Chromosomal crossover happens every time a human makes a cell that can become an egg or sperm cell. Its one of the two major things that make every sperm cell different from every other sperm cell, and make an egg cell different from every other egg cell. Chromosomal crossover is not a form of mutation. It does not make us any less human. Its just a way of shuffling our human genes around so that each person has a different set of genes (unless they are an identical twin).

We each have 23 pair of chromosomes. One set we got from our mom, the other set we got from our dad. If a person passed on only the dad chromosomes to one cell, and only the mom set to the other cell in meiosis, they would end up making only two different kinds of eggs or sperm. So two people could only make 4 different types of kids. Some of their kids would be genetically identical. If nothing in the world changed, there would be no problem with that. However, the world does change. Diseases are mutating all the time, the climate can change, and other species change. If all of our kids were pretty much alike, something that was harmful to one would probably be harmful to all. Shuffling the genes means that if things get bad, a set of parents would have a better chance that at least one of their kids would do well.

Here are the two ways things get shuffled. One is shuffling the chromosomes. We can look it this literally. Take a deck of cards. Put all of the black cards in one pile and the red ones in another pile. Now set aside 3 of the cards from each pile. You now have 23 red cards and 23 black cards. These are standing in for your chromosomes. You have 23 pair of them. One set you got from your mom, the other set you got from your dad. You will pass on one chromosome of each pair to any future children you have. The question is, which ones?In the first cell division in meiosis, the cell with 23 pair of chromosomes divides so that the chromosome #1 you got from your dad goes into one cell, while the chromosome #1 you got from your mom goes into the other cell. This happens with every pair of chromosomes, but which one goes into which cell is completely random. The #4 from mom may go into the same cell as the #1 from mom, but it may not. This gives us many possible combinations, just like taking your pile of red and black cards and randomly dividing them between two new piles. You still have the same cards, they are just divided differently. We call this independent assortment, and it is the other thing that makes every egg or sperm different from every other one that a person makes.

The second process is the crossover that you asked about. This means that chromosomes of the same type, for example chromosome #5, swap pieces of each other. So the chromosome from Dad actually has a piece of chromosome from Mom, and the chromosome from Mom has a piece of the chromosome from Dad. This would be like putting a red 5 on top of a black five, cutting off a big corner of the stack of 2 cards, swapping the corners and taping them onto the opposite color card. (I dont suggest doing this to your cards, just think about it.) Now think of how many possible combinations you could make. Would you ever have two children that were genetically identical?

Human beings won't ever "stop" from chromosomal crossover.Chromosomal crossover is how new genetic diversity is introduced -- this is why children don't look exactly like either one of their parents, but have some traits from one parent and some from the other. Everyone is a mixture of traits inherited from their parents. If anything, chromosomal crossover ensures that human beings "continue" living for a long time. For example, a certain disease/flu/cold doesn't always make everyone very sick -- there are often some people who don't get very sick. If people did not have any diversity and were all clones of each other (and everyone was the same), then there could be one single disease that could make everyone very sick, and potentially kill them. Because we are so diverse, this doesn't happen, and over time this diversity leads to a healthier, more disease-resistant population because the people living are descendants of people who all survived the particular disease. For more on chromosomal crossover, check out this Wikipedia article:

And here's a good website on genetic diversity:

genetic-diversity

Chromosomal crossovers occur during prophase I during meiosis, which produces games. After the chromosomes duplicate in inter phase S and take on looking like "X"s, the pairs of chromosomes (one from the mother, one from the father) get close enough to each other that parts overlap. It is at these overlapped parts that pieces of the chromosomes can switch from the mother to the father chromosome and vice versa.

There is no rule or requirement that a certain number of crossovers have to occur, it is just a result of what portions of the chromosomes overlap. Each of the 23 pairs of chromosomes may have multiple crossovers or none may occur.