Part A. The pedigree is as follows:
Galactosemia is a human genetic disorder caused by a deficiency of the enzyme galactose-1-phosphate uridylyltransferase (commonly called GALT). Children with galactosemia seem normal at birth, but begin to show symptoms as soon as they drink milk. This is because they cannot metabolize galactose, a simple sugar that results from the breakdown of lactose (the sugar found in milk). Early symptoms of galactosemia include vomiting and diarrhea. If untreated, affected babies will develop enlarged livers, cataracts, and mental retardation. Depending on the alleles the baby has inherited, early death may result.
Part A - Construct a pedigree
John and Jane Jones’ newborn baby girl, Leah, has just been diagnosed with galactosemia. Prior to their genetic counseling appointment, they submitted this family history:
John and Jane’s older child, a son named Lee, does not have galactosemia.
John is the only child of Hanna and Harry.
Harry was an only child. Hanna has two older sisters, Hope (the oldest) and Holly.
Both Harry and Hanna’s parents lived in good health into their 80s.
Jane’s brother, Joe, is married to Jen. They have a son, Les, who is a healthy nine-year-old with myopia (nearsightedness) but no sign of galactosemia.
Jane’s mother, Hilda, is alive and healthy; Jane’s father, Henry, passed away last year of a sudden heart attack. Hilda and Henry have no family history of galactosemia.
Use the information in the family history to construct a pedigree for this family. Drag the correct symbols to their appropriate locations on the pedigree below. Not all symbols will be used.
Part B - Identify the mode of inheritance
Look over the pedigree you constructed in Part A. Based on the inheritance pattern, which mode of inheritance must be the cause of galactosemia?
a. autosomal recessive
b. autosomal dominant
c. sex-linked recessive
d. sex-linked dominant
Part C - Predicting the probability of inheritance
Jane and John are considering having another child. Given the pedigree you constructed and the mode of inheritance for galactosemia, what is the risk that their next child will have the disorder?
a. 1/4 (because they are both heterozygotes)
b. 0 (because they already have one child with the disorder)
c. 3/4 (because they are both heterozygotes)
d. 1/16 (because they each have 1/4 chance of passing on the allele)
Part D - Genetic testing
If Jane and John want to have another child, they plan to see a genetic counselor to find out when it would be best to test for galactosemia. A newborn with galactosemia must be put on a lactose- and galactose-free diet as soon as possible after birth. Even on this diet, affected individuals may still suffer from learning disabilities, ovarian failure (in young women), late-onset cataracts, and early death.
Which of the following tests would be most useful for Jane and John to have?
a. no testing
b. newborn screening (either assaying for the GALT enzyme or measuring excess galactose in the newborn’s blood)
c. carrier screening of Jane and John (to identify whether either or both of them carry the allele for galactosemia)
d. fetal chromosome analysis (examining fetal chromosomes for abnormalities as early as 11 weeks into the pregnancy)
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