Emily and Jack were so excited at the birth of their baby Mark. “Jack, he’s just so perfect! Just one problem though, it looks like he has your hairline!” Emily teased her husband who, though only 32, was balding. “Emily, I spent all that time painting the baby’s room and I

Emily and Jack were so excited at the birth of their baby Mark.
“Jack, he’s just so perfect! Just one problem though, it looks like he has your hairline!” Emily teased her husband who, though only 32, was balding.

“Emily, I spent all that time painting the baby’s room and I just hope that he’s not color blind like your father or he won’t be able to see it!” Jack responded.

Both the pregnancy and delivery had been uneventful. But in the back of their minds, they really were worried
because their first child, Sean, died at the age of nine days.

By the fifth day after birth, Mark began to have trouble nursing and by the seventh day he had completely stopped feeding. Emily and Jack were frantic because it seemed to them that Mark might also die.

“What is going on with our family? Another sick baby?” Jack thought to himself.

Emily and Jack rushed him to the emergency room. Although Mark's limbs were rigid and he had had a seizure, the examination showed no infection and his x-rays were normal. The doctor also did routine lab tests on his blood and urine.

“Doctor, do you think that this funny smell in Mark's diapers has anything to do with his problem?” Emily asked. “I brought one along so that you could smell it too.”

Mark's urine did have a sweet, maple syrup smell and lab results revealed elevated levels of the branched chain amino acids (BCAA)—valine, isoleucine, and leucine.

Skin biopsies from the baby and his parents were taken and cultured. The ability of the cultured skin fibroblasts to metabolize BCAA was determined. While his parents’ enzyme activity levels were nearly normal, Mark's was less than 2% of normal.

“Given the medical information and the smell of the urine, Mark has Maple Syrup Urine Disease. The disease is due to a recessive gene,” reported Dr. Martin. “He will not be able to breast feed or drink regular formula. What is really important is that Mark eats a low protein diet. This diet must continue for the rest of his life or else the amino acids will accumulate in the body creating a situation that leads to brain swelling, neurological damage, and death. In spite of dietary intervention, the disease may cause several complications, the most notable being mental retardation. You need to know that dietary intervention does not cure the disease.”

Emily and Jack were Mennonites and their family history revealed that Emily's mother had two sisters who died in their first year of life; no one knew why. Jack's father had a sister who died at seven months of age from unknown causes. Could the gene for Maple Syrup Urine Disease run in both of their families?

**What is the probability that Jack and Emily would have a child who is a carrier of the disease?**