Biology: Concepts and Investigations 2nd Edition by Mariëlle Hoefnagels

Exercise 7

Huntington disease is caused by a faulty gene on chromosome 4. Before researchers discovered the actual gene in 1993, people at risk for the disease were tested for the presence of a nearby "marker" on chromosome 4. The marker was not the gene itself, but it reliably predicted who would develop the disease later in life. Sequences farther away from the disease gene, however, were not good predictors. How do the events of crossing over explain this observation? Create a sketch to accompany your answer.