Extra copy of 21st chromosome causes _________ in human.

Down's syndrome is a genetic disorder caused by the presence of an additional copy of chromosome $21$, also known as Trisomy $21$.

Turner's syndrome is caused by the absence of one of the X chromosomes, resulting in a $45$, X$0$ karyotype.

Klinefelter's syndrome is caused by the presence of an additional X chromosome, resulting in a $47$, XXY karyotype.

Thalassemia is an autosomal recessive blood disorder.

Therefore, an extra copy of the $21$st chromosome causes Down's syndrome.

Answer: Down's syndrome