|MadSci Network: Genetics|
Dear Awal Thanks for asking these questions. At one level, the answers are simple. In the human body there are two copies of almost every gene in almost every cell. These are located, as you say, on the two sister chromosomes. Of course that means that cells that have only one set of chromosomes, the sperm and egg cells, have only a single copy of each gene. Sometimes versions of a gene may be present on other chromosomes, but strictly speaking these are separate genes, and they will generally be on both members of the pair of chromosomes. Every gene is not present in every cell. Red blood cells, for example, contain no nucleus and therefore no genes. I hope that answers your questions. You might like to look here for an introduction to very basic genetics, which might provide more information. Best wishes Jeremy Cherfas
Try the links in the MadSci Library for more information on Genetics.