This specific learning problem is referred to as Turner neurocognitive phenotype and appears to be due to loss of X chromosome genes important for selected aspects of nervous system development.
Identical twins will have the same DNA, genetic material (genotype), but it may be expressed differently (phenotype).
Although PKU conforms to these basic genetic patterns of inheritance, the actual expression, or phenotype, of the disease is not strictly an either/or situation.
Siblings of autistic individuals are more likely to have a pervasive developmental disorder, and there is a broader autism phenotype that may tend to run in families affected by the spectrum.
You may come across the term broader autism phenotype referring to this population.