Introduction

Down syndrome, the single most common condition causing learning disability, was first systematically described in 1866 by John Langdon Down. He named the condition ‘mongolism’ a term that was in use until the early 1970s, when it was superseded by ‘Down's (now Down) syndrome’. A variety of physical features occurs in people with Down syndrome although usually not all are present in any one individual. They include short stature, a rounded head, characteristic folds at the inner corners of the eyes, broad hands with a single palmar crease and a short incurved little finger. About half of all infants with Down syndrome are born with a heart defect (Hallidie-Smith, 1996). A major defining feature is intellectual disability, though its degree varies widely. No consistent relationship has been shown between the number of physical features present and severity of disability (Cunningham et al., 1991). Women with Down syndrome are potentially fertile and 32 pregnancies (one of twins) have been noted in the literature worldwide (Rani et al., 1990). Of these infants, about one-third themselves had the Down syndrome karyotype, about onefifth had disabilities other than Down syndrome and about two-fifths were normal. None of the 25 surviving infants appears to have been brought up by his or her own mother. Men are generally subfertile and only one authenticated case of a man with Down syndrome fathering a child has been reported (Sheridan et al., 1989).