Inherited characteristics are carried from one generation to the next by chromosomes. It is usual to have 46 chromosomes (23 pairs). The sex chromosome pair determines the sex of an individual.

Females have two X chromosomes and males have one X and one Y. These chromosomes are made up of DNA sequences or genes, which act as templates for making proteins with specific functions. It has been estimated that we have between 80,000 – 100,000 genes.

A change in a gene is called a mutation. Some mutations have no effect, but others, such as in the FMR-1 gene, cause changes in the structure or function of our bodies.

In front of the FMR-1 gene is the DNA sequence ‘CGG’. This sequence is normally repeated between 6 and 50 times. In some people, this sequence is repeated between 50 to 200 times and is called a premutation, which generally causes few or no symptoms of fragile X syndrome.

In a woman with the premutation, the size can expand to over 200 repeats when that X chromosome is passed on in her egg. This is called a full mutation. In the full mutation, the gene for FMR-1 switches off and no protein is produced. Fragile X syndrome is the result. The expansion does not occur from males to their children. Males with the fragile X gene will pass that X chromosome without a change to all of their daughters.

More information

Factsheets by The Centre for Genetics Education (CGE), Sydney

• GENES AND CHROMOSOMES Factsheet by The Centre for Genetics Education (CGE), Sydney
• X-LINKED INHERITANCE Traditional Patterns of Inheritance 3
• X CHROMOSOME INACTIVATION
• Learn.Genetics by the Genetic Science Learning Centre, Utah University. Includes sections on The Basics and Beyond and Understanding Genetic Disorders

Image – The X chromosome in fragile-X syndrome. The arrow indicates the fragile site. Source SGUL