Chapter 7, Problem 36P

In Problem 24, you learned that Bloom syndrome is an autosomal recessive disease characterized by the high frequency of harlequin chromosomes (as detected after growth in BrdU). These chromosomes are caused by high levels of chromosome breakage followed by repair through homologous recombination. In some patients, every cell has many harlequin chromosomes. In other patients, the majority of cells have many harlequin chromosomes, but about 10% of the cells surprisingly have none.

a.	What kinds of events produce the 10% of the cells in certain Bloom syndrome patients with no harlequin chromosomes? (Hint: Think about recombination.) What does the existence of these cells lacking harlequin chromosomes say about the alleles of the Bloom syndrome gene carried by these patients?
b.	In what way do Bloom syndrome patients of both classes reflect the results of complementation tests?
c.	Why does it make sense that the events you described in part (a) might occur in Bloom syndrome patients?
d.	What is different about the events you described in part (a) from the events that give rise to harlequin chromosomes?
e.	Could the events you described in part (a) occur during G1 of the cell cycle? During G2?
f.	The events that give rise to the cells without harlequin chromosomes are very rare, occurring in less than one in a million cell divisions even in Bloom syndrome patients. Surprisingly, however, roughly 10% of the cells in certain patients lack harlequin chromosomes. How can these two statements be true simultaneously?