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Each of the affected individuals on this pedigree inherited the disorder from asymptomatic carrier parents, which is consistent with a recessive inheritance pattern.  Because both males and females inherit the condition, it is an autosomal recessive disorder.  Based on this inheritance pattern, 50% of offspring will be asymptomatic carriers, 25% will be unaffected, and 25% will express the disorder.

Classic galactosemia is the most common and most severe of the galactosemic disorders.  It is an autosomal recessive disorder leading to complete enzymatic absence of galactose-1-phosphate uridyl transferase.  Newborns present within days of birth with jaundice, vomiting, and hepatomegaly.

In general, most enzyme deficiency conditions follow an autosomal recessive inheritance pattern whereas diseases due to defective noncatalytic proteins follow an autosomal dominant pattern.

(Choice B)  Hemophilia B (Christmas disease) is an X-linked recessive disorder affecting males that causes factor IX deficiency with easy bruising and bleeding (eg, hemarthrosis, oral bleeding, intracranial hemorrhage).  Affected males will have asymptomatic carrier mothers.

(Choice C)  Huntington disease is an autosomal dominant disorder affecting males and females equally.  It causes progressive neurodegeneration of the caudate and putamen, leading to chorea, dementia, and death.  Patients usually have an affected parent.

(Choice D)  Leber hereditary optic neuropathy is a mitochondrial inheritance disorder affecting all offspring of an affected mother; there is no father-child transmission.  It is characterized by progressive bilateral optic neuropathy leading to blindness.

(Choice E)  Rett syndrome is an X-linked dominant disorder affecting females (affected males die in utero) that presents in early childhood with progressive neurodegeneration and stereotypical hand movements.  X-linked dominant conditions are characterized by a lack of father-son transmission whereas all daughters of an affected father are affected.  Half of all offspring of an affected mother are also affected.

Educational objective:
Autosomal recessive disorders affect 25% of offspring of asymptomatic heterozygous carrier parents.  Classical galactosemia is the most common and severe galactosemic disorder and presents within days of birth with jaundice, vomiting, and hepatomegaly.