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trisomic Applied to a genome that is diploid but which contains an extra chromosome, homologous with one of the existing pairs, so that one kind of chromosome is present in triplicate. The chromosome number is thus of the form 2n + 1. Trisomies give multiple doses of genes and may have deleterious effects, as in the human condition of Down's syndrome, which results from a trisomy of chromosome 21.

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