Fragile X Syndrome

Fragile X Syndrome is a condition in which a strand of DNA in the longest arm of the X chromosone becomes stretched out, causing the gene to stop producing proteins.
Fragile X is a change in a single gene, the Fragile X Mental Retardation 1 (FMR1) gene.
Down Syndrome, due to an error in the number of chromosomes Fragile X, due to a repeat in the genetic code that leads to a lack of production of a certain protein PKU (phenylketonuria), due to an error in a single gene that makes a defective enzyme.