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

(a) Chromosome mutation, types of polyploidy and their phenotypic effects

A chromosome mutation is a heritable change in the number or structure of chromosomes, as opposed to a gene mutation that alters bases within a gene. Structural changes are deletion, duplication, inversion and translocation; numerical changes are aneuploidy (loss or gain of single chromosomes) and euploidy (whole sets), which includes polyploidy.

Types of polyploidy:

• Autopolyploidy: extra sets of the same genome, e.g. autotriploid (3x, as in seedless banana and watermelon) and autotetraploid (4x, as in potato). Colchicine, which blocks spindle formation, induces it experimentally. In odd-numbered sets the gametes are unbalanced and fertility is low.
• Allopolyploidy: hybridisation of two species followed by doubling of the chromosomes, which makes the hybrid fertile. Bread wheat, Triticum aestivum, is an allohexaploid (2n = 6x = 42, AABBDD); Karpechenko's Raphanobrassica (2n = 36) is an amphidiploid of radish and cabbage, although it is not commercially useful.
• Aneuploid-like polyploids, as in modern sugarcane (2n about 100–130), a complex hybrid derived from Saccharum officinarum and S. spontaneum.

Phenotypic effects: polyploids typically have larger cells, nuclei and stomata and are bigger (gigantism) with thicker leaves, larger flowers and fruits, and often slower growth and later flowering. Odd polyploids are sterile and seedless, which is used in horticulture. Allopolyploids show hybrid vigour and buffering of mutations. Polyploidy is common in plants and rare in animals, where it upsets sex determination. It can create a new species in one generation because the polyploid is reproductively isolated from its parents.

(b) Chromosome theory of linkage and determination of linkage

Sutton and Boveri proposed that genes lie on chromosomes. Morgan and his colleagues showed from Drosophila that genes on the same chromosome are inherited together (linked) and do not assort independently. Linked genes stay together unless separated by crossing over in meiosis; the closer two genes lie, the less often they are separated. Linkage may be complete (no crossing over, as in male Drosophila) or incomplete (recombinants formed).

Determination of linkage:

• Two-point test cross: a double heterozygote is crossed to a double homozygous recessive. Independent genes give four classes in equal numbers (1:1:1:1). Linked genes give an excess of the parental classes and a few recombinants. Example: in maize, coloured aleurone (C) and shrunken endosperm (sh) on chromosome 9 gave about 3.6% recombinants.
• Recombination frequency = (recombinants/total progeny) × 100; 1% recombination is 1 map unit (centimorgan), which gives the map distance.
• Three-point test cross: three linked genes are followed together. The rarest class gives the double crossovers, which fixes the gene order, and the distances are summed (with double crossovers added) to build a linkage map, with the coefficient of coincidence showing interference.
• A chi-square test tells linkage from independent assortment.

(c) Mendel's dihybrid cross and independent assortment

A dihybrid cross follows two pairs of contrasting characters. Mendel crossed pure yellow round peas (YYRR) with green wrinkled (yyrr). The F₁ was all yellow round (YyRr). Selfing the F₁ gave 9 yellow round : 3 yellow wrinkled : 3 green round : 1 green wrinkled in the F₂, in which yellow wrinkled and green round are new combinations. The F₁ forms four types of gamete, YR, Yr, yR and yr, in equal numbers, and 16 random combinations give the 9:3:3:1 ratio, which is the product of two monohybrid 3:1 ratios.

Mechanism: independent assortment occurs because the genes are on different chromosome pairs. At metaphase I each bivalent lies on the equator and faces either pole at random, independently of other bivalents, so at anaphase I the maternal and paternal chromosomes of each pair go to the poles in all combinations. The result is four equal kinds of gametes, so the alleles of one gene pair separate independently of those of the other. The rule fails for linked genes on the same chromosome. The chi-square test checks the fit of the observed numbers to 9:3:3:1.