<prompt> Downs syndrome isTriploidyMonosomyTrisomyAneuploidy  <response>: Ans. c Trisomy. Ref Harrison, Medicine, 18thCh. 62. Chromosome DisordersClinically, the most important of numerical abnormality is trisomy 21, the most frequent cause of Down syndrome.Depending on the maternal age structure of the population and the utilization of prenatal testing, the incidence of trisomy 21 ranges from 1600 to 11000 livebirths, making it the most common chromosome abnormality in live-bom individuals.Like most trisomies, the incidence of trisomy 21 is highly correlated with maternal age, increasing from about 11500 livebirths for women 20 years of age to 130 for women 45 years.In addition to trisomy 21, only two other autosomal trisomies, 13 and 18, occur with any frequency in livebirths.Incidence rates for trisomies 13 and 18 in livebirths are 120,000 and 110,000, respectively. Unlike trisomy 21 that is associated with near-normal life expectancy, both trisomies 13 and 18 are associated with death in infancy, typically occurring during the first year of life.Three sex chromosome trisomies the 47,XXX, 47,XXY Klinefelters syndrome, and 47,XYY conditions are quite common, with each occurring in about 12000 newborns. Of all the trisomic conditions, three have fewest phenotypic complications. In fact, with the exception of infertility in klinefelters sundrome, it is likely that most individuals with such trisomies may go undetected.TableNumerical chromosomal abnormalities includePolyploidye.g. triploidy haploid x 3 69XYY 23x3 common but lethalAneuploidyAny chromosome number, which is not an exact multiple of haploid number e.g., Down syndrome 47 XY 21 or Klinefelter syndrome 47 XXY.TrisomyState of having three representatives of a given chromosome instead of the usual pair, as in trisomy 21 Down syndrome. 