Fragile X Syndrome: Genetic Risks for Future Generations

Osteogenesis Imperfecta Issues

Hello, the younger brother in the family has Fragile X syndrome, which is associated with intellectual disabilities.
However, the older brother does not have Fragile X syndrome and is a normal individual.
If the older brother has children with his wife, who is healthy and has no history of Fragile X syndrome, will he pass on the gene to the next generation? If so, will the daughters exhibit symptoms? Will the sons exhibit symptoms as well?

Hsun, 20~29 year old female. Ask Date: 2023/05/22

Dr. Lin Xinwang reply Obstetrics and Gynecology

Hello, X-linked Fragile Syndrome is characterized by a fragile site on the X chromosome and is inherited in an X-linked dominant manner.
Therefore, a brother with a normal X chromosome will not pass this condition to the next generation.
Both males and females can exhibit symptoms, but males tend to have more severe manifestations.
Symptoms may include intellectual disabilities, physical abnormalities, and social or emotional challenges.

Reply Date: 2023/05/22

More Info

Fragile X Syndrome (FXS) is a genetic condition that is primarily caused by a mutation in the FMR1 gene located on the X chromosome. This condition is known to be the most common inherited cause of intellectual disability and is associated with a range of developmental and behavioral challenges. Understanding the genetic risks associated with FXS is crucial for families with a history of the condition.

In your case, you mentioned that your brother has Fragile X Syndrome, while your other brother (the one in question) does not have the condition. Since FXS is an X-linked disorder, it is important to note that males have one X and one Y chromosome (XY), while females have two X chromosomes (XX). The FMR1 gene mutation can be passed from mother to child, but since your brother is unaffected, he does not carry the mutation on his X chromosome.

When your brother (the unaffected male) has children with a partner who is also unaffected and has no family history of FXS, the risk of passing on the condition is very low. Males cannot pass on their Y chromosome to daughters, and since your brother's X chromosome is normal, he will not transmit the Fragile X mutation to his children. Therefore, any daughters he has will inherit his normal X chromosome, and any sons will inherit his Y chromosome, which does not carry the FMR1 mutation.

However, it is important to consider that if the mother (your brother's partner) is a carrier of the Fragile X mutation (which is possible even if she does not show symptoms), there is a risk that she could pass the mutated X chromosome to her children. In such a case, there is a 50% chance for each child to inherit the mutated X chromosome if the mother is a carrier. Daughters who inherit the mutated X chromosome may exhibit symptoms of Fragile X Syndrome, while sons who inherit it are more likely to show more severe symptoms due to having only one X chromosome.

In summary, if your brother is confirmed to be free of the Fragile X mutation, and his partner is also unaffected, the likelihood of their children having Fragile X Syndrome is minimal. However, if there is any concern about the mother's family history or potential carrier status, genetic counseling and testing may be beneficial. Genetic counselors can provide valuable information and support for families navigating these complex issues, helping them understand their risks and options for family planning.

In addition to the genetic aspects, it is also important to consider the broader implications of Fragile X Syndrome. Children with FXS may face challenges in learning, social interactions, and behavior. Early intervention and support services can significantly improve outcomes for affected individuals. Therefore, if there is any concern about the potential for Fragile X in future generations, it is advisable to seek genetic counseling to discuss testing options and the implications for family planning.

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