Down's syndrome is
Ans. c (Trisomy). (Ref Harrison, Medicine, 18th/Ch. 62. Chromosome Disorders)Clinically, 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 1/600 to 1/1000 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 1/1500 livebirths for women 20 years of age to 1/30 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 1/20,000 and 1/10,000, 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 (Klinefelter's syndrome), and 47,XYY conditions are quite common, with each occurring in about 1/2000 newborns. Of all the trisomic conditions, three have fewest phenotypic complications. In fact, with the exception of infertility in klinefelter's 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 lethal)AneuploidyAny 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).
Triploidy
Monosomy
Trisomy
Aneuploidy
Genetics