Fragile X

Inheritance	X-linked recessive
Occurrence	1 in 1,200 males is affected and 1 in 300 women carries the fragile X mutation
Description	A disorder that causes various levels of mental impairment-- from learning disabilities to severe retardation, both combined with delayed speech and language development.
Gene and location	The gene causing fragile X is called FMR-1 and is found on the X chromosome. This gene contains repeats of the trinucleotide CGG. On average unaffected individuals have 29 repeats of this sequence. Some individuals have what is called a premutation, possessing between 50 and 200 copies of this sequence. These individuals are intellectually normal but the sequence is unstable and the number of repeats may increase when passing from mother to child. Individuals with more than 200 repeats have Fragile X Syndrome and some degree of mental impairment. (There can be as many as 1,000 repeats of this sequence).
Testing	is available to women who have fragile X or may have a family history of fragile X; there is also a prenatal test

Chromosomes and Karyotypes

Fragile X is identifiable by karyotype analysis. A karyotype is essentially a photograph of all the chromosomes in cell. For humans a karyotype shows 22 pairs of autosomes and the sex chromosomes. Karyotypes of individuals with Fragile X Syndrome appear to be missing a small piece of the X chromosome near the end. Karyotypes make it possible to determine sex, by seeing if there are two X chromosomes present or an X and a Y chromosome present. Other chromosomal abnormalities like Trisomy 21 are also detectable.

In most biology texts there are several available karyotypes for students tostudy. Teach them what things to look for and what information is found in a karyotype. Have them draw their own karyotype (making sure to have the right sex chromosomes depending on their gender) or analyze a karyotype you give them. For many students the visual image of chromosomes help to solidify many other biology concepts, such as how gametes are formed, how many chromosomes are normally in their cells, and how the chromosome sets are combinded at fertilization.