Practice question

Allopolyploids arise when two distinct species hybridize and their combined chromosome sets undergo doubling, producing new species containing both parental genomes in duplicated form. Initial interspecific F1 hybrid is typically sterile because homoeologous chromosomes from different species lack sufficient homology for regular bivalent pairing at meiosis I, causing univalents and aborted gametes due to irregular segregation. Chance somatic doubling via nondisjunction or unreduced gamete fusion doubles each parental complement, providing each chromosome a homologous partner to pair as bivalent, restoring fertility and disomic inheritance. Genome formulas illustrate: species A 2n=AA, species B 2n=BB, hybrid AB sterile, doubling yields AABB allotetraploid fertile. Classic examples include Brassica napus AACC from B. rapa AA × B. oleracea CC, wheat AABBDD hexaploid, and tobacco. Allopolyploidy drives speciation, fixing heterosis and combining advantageous traits from divergent lineages important for crop evolution. Molecular cytogenetics with genomic in situ hybridization GISH distinguishes parental chromosomes in allopolyploids, confirming genome composition and detecting intergenomic translocations; this technique validated origins of many natural allopolyploids and supports introgression breeding by tracking alien chromatin segments transferred from wild relatives for trait improvement.