Practice question

Common bread wheat Triticum aestivum is an allohexaploid that arose through two successive natural hybridizations followed by spontaneous chromosome doubling. First hybridization between einkorn relative Triticum urartu carrying AA genome 2n=14 and Aegilops speltoides related BB genome 2n=14 produced tetraploid wild emmer AABB 2n=28. Second hybridization of tetraploid emmer with diploid goat grass Aegilops tauschii DD genome 2n=14 and subsequent doubling yielded hexaploid ABD genome complement with basic chromosome number x=7. Therefore 6x = 42 chromosomes in somatic cells, 21 pairs, with 7 chromosomes per subgenome. Meiosis is regulated by Ph1 gene on chromosome 5B ensuring strict homologous pairing, so wheat behaves as diploid genetically despite polyploidy. Genome size is large 17 Gb. Hexaploidy provides buffering, adaptability, and broader quality traits essential for bread-making properties and worldwide adaptability of modern wheat cultivars under diverse environments. Gene balance hypothesis explains stability of hexaploid due to balanced duplication of interacting genes; allopolyploidy buffers deleterious recessive mutations, allows subfunctionalization and neofunctionalization of homoeologs, contributing to evolutionary success and broad adaptability of bread wheat across temperate and subtropical environments worldwide today.