An individual with an extra chromosome on the 21st pair of chromosomes has tourette syndrome fragile x syndrome down syndrome nearsightedness previous next

The individual has the Down syndrome.
Trisomy 21 is the most common form of Down syndrome, where an individual has 47 chromosomes in each cell instead of the normal 46 chromosomes. It is caused by an error during cell division call non-disjunction which leaves gametes (sperm or egg cell) with an extra copy of chromosome 21 before or at conception. 

When both parents are carriers for a recessive disorder, each child has a 1 in 4 (25 percent) chance of inheriting the two changed gene copies. A child who inherits two changed gene copies will be “affected,” meaning the child has the disorder.