Pedigree Case Studies



Pedigree Case Studies

Directions: Read the pedigree case studies provided and answer the questions. In some cases you will be provided the pedigree and in other cases you will have to construct the pedigree using given information.

Case Study 1:

In this story, a couple is concerned about their child who appears normal at birth, but develops problems, so that by the seventh day he has completely stopped feeding. The child is diagnosed with Maple Syrup Urine Disease (MSUD), an autosomal recessive metabolic disorder. Interestingly, the urine of affected individuals smells like maple syrup due to the accumulation of branched chain amino acids (leucine, isoleucine, and valine). The current standard treatment protocol is a special low protein diet. Even with strict adherence to the restricted diet, several times a year many patients have crises during which protein levels increase and branched chain amino acids accumulate. This can result in swelling of the brain, neurological damage, and death. Mental retardation is a common complication.

The quality of life for individuals with MSUD may be affected by the complications of the disease. Most students can empathize with parents who are struggling to care for a child with special needs. In the future, gene therapy might be a useful treatment, but currently studies are only being done in mice. Orthotopic liver transplantation has been a successful and yet risky “cure.” This case allows for use of basic genetic principles that can then be weighed against the parents’ concerns about their child. It allows for critical thinking in the context of a decision for medical care that students may face as parents (or aunts or uncles).

Matthew’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, Matthew’s was 200 times lower than normal.

“Given the medical information and the smell of the urine, Matthew has Maple Syrup Urine Disease (MSUD),” reported Dr. Morton of the Clinic for Special Children. “He will not be able to breast feed or drink regular formula. What is really important is that Matthew 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.”

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

MSUD is due to a recessive gene. For an individual to be affected, he or she would need to inherit a defective nonworking copy from each parent. The individual would then be described as being homozygous recessive.

Case Study II

THE DEATH OF BABY PIERRE

by Clyde Freeman Herreid

SUNY/Buffalo

On March 7, 1964, the baby known as Pierre was born in a remote part of Quebec Province in Canada. He appeared to be a healthy six-pound twelve-ounce child, except he did not eat well. Over the weeks after his birth, he became progressively more lethargic, vomiting periodically. Most peculiarly, his urine smelled of rotten cabbage, and soon the smell permeated his clothes and body. By the time he was admitted to the hospital on September 14, his muscles were weak and his ribs were showing. Baby Pierre had only gained half a pound in the six months since his birth. The doctors kept him alive by feeding him through a tube threaded through his nose and into his stomach. He gained weight and strength for a while, then suddenly took a turn for the worse. On November 30, baby Pierre vomited blood and died.

It soon became increasingly apparent that other babies in the Chicoutimi area of Quebec Province had similar symptoms, and people recalled similar deaths in this remote area 120 miles north of Quebec City. Some families lost several children to Pierre's disease. In those families stricken, it soon became clear that the parents were normal, but about one quarter of their children were afflicted. Boys and girls were equally afflicted. Specialists soon concluded that all of the facts indicated that this was a genetic disorder.

1. If a genetic disorder was the cause, which answer is most likely correct?

(A) A pollutant is causing mutations.

(B) Multiple alleles are involved.

(C) The disease is caused by a dominant allele.

(D) Baby Pierre's parents are homozygous for a recessive allele causing the disease.

(E) The disease is due to an autosomal recessive.

Explain the reason for your choice.

(To be answered after Part 1 is handed in)

Baby Pierre and the other stricken children were victims of hereditary tyrosinemia. This is caused by an autosomal recessive disease. The children lacked the normal gene which produces a liver enzyme that breaks down the amino acid tyrosine. Without the enzyme, tyrosine builds up in the liver and kidneys leading to the cabbage-like smell of the urine. Lethal side-effects follow. (A liver transplant is the only long-term treatment of the disease as of 1997.)

Both parents avoid this fate because, although they carry one copy of the defective gene, they carry a normal gene which produces more than enough enzyme for normal liver function. The parents are unwitting carriers of the disease. In genetic terminology they are heterozygotes, while Baby Pierre was a homozygous recessive.

Below is a pedigree of three generations of Canadians.

[pic]

2. Using the symbols of "a" for recessive allele and "A" for the dominant normal allele write the genotype for the following individuals.

|A _______________________ |I _______________________ |

| | |

|D _______________________ |K _______________________ |

| | |

|E _______________________ |R _______________________ |

3. Is the above pedigree consistent with a sex-linked trait?

4. What is the likelihood that female K will have a normal child if she marries a normal person who is a carrier for tyrosinemia?

5. What is the likelihood of female K having a normal child if she marries her cousin M?

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