Meiosis, including crossover, definitely
happens before fertilization. It’s the process
that starts off the making of eggs or sperm
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
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
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.
Why is it so important that each gamete be different?