In the following human pedigree, the filled symbols represent affected individuals. Which of the following disorders does the chart most probably represent?

 

The disease is autosomal recessive. It is seen in the first and third generations but not in the second generation. Dominant traits never skip generations. Phenylketonuria is an autosomal recessive disorder. Had the disease been sex-linked recessive (e.g., red-green colourblindness), all the sons of the affected woman (generation 1) should have been affected, which is not the case here. Myotonic dystrophy is autosomal dominant. Hypertrichosis is a Y-linked trait, found only in males.