Is the gene present on one of the homologous chromosomes transcribed or both the homologous chromosomes?
Usually it’s both that are transcribed. This is how a dominant trait can “overcome” a recessive one. For example, in a person heterozygous for cystic fibrosis, the “working” version of the chloride pump protein is made as well as the “broken” (mutated) version – but enough of the working version is around to overcome the broken one. The regulatory sequences associated with the genes are the *same* on both homologous chromosomes – so the same promoters and transcription factors will bind to both genes, and turn them both on. However – there are exceptions: Barr bodies – in a woman’s cells, one of the X chromosomes is deactivated in each cell (visible under the microscope as a “barr body”) – so only one chromosome is active in each cell. However – *which* X chromosome is randomly chosen, so 50% of the cells have each of them active, and that is enough to prevent sex-linked traits appearing in women. http://en.wikipedia.org/w
