Trisomy-21 is seen in Down syndrome. Trisomy 21 is a genetic disorder caused by the presence of all or part of a Klinefelter syndrome. A Trisomy 21 affected baby will have one extra chromosome at 21st position out of 23. This dramatically impacts the development of children and can cause serious illnesses like epilepsy, hearing loss, etc. This trisomy is found in about 1 infant in every 600-700 live births. 

Down syndrome  

• It is one of the well-known and best-studied examples of chromosome-caused abnormalities in human beings.
• This condition, which is an autosomal trisomy, was first described by Langdon Down in 1866 and was originally called mongoloid idiocy or mongolism.
• Down chose this name because of the prominence of the epicanthic fold in the eyelid a phenotype resembling members of the mono gold race. 

• Turner syndrome: This genetic disorder generally affects the females where the person has a missing/partially missing X chromosome, ( a healthy girl child has 2'X' chromosome.)
• Klinefelter syndrome: This genetic disorder generally affects the male child. A healthy male child has one 'X' and one 'Y' chromosome but in the case of Klinefelter syndrome, the child is born with the extra X chromosome. This affects the growth of the child as a normal adult male. 

Hence, we can conclude that Trisomy-21 is cause of Down syndrome.