I looked high and low for a simple explanation of inheritance, but found only sites full of technical jargon, so I decided to write my own as my sister explained it to me.

1^st, You have 23 pairs of chromosomes that make up your DNA and make you uniquely you. One of those chromosomes comes from your father in his sperm and the other from your mother in her egg.

2^nd, Your parents each had 23 pairs of chromosomes from their parents, your grandparents. So the trick is to understand how their pairs of chromosomes became the one that you inherited from each of them.

3^rd, The scientific term for this process is called meiosis (click here). My sister explained that in the egg, for each of your 23 chromosomes, the two paired chromosomes that your mother inherited from her mother and father lie side by side and attach to each other in places (called cross over (cick here)). The same is true in your father's sperm. Later in the fertility process, those paired chromosomes separate but most of the resultant chromosomes contain segments from both your mother's mother and your mother's father, starting and ending at these "cross over" spots. These cross over spots are random, and so is the length of the segments that are created. Still later in the process, the cell divides into two eggs, each of which has only one set of 23 chromosomes, instead of a pair. Now the egg is ready to mate with a sperm cell to create a human. The sperm cell goes through the same process of lying side by side, attaching, splitting and splitting again into sperm cells that have only one set of 23 chromosomes.

4^th, Because the cross over attachment spots are random and the lengths of the segments are random, you will not inherit exactly ¼ of your DNA from each grandparent. In fact that seldom happens. Similarly you will not inherit 1/8^th of your DNA from each great-grandparent. In fact you might get quite a lot of DNA from one great-grandparent and not much from another.

Because this is all random, your sibling or your cousins might have inherited quite a lot of DNA from a great-grandparent from whom you received very little. And that is why it is so very important to have everyone tested who is willing to do a DNA test.

Do note that if your grandparents are willing to do a DNA test, you don't need to test any other of their children or grandchildren.