This is one of the simpler explanations I have come across to answer your question, "The two halves of each chromosome come from the two parents. One half of the "X" in each chromosome comes from the mother, and the other half comes from the father. The two halves are bonded together at the middle of the "X." Each half of the "X" contains a complete set of genes, so each chromosome has two copies of every gene -- the "dominant" gene of the two is the one that is expressed." http://science.howstuffworks.com/life/genetic/question92.htm