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A Sickeningly Sweet Baby Boy

A Case Study of Recessive Inheritance in Inbred Populations

Jacqueline Washington
Department of Biology and Chemistry
Nyack College
Anne Zayaitz
Department of Biology
Kutztown University


When a newborn develops symptoms eerily similar to those of an older sibling who died shortly after birth, his Mennonite parents are understandably alarmed. They soon discover that their son has Maple Syrup Urine Disease (MSUD), a recessively inherited metabolic disorder. This case explores the genetics of the disease and the ultimate dilemma of treatment options. Developed for an introductory non-majors biology course, the case can also be used in other science or health related courses such as human genetics and biochemistry.


  • Explain the relationships between family members and the disease incidence using a pedigree chart.
  • Understand the rules governing the inheritance of a single gene recessive disorder.
  • Calculate the probabilities of inheriting recessive traits and passing them on to offspring.
  • Explain the dietary restrictions associated with MSUD.
  • Discuss the risks associated with liver transplants.
  • Discuss the options of genetic screening and genetic testing in this case.
  • Compare the advantages and disadvantages associated with gene therapy and more conventional treatment means, i.e., transplants.
  • Explore genetic drift effects on small populations (e.g., the Mennonite community), including other possible conditions resulting from a founder effect in similar communities.


Metabolic disorder; recessive disorder; genetic disease; genotype; phenotype; homozygous; heterozygous; Maple Syrup Urine Disease; MSUD; pedigree chart; founder effect; liver transplantation; Mennonites; Pennsylvania

Educational Level

High school, Undergraduate lower division, Undergraduate upper division



Type Methods

Interrupted, Dilemma/Decision



Subject Headings

Biology (General) Genetics / Heredity Biochemistry Medicine (General)

Date Posted


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Janice Carpenter
Science - Middle School
McGee Middle School
Berlin, CT
Over the past two years, I have used this case with 10 eighth grade classes as the last activity in a reproduction/genetics unit. We need three 40-minute class periods to complete the case. Each day we read one part of the case as a whole class, students work in small groups to answer the questions in the section we just read, and the students then come back together as a whole class to discuss the answers and share their thoughts. The students love it! Thank you for a clear and interesting case.