a) Mendelian disorders are mainly determined by the alteration or mutation in the single gene. These mutations are transmitted to the offspring. On the other hand, Chromosomal disorders are caused due to absence or excess of one or more chromosomes. b) The two chromosomal aberrations that are most commonly seen are Down's syndrome ( trisomy of 21st chromosome ) and Turner's syndrome ( absence of one of the X chromosome i.e. 45+X0) . c) The characteristics of the two chromosomal disorders are as follows- Characteristics of Down's syndrome are furrowed tongue, partially opened mouth, broad palm with characteristic palm creases, short-statured, small round head physical, psychomotor and mental development is retarded. Characteristics of Turner's syndrome are sterile or rudimentary ovary , lack of secondary sexual characteristics . It occurs only in females.