Doctor Gul Dolen explains that Fragile X syndrome is not a mendelian disorder, because the inheritance pattern in slightly different.

Fragile X is a little bit different from a Mendelian disorder because when we say Mendelian genetics we mean usually that you have a mutation on one gene, and it’s passed directly down to the next generation and Fragile X is a little bit different. What’s passed down is the tendency for the gene to be methylated. So, every person has parts of their chromosome that have repeated areas. They are sequences that are called CG repeat areas, and those are found scattered throughout the genome in everybody. But parents of children with Fragile X, the mother, will have little islands where those repeats are more than is normal in the general population and those mothers are called premutation carriers. When you have an expansion of that repeat there is a tendency for it, in the next generation, to expand further. When that repeat-expansion goes beyond 200, then you get Fragile X. So it’s not a Mendelian disorder because there is a change in one generation, that means that the mother doesn’t have it but the child does and so strictly speaking it’s not a Mendelian inheritance pattern.

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