where do your genes come from?
Chromosomes are Inherited From Your Parents
One chromosome from each of your 23 pairs came from each of your parents. The two chromosomes of a pair (except for the sex chromosomes) contain the same genes, but the genes have small differences. Things like SNPs make each copy of a gene uniquely Mom's or Dad's.
The X and Y Chromosomes Determine Your Sex
One chromosome pair - the sex chromosomes - is unique. You don't necessarily end up with a matching pair. Typically females have two X chromosomes and males have an X and a Y. Mothers always pass an X chromosome on to their children. Whether your father passes on his X chromosome (leading to a pair of X chromosomes) or his Y chromosome (making a mixed set) determines your sex.
Chromosomes Get Shuffled When Eggs and Sperm are Made
Though most adult cells contain two sets of chromosomes, sperm and egg cells are different. These special cells have just one chromosome from each pair. Which chromosome they get from each pair is random, making each sperm or egg cell unique. There is also a bit of mixing before the chromosomes are sorted into individual sperm or egg cells. Chromosomes from each pair in a mother or father, respectively, make contact and exchange pieces of DNA, creating hybrid chromosomes.
Egg and Sperm Combine to Create a Child
When a sperm and egg cell join together at fertilization they create a single cell with two complete sets of 23 chromosomes. This single cell divides to create new cells, over and over, forming the body of developing child. This is how you came to be you. If you have siblings, the same events created them. But because the egg and sperm production processes are random, your siblings didn't get the same set of chromosomes from each of your parents that you did (unless you happen to be an identical twin).
Using DNA to Trace Ancestry
Because the chromosomes mothers and fathers pass on to their children are reshuffled versions of the ones they inherited from their own parents, it is hard to use most chromosomes to trace genealogy back very far. Most of the Y chromosome, however, is handed down from father to son without changes. Likewise, in humans, the tiny bit of DNA contained in an unusual package of genetic material known as mitochondrial DNA is passed down from mother to child without any recombination. For this reason, these types of DNA can be used to trace your ancestry.