Chapter 19, Problem 36P

It is crucial to the development of Drosophila that the Hox genes must not only be on where they should be expressed, but also that they must be off where they should not be expressed. Thus, in addition to enhancers, the Hox genes are controlled by silencers—elements like enhancers that bind only repressors. This fact was discovered by zygotic mutations in about 30 so-called Polycomb group (PcG) genes. Embryos homozygous for loss-of-function mutations in any one of the PcG genes have a similar mutant phenotype: As seen in the accompanying figure, parasegments whose identities are controlled by BX-C genes are all transformed into the most posterior abdominal parasegment (PS13):

a.	What types of proteins do you think that the PcG genes encode?
b.	Explain the mutant phenotype of the embryos lacking PcG proteins..