Your genes come from your parents of course and from their parents and from their parents parents and well you get the idea. You have two sets of 23 chromosomes, one set from each parent. For almost every chromosome you inherit from your mother, you also inherit one from your father that has his versions of the same genes. Together they form a pair of homologous chromosomes. The X and Y chromosomes however are special. Usually females have two X chromosomes while males have an X and Y. You get one of your X chromosomes from your mother. Whether you get your father's X chromosome or his Y chromosome determines your sex. Most adult cells contain two sets of chromosomes. But sperm and egg cells have only one set of 23 chromosomes each. When the body forms sperm or egg cells, a cell divides and pairs of chromosomes separate. A random member of each pair moves into each new cell. This is why when you are conceived you obtained half of your mother's genes and half of your father's genes. But your siblings didn't necessarily get the same versions of your parents' genes that you did. Unless you happen to be an identical twin. To form sperm or egg cells your chromosomes double, like so. When the homologous pairs separate, sometimes they cross over. And at seemingly random points, exchange DNA. This is called genetic recombination. Because your genes get shuffled during recombination, the chromosomes you pass along to your children are not exactly the same as the ones you inherited from your parents. This makes it hard to use most of your chromosomes to trace your genealogy back very far. However, most of the Y chromosome is handed down from father to son entirely intact. Likewise, in humans, DNA in the mitochondria is passed down only from mother to child. For this reason, ancestry along your father's line or your mother's line is easier to trace using the Y chromosome or mitochondrial DNA. When sperm and egg cells join together at fertilization, they create a single cell with two complete sets of 23 chromosomes, one set from your mother and one from your father. This single cell will divide to create new cells over and over until it creates a child.