Human Heredity 9th Edition by Michael Cummings

Exercise 1

A genetic counselor was called to the pediatric ward to examine a 3-week-old infant who was diagnosed with a genetic disorder of sugar metabolism called galactosemia. The infant was admitted to the hospital because of failure to thrive and severe jaundice (yellowing of the skin resulting from liver problems). Upon examination, the physician determined that the infant had an enlarged liver, cataracts, and constant diarrhea and vomiting when fed milk. Escherichia coli infection is a common cause of death in infants who have galactosemia, and cultures were drawn from the infant. Laboratory results confirmed that the infant had a deficiency of the enzyme galactose-1-phosphate uridyltransferase and was infected with E. coli.
The counselor took a detailed family history and explained the condition to the parents. She indicated that the condition is due to the inheritance of a mutant gene from each parent (the trait is autosomal recessive) and that there is a 25%, or one in four, chance that each pregnancy they have together will produce a child with this condition. The counselor explained that there is wide variability in phenotype, ranging from very mild to severe. A blood test could determine which variant of the disease they carry.
What exists in blood that can be tested for a variant of a disease-causing gene?