Down Syndrome
Down syndrome is caused by a change in chromosome number. Specifically, the syndrome results when the child has an extra copy of the 21st chromosome. This typically occurs through a mistake in the copying process called meiotic nondisjunction.  The syndrome also can result through a process called translocation in which all or a portion of the extra 21st chromosome attaches to another chromosome.  Finally, a third variety of Down syndrome is called mosaicism. This occurs when a copying error occurs after fertilization. The child therefore has some cells in the body that have the proper number of chromosomes and some cells in their body with an extra 21st chromosome. Mosaic Down syndrome typically is milder in terms of intellectual functioning, physical features, and behavioral manifestations than the other two varieties of Down syndrome.

Fragile X Syndrome
Fragile X syndrome is the leading inherited cause of intellectual disability. This syndrome results when a section of the X chromosome becomes elongated through a process called repeat expansion. The syndrome results when the number of repeats is greater than 200. Milder forms of disability may result with lesser numbers of repeats. Because this syndrome is X-linked, it is more typically more severe in males because they only have one copy of the X chromosome. A syndrome called permutation is diagnosed when the number of repeats is less than 200 but greater than 50. This syndrome is typified by below average levels of intelligence, speech disorders, and behavioral problems. The symptoms of Fragile X syndrome overlap with those of autism and therefore individuals may have both diagnoses.