Methylation Used with permission of Richard Roberts, New England Biolabs One of the ways the cells control which genetic information they will use is to chemically modify the DNA . This inactivates that part of the chromosome. In females methylation is used regularly to solve a problem.
Men have only one X chromosome and women have two.
As a result, female cells might be expected to make twice as much protein from the information on X chromosomes as males do. Instead, women's cells randomly pick one of the X chromosomes and turn it off by methylation. Thus both males and females have one working X chromosome in each cell and as a result, one working unit of all the genetic information on the X chromosome. As noted above, methylation is generally a useful method for turning off chromosomal information. However, in fragile X syndrome, methylation is involved in causing the disease.