Feature of Down syndrome is

Ans) c (Trisomy 21) Ref Nelson 18th ed p 508Down syndrome is the most common chromosome disorder and the single most common genetic cause of moderate mental retardation. The incidence of Down syndrome in live births is approximately 1 in 750. The occurrence of trisomy 21 as well as other autosomal trisomies increases with advanced maternal age (> 35 yr)In approximately 95% of the cases of Down syndrome there are 3 copies of chromosome 21. The parental origin of the supernumerary chromosome 21 is maternal in 97% of the cases. Approximately 1 % of individuals are mosaics with some cells having 46 chromosomes, while another 4% of individuals have a translocation that involves chromosome 21.The majority of translocations in Down syndrome are fusions at the centromere between chromosomes 13, 14, 15, or 21, known as Robertsonian translocations. The translocations can be de novo or inherited.Down syndrome patients without a visible chromosome abnonnality are the least common.It is not possible to distinguish the phenotypes of individuals with full trisomy 21 and those with a translocation.Patients who are mosaic tend to have a milder phenotype.