Chapter 13, Problem 38P

Down syndrome is usually caused by having a complete extra copy of chromosome 21. The syndrome encompasses many different traits including low IQ, heart disease, characteristic facial features, gastrointestinal tract abnormalities, short stature, poor muscle tone, and increased risk of leukemia and dementia. Some people diagnosed with Down syndrome display only a subset of these anomalies. These people often have one normal chromosome 21 and one chromosome 21 containing a duplication; the region of chromosome 21 that is duplicated can differ among individuals.

Some scientists think that one critical region of chromosome 21 exists (containing one or a few genes) that is responsible for Down syndrome. Researchers have used individuals with duplication Down syndrome to test this idea. Suppose that using genome sequencing, scientists determined which parts of chromosome 21 were present in three copies in eight individuals with duplication Down syndrome. A summary of the data is shown in the table that follows. An X indicates that the given individual is affected by the particular abnormality listed in the column heading.

 

a.	Do the data support or refute the hypothesis that chromosome 21 contains a single Down syndrome critical region? Explain.
b.	Do the data support the idea that each Down syndrome abnormality is caused by overexpression of a different region of chromosome 21? Explain.