Trisomics are aneuploids, carrying an extra chromosome to the normal somatic complement. The nature of this additional chromosome determines its types; normal in primary, isochromosome in secondary, and translocated in tertiary or interchange types. The extra chromosome causes meiotic irregularities manifested by its abnormal associations at metaphase I and unequal separation of chromosomes, laggard and bridge formation at anaphase I. This leads to the formation of unbalanced gametes (n+. 1) in trisomic individuals. Poor viability and vigor of n+. 1 gametes severely reduce the transmission rate of trisomics through the disturbances in prezygotic, zygotic, and postzygotic stages. Since the trisomics carry an extra chromosome, the modified segregation ratios have immense significance in comprehensive genome mapping using both classical and molecular markers. © 2013 Elsevier Inc. All rights reserved.