Meiosis, including crossover, definitely happens before fertilization. It’s the process that starts off the making of eggs or sperm (gametes).

Here’s the deal. We each get (roughly) half our DNA from each of our parents. Each parent gives us a full set of 23 chromosomes, so we have 23 PAIRS of chromosomes (2 of chromosome #1, 2 of chromosome #2, etc.). This is what we mean when we say we’re diploid. All of our cells that aren’t gametes have that full set of 23 pair of chromosomes, half from Mom, half from Dad. Meiosis takes a diploid cell and splits it into haploid cells, which only have 23 chromosomes, not 23 pairs. The haploid cells become the gametes.

When I talk about this for my college students, I bring in two cookbooks that started off identical, but are clearly used. They were published in 1960. My analogy is that you got a cookbook from each of your parents. They both contain recipes for the same things. BUT, the recipe in one book may have been changed over time. So the recipe for “mushroom spuds” may say to bake for an hour and 15 minutes in one book, but maybe some damage makes it say one hour in the other book. In the same way, the “recipe” from one parent may code for an earlobe that is attached to your head, while the “recipe” from the other parent codes for a free-moving earlobe.

In my analogy, the cookbook is a set of 23 chromosomes. The recipes are like genes because genes are basically recipes for proteins. Two different variations of the same recipe are like 2 alleles, or different varieties of the same gene.

When the time comes for a female’s body to make cells that will become eggs, or a male’s body to make cells that will become sperm, the DNA has to be cut in half. Otherwise, it would be like a mother giving the child her 2 cookbooks and the father giving 2 cookbooks. The baby would have 4 cookbooks (and the number would double every generation). That doesn’t work. Even having one extra chromosome causes some big problems.

Meiosis is the process of taking the two cookbooks that an individual has in a cell, and dividing them between two cells. But we don’t pass along the entire set of chromosomes from our mom into one cell and the entire set of chromosomes from our dad into the other.

A chapter would be like a chromosome, because it has a set of recipes. Ignore crossover for a minute, and let’s say that we have 23 chapters in each cookbook and want to randomly put each chapter into one of two binders. That would be like dividing up the one diploid cell into 2 haploid cells. The haploid cells will have 23 individual chromosomes. Maybe 11 were from the person’s dad and 12 from their mom in one cell and the opposite in the other. Since it’s random, it’s possible that one cell might get all 23 chromosomes from their mom, while the other cell got 23 from their dad. So each haploid cell is likely to be different from other ones. In biology, this is independent assortment.

I have to stretch my analogy here a bit, but pretend that each recipe is on a different page. Now, before I divide that diploid cell into two haploid ones, I set the chapters from Mom and Dad next to each other and swap some recipes between them. They both still have a recipe for mushroom spuds, but the one from Dad may be in the chapter from Mom (and vice versa). That’s what happens in crossover. It’s another process that makes each egg cell a female makes different from every other egg cell she makes. For a male, every sperm he makes is different from every other sperm he makes.

In real life, things are more complicated. Before the cells divide, their DNA replicates, so it actually takes two divisions, but they’re haploid after the first division. The two divisions lead to 4 sperm cells. Egg cells give all the cytoplasm (cytosol + organelles) to the embryo, so they don’t divide evenly. Each division, one cell takes all the cytoplasm and the other is just a throwaway nucleus, leaving one egg cell. That’s probably enough complication for today.

So the DNA that is being divided when gametes form came from the parents of the individual making the gametes. The mixing is the shuffling and dividing of the DNA that person got from their own parents. Meiosis takes a diploid cell and divides it to make haploid gametes that contain a mix of information from each of that person’s parents.

Cells divide and reproduce in two ways: mitosis and meiosis. Mitosis results in two identical daughter cells, whereas meiosis results in four slightly different daughter cells. We will be talking about meiosis, which occurs before fertilization.

During meiosis, one cell (from your mom or your dad) divides into two identical daughter cells like mitosis, but then those two cells divide again to create four daughter cells. Meiosis is also different from mitosis in that the cells that are created have HALF the chromosomes (DNA) as the parent cell, whereas in mitosis the daughter cells have the same amount of DNA as the parent cells.

When a daughter cell has half the DNA, it’s called haploid (23 chromosomes), and when it has the same amount, it is called diploid (46 chromosomes). So at the end of meiosis, 4 haploid (23C) daughter cells are created, in the form of either sperm or egg cells. Since these cells have only half the DNA, when a sperm and egg combine, they are restored to the diploid number of 46 chromosomes and we call that a “zygote”. After that, the zygote divides by mitosis over and over again, creating daughter cells that have all 46 chromosomes and over time this develops into a multicellular human!

So to answer your questions, meiosis occurs before fertilization. The DNA comes from the cells of your parents. Your mom and your dad each undergo meiosis and create their 4 haploid cells. During all that splitting, the chromosomes share some DNA so that each of the 4 daughter cells is slightly different. But when the final sperm and the final egg cells combine to form a zygote, half of the DNA that makes you up is from your dad, and half is from your mom. I’ll attach a diagram of meiosis below so you can see how the chromosomes cross over and share DNA and also what the end product haploid cells look like.

Here is the meiosis diagram.

The cell undergoing meiosis duplicates its genome before undergoing meiosis, and during crossing over, the chromosomes making the exchange swap their DNA with each-other. Since one chromosome came from the mother and the other from the father, this mixes up the DNA that came from each parent, thereby ensuring Mendel's law of independent assortment even for linked genes.

Meiosis happens before the egg and sperm are formed. There are 4 copies of each gene in our cells - 2 from our mothers and 2 from our fathers. These genes are on the chromosomes we get from our mothers and fathers - 2 copies of each gene from each parent. Here - this picture describes it!

The 4 copies of each gene, 2 from our mother and 2 from our father, and how the 4 chromosomes pair up and cross over.