My sister married my husband's brother (so we have two brothers who married two sisters). We are all curious about how related our children will be. (I'm pregnant with our first child right now.)

I understand siblings share 50% of the same genes. In our case would the first cousins be as genetically related as siblings? If no - can you tell us exactly how related they will be?

-A curious adult from Canada

February 24, 2005

What a fun question. And really hard too.

The quick answer is that I think the kids would end up being somewhere between siblings and first cousins. This wasn't as easy to figure out as it might seem...

First off, no DNA can pass through a generation unscathed. Even if your sister and you were identical twins, there would be at least 100 differences between her and your DNA.

We all build up mutations in our DNA over time. Most of these DNA changes are harmless although some can lead to diseases like cancer.

Where do these changes come from? Some come from the stuff our body does everyday. For example, we all start out with a single cell and end up with somewhere around 50 or 100 trillion cells.

The DNA in all of these cells needed to be copied (not 100 trillion times but a lot). The machinery in our cells that copies our DNA is incredibly good at what it does, but not perfect. Occasionally, it makes a mistake that is not fixed.

Our DNA also changes in response to things like sunlight or the food we eat. Both can damage the DNA causing mistakes to happen.

If any of these changes happen in an egg or sperm cell, then that change will be passed on. This would mean you and your sister's kids would be just a bit different.

Of course, 100 changes out of 3 billion isn't much. Are there any other differences?

Sometimes when a sperm or an egg gets made, a gene can get broken or fixed. This can result in a new trait not seen before in the family. How might this work?

If we think about you and your sister, you each got half your DNA from your mom and half from your dad. What does that really mean, though?

Remember, almost all of our genes are found on 23 different chromosomes. We each have 2 copies of each of these chromosomes (except for males who have an X and a Y instead of two X's). This also means that we have 2 copies of nearly all of our genes.

When an egg or a sperm gets made, each of the two chromosomes is randomly sorted so that the sperm and egg only get 1 of each chromosome. Does this mean that you have mom or dad's exact same chromosome?

No. We are all much different than that. Before the chromosomes get sorted out, there is a lot of mixing and matching between two of the same chromosomes. This is called recombination.

For example, your chromosome 5 is not an exact copy of mom or dad's chromosome 5. The chromosome 5 your dad gave you is a mix of both copies of his chromosome 5 -- it isn't exactly the same as either one by itself.

Sometimes this mixing can change genes so that new traits arise. Let's think about blue eyes as an example.

Imagine dad has blue eyes because of a mutation at the front end of one copy of his brown eye color gene and a different mutation at the back end of the other copy of the gene. Each gene has a single mutation but at different places in the gene.

Now imagine that when his sperm is being made, the middle part of the eye color gene is switched between the two genes resulting in one brown eye gene and one blue eye gene with two mutations. Now dad can produce a brown-eyed child. (For other examples, see /ask/ask29)

Again, this is most likely a pretty minor difference. Are there any big differences?

This was the hard part. It helped me to think about you and your sister each having two kids. Would the true brother and sister be more related than the first cousins? The answer, I think, is yes.

You got a random mix of genes from your mom and dad and so did your sister. The same is true for your husband and his brother.

What this means is that your kids will get a subset of genes from their grandparents. Your nieces and nephews will get a different subset.

So, your kids will be created from a different set of genes than will your nieces and nephews. This suggests to me that your kids will be more closely related to each other than to their cousins. But maybe not by much...

And I haven't even started on how the environment can affect DNA and what changes might come from that! Tough question, but it really got me to thinking about all of this. Thanks!

by Dr. Barry Starr, Stanford University

Recombination can change genes