Practice question

Triploids carry three complete sets of chromosomes, 3x, resulting from cross between tetraploid 4x and diploid 2x parents or from failure of reduction in meiosis producing unreduced 2n gamete fertilized by normal n gamete. During prophase I homologous pairing must partition three copies of each chromosome; trivalent formation, or bivalent plus univalent, is common, leading to unbalanced segregation and aneuploid gametes with variable chromosome numbers ranging from n to 2n. Gametes receiving incomplete complement are typically non-viable, pollen abortion high, embryo sac degeneration frequent. Consequently seed set is extremely low, plants are effectively sterile, though vegetatively vigorous due to heterozygosity and gigas effect. This sterility is exploited commercially to produce seedless fruits like banana Cavendish, watermelon, and citrus, where parthenocarpy or vegetative propagation bypasses need for viable seeds. Triploid block also reinforces reproductive isolation in nature and speciation barriers. Cytological analysis using acetocarmine staining reveals irregular chromosome configurations in triploid PMC, leading to lagging chromosomes, micronuclei formation, and tetrads with unbalanced chromosome numbers, cytological mechanisms underlying sterility that breeders exploit intentionally for seedless fruit development and reproductive isolation between ploidy levels in nature.