DNA in a human ovum

Some coworkers and I had a friendly argument yesterday about the DNA in a human ovum. I had always thought that each of a woman’s ova contained the same DNA, and that there was some random aspect to the way it combined with the sperm’s DNA, thus accounting for variation among children. In other words, if two people have multiple children, it’s the same DNA combining each time, but it combines differently.

My coworkers were of the view that each of a woman’s ova contained different DNA, so that children are destined to be different even before fertilization. (I recognize that in my scenario, children are almost certainly going to have different DNA anyway; the question is when the variation arises).

Does anyone know how this works? Also, if you happen to have a link, it would help to settle the argument.

Each ovum (and each sperm, FWIW) has half the chromosomes of a human, and therefore half the DNA of the parent. The combination of the half from each parent is what creates unique DNA for each offspring.

We have two copies of each gene in a full genome, and each ovum gets only one copy of each gene. Which copy any particular egg gets, is completely random. For the purposes of illustration, let’s say someone has three gene loci and is heterozygous for each one, ie. they have two different copies of each of three genes.

If I’m AaBbCc, then my ova can be ABC, ABc, Abc, AbC, abc, aBc, abC, or aBC. That’s eight distinct genetic makeups for my eggs. Multiply that out by however many hundreds of gene loci humans have, and you’ve got endless possibilities.


Answer to the OP is that your coworkers are right. I quote from HowStuffWorks: “When your body produces sex cells (sperm or egg, depending on whether you are a male or female), your body must reduce the number of chromosomes by half to go into the sex cells. To do this, it randomly sorts chromosomes from both sets in one cell division and then reduces them by half in another. Therefore, each sperm or egg that your body produces is unique and different – it contains a different mix of your mother’s and father’s genes. This is why two brothers in the same family can look and act totally different from one another even though they come from the same parents – it all depends on which genes (chromosomes) were randomly chosen when producing the sex cells of the mother and father.” In other words, the variation in siblings can be traced to the uniqueness of each egg and sperm.

Thanks. I looked for the information online, but didn’t find it; I didn’t think of HowStuffWorks.