on phenotypic effects…

Chromosome mutation is a heritable change in chromosome number or structure, unlike gene mutation which alters a base pair within a gene. In polyploidy, whole sets of chromosomes are added. Autopolyploidy arises from duplication within one species: autotriploids (3x) and autotetraploids (4x). Potato (Solanum tuberosum) is a naturally occurring autotetraploid, and colchicine can experimentally induce tetraploidy in other plants. Allopolyploidy arises by hybridisation between related species followed by chromosome doubling, giving a new fertile species. Bread wheat (Triticum aestivum) is an allohexaploid with 2n = 6x = 42, genome AABBDD. Raphanobrassica (2n = 36) is an amphidiploid from Raphanus and Brassica. Modern commercial sugarcane, based on Saccharum officinarum, is a high polyploid, often octoploid with 2n = 80, and is best treated as a complex allopolyploid/aneuploid hybrid rather than a simple autotetraploid. Phenotypically, polyploids show gigantism, larger cells, thicker leaves, delayed flowering, and sometimes reduced fertility in odd ploids. Their evolutionary significance is high in plants, because reproductive isolation after chromosome doubling can produce instant speciation. Applied significance includes crop improvement in Indian wheat and cotton breeding, where polyploidy increases vigour, yield potential and stress tolerance.

Chromosome theory of linkage. Sutton and Boveri postulated that chromosomes are the physical carriers of genes, that homologues segregate during meiosis, and that linked genes remain together because they occupy the same chromosome. Complete linkage occurs when two loci are so close that no crossing over is detected between them; incomplete linkage occurs when crossing over produces recombinant gametes. The strength of linkage depends on distance along the chromosome. Methods of determination begin with a two-point test cross: a heterozygote for two markers is crossed to a homozygous recessive tester, and recombinants are counted. Recombination frequency equals recombinants divided by total progeny, multiplied by 100; one per cent recombination is one map unit or centimorgan. In maize, shrunken (sh) and waxy (wx) are close on the same chromosome and give a small map distance, about 2.8 map units. In Drosophila, singed (sn) and lumpy (lp) on chromosome IV can be scored similarly. Three-point test crosses use three markers to detect double crossovers, determine gene order, and construct a genetic map. If the order is A-B-C, the map distance is the sum of adjacent intervals corrected for double crossovers. This distinguishes linkage from independent assortment: linked genes travel together unless separated by crossing over, while unlinked genes on different chromosomes assort independently.

Mendel's dihybrid cross and independent assortment. Mendel crossed true-breeding yellow round (YYRR) with green wrinkled (yyrr) peas. The F1 was yellow round (YyRr). Selfing F1 gave the 9:3:3:1 phenotypic ratio: 9 yellow round, 3 yellow wrinkled, 3 green round, 1 green wrinkled. The parental phenotypes are yellow round and green wrinkled; the recombinant phenotypes are yellow wrinkled and green round. The ratio is derived from four equally probable gametes, YR, Yr, yR and yr, produced by independent segregation of the two gene pairs. Cytologically, independent assortment occurs at metaphase I of meiosis: each homologous pair aligns randomly at the equator, and at anaphase I homologues separate. For two pairs, the orientation of one pair is independent of the other, giving four gamete types in equal numbers. A simple annotated diagram would show two bivalent pairs, one vertical and one horizontal, with four possible orientations and resulting gametes. Statistical validation uses chi-square tests to compare observed and expected 9:3:3:1 counts. The mechanism explains why unlinked traits show Mendelian ratios, whereas linked traits deviate. Its applied significance is in breeding programmes, where independent assortment generates new combinations and contributes to hybrid vigour, while linkage mapping helps breeders select marker-trait associations.