Why is an arm an arm? Why is a leg a leg?

Every organism has a unique body pattern. Although specialized body structures, such as arms and legs, may be similar in makeup (for example, both are made of muscle and bone), their shapes and details are different. During growth of the embryo, arms and legs develop differently due to the actions of special genes, called homeotic genes, which specify how structures develop in different segments of the body.

• How did scientists discover genes that determine body pattern?
  
  
• Many organisms have similar sets of homeotic genes
  
  
• Homeotic gene organization is conserved through evolution
  
  
• Mutations in mammalian homeotic genes
  
  
• Homeotic gene mutations may play a role in evolutionary change
  
  

Funding provided by a Howard Hughes Medical Institute Precollege Science Education Initiative for Biomedical Research Institutions Award (Grants 51000125, 51000176)