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Down syndrome in humans is due to ADDITIONAL, PARTIAL OR FULL COPY of the 21 st chromosome.

Explanation:

Down syndrome is a genetic disorder in which an individual has a total of 47 chromosomes instead of 46 that is, 23 pairs of chromosomes which is found in normal individuals. This occurs as a result of the presence of additional, partial or full copy of chromosome 21. There are generic variations that occurs in chromosome 21 leading to down syndrome, these include:

-->Trisomy 21 : the individual has three copies of chromosome 21 instead of two copies due to abnormal cell division that occurred during development of sperm or egg.

--> Mosaic Down syndrome: the individual has some cells with an extra copy of chromosome 21 and it's caused by abnormal cell division after fertilization.

--> Translocation down syndrome: This type of down syndrome occurs when chromosome 21 gets attached to another chromosome which occurs during or after conception.

Generally the symptoms due to the presence of this extra genetic material includes:

--> short heights

--> poor muscle development

--> short neck

--> short fingers with small hands and feet

--> increased risk to development of dementia

--> greater tendency to develop obesity