Mouse autosomal trisomy company, three's a crowd

neuploidy is a condition in which there are more orfewer chromosomes than an exact multiple of the hap-loid number for that species. In diploid species, such asmouse and human, if an individual chromosome is presentin three, rather than the normal two, copies, it is ‘tri-somic’, or if present in one copy it is ‘monosomic’. Whole-chromosome aberrations generally occur because of meiotic non-disjunction. Duplications, deletions and otherrearrangements of parts of chromosomes can also producean unbalanced genetic complement, such as three copiesor one copy of a particular region; in these cases, the indi-vidual can have an aneuploid or a euploid number ofchromosomes.In mouse and human, the sex chromosome aneu-ploidies are less severe than the autosomal aneuploidies,providing the individual has at least one X chromosome.No autosomal monosomies are viable, presumablybecause at least two ‘doses’ of a subset of the genes oneach autosome are required for normal development. Thehuman autosomal trisomies are relatively common – it isestimated that 20% of human conceptuses are trisomic –but all have a very high degree of morbidity and mortalityand most abort early in embryogenesis