Fragile X Syndrome: Understanding Parental Genotypes & Inheritance

The inheritance pattern of fragile X syndrome is complex and can best be explained using Punnett squares, however, here is a simplified explanation of the possible genotypes of parents:

1. Carrier mother and unaffected father: If a mother is a carrier of the fragile X mutation (premutation) and the father does not have the mutation, there is a 50% chance with each pregnancy that the mother will pass on the premutation to her sons, who will then be affected by fragile X syndrome. Her daughters have a 50% chance of being carriers like their mother.

2. Affected mother and unaffected father: In rare cases, an affected mother (full mutation) can also transmit the fragile X mutation to her children. Sons of an affected mother will always be affected by fragile X syndrome, while daughters have a 50% chance of being affected and a 50% chance of being carriers.

3. Affected father: It is rare for males to have fragile X syndrome due to the fact that males with the full mutation usually have severe intellectual disability and reproductive issues; however, males with fragile X syndrome can pass on the premutation to their daughters, who have a 50% chance of being carriers.

It's important to note that genetic counselling plays a crucial role in fragile X syndrome to understand the recurrence risks and reproductive options for families affected or at risk for this condition.