Concepts of Genetics 11th Edition by William Klug,Michael Cummings,Charlotte Spencer,Michael Palladino

Exercise 1

A newborn screening program identified a baby with a rare autosomal recessive disorder called arginosuccinic aciduria (AGA), which causes high levels of ammonia to accumulate in the blood. Symptoms usually appear in the first week after birth and can progress to include severe liver damage, developmental delay, and mental retardation. AGA occurs with a frequency of about 1 in 70,000 births. There is no history of this disorder in either the father's or mother's family. The above case raises several questions:
Since it appears that the unaffected parents are heterozygotes, would it be considered unusual that there would be no family history of the disorder? How would they be counseled about risks to future children?