You Have a Dominant Trait that Tends to Cause Disease. Should you Tell Your Partner?

Editors' Note: Over Memorial Day, we put out a call on Twitter and Facebook for readers to share their niggling questions about genetics using the hashtag #futureofyou. And boy, did we get some fantastic responses. Your questions ranged from wondering why spinach tasted like tinfoil to the potential of administering genetic tests to pregnant women and its possible harm.

We passed on these questions to Dr. Barry Starr, who writes regular columns for KQED about the ever-evolving field of genetics. He's selected three to tackle this week, but will return to some of your questions in upcoming posts. More on Starr and our experiment here. 

Over to you, Barry...

Question 1: "What's your responsibility as spouse if you know you've a dominant trait that tends to cause disease?

Barry Starr: That's a very tricky question! This will be a big discussion going forward as we continue to gain more and more information about our genetic make-up.

As I wrote recently, a new genetic test makes it easier than ever to see if you are at a higher risk for breast cancer. There are also diseases like Huntington’s disease that are fatal but not until later in life. Each child of a person with the gene has a 50  percent chance of having it as well. Should you notify your spouse or spouse-to-be?

After digging around the Internet, I found a fascinating study from 2011 on what people actually do with this information in dating situations. The study involved 64 people who have dominant conditions that could either affect them or their future children.

Some decided not to date at all to avoid the complication. Some opted to be straight with their partner, while others didn’t share the truth about their disease risk because they were afraid of rejection. Those who kept this information under wraps struggled with when and how to bring it up.

As a scientist, it's difficult for me to generalize about how people should act. So I turned to my colleague, Hank Greely, the director of the Center for Law and the Biosciences at Stanford, for a more thorough explanation:

"I don’t think you’ve got any legal responsibility unless you affirmatively misrepresented your status to your spouse and he or she relied on that misrepresentation in some way that caused harm,"  Greely told me.

To win a court case, Greely said there would likely need to be evidence that you didn't share the information. "I think you might need a situation where the other spouse asked you something like, “So, your father has Huntington’s disease, which I’ve read puts you at 50 percent risk.  Have you been tested?" Even in a situation like this, Greely isn't sure what the damages would amount to.

Ethically, it's a different story. "As an ethical matter, I think you need to reveal any serious health risks you’ve got to your partner before marriage, domestic partnership, or a decision to have kids together," he said.

Question #2: How can my parents and I be on the shorter side but my 2 younger brothers be 6 foot tall?

That's a question I'm sure a lot of folks are wondering! Height is a surprisingly complex genetic trait. Your final height is the result of the combination of genes you get from your parents and the environment.

Like lots of other complex traits, height is the result of many different genes working together. In the most recent tally I could find, scientists found 697 gene variants in 424 gene regions (with more sure to follow).  Each of these contributes a bit towards your final height. This makes for a lot of possible combinations! So it could be that your parents have a mix of many variants that make them shorter and a few that make them taller. By chance you ended up with more of the “shorter” versions and your brothers more of the “taller” ones.

As it turns out, around 80 percent of height is hereditary and the rest is environmental. An example of this is the huge changes in the average height of the Japanese since World War II. In the last 50 years, the average eleven year old in Japan has gained 5.5 inches. This is way too fast for genetics and is probably explained by better nutrition. Another possibility is that one or both of your parents are shorter than their DNA says because of poor nutrition growing up. They did not reach their height potential, but passed that potential on to your brothers. (You still got the short end of the genes.) More on the genetics of height here.

Question #3: How come I didn't have any red headed children? My husband has red hair, I have brown hair. Am I not a redhead gene carrier?

Most of the time redheads have red hair because they got the necessary gene from both parents, a classic recessive trait. Since your husband has red hair, we know he passed a red hair gene to his kids. Because red hair is usually recessive, most likely that is all he has to give.

If you do not have a red hair gene, then your kids obviously can’t get one from you. They will get one red from your husband and one not-red from you. They will all be carriers because of your husband.

They will not have red hair but can pass a gene for red hair down to their kids. If their partners also pass one down, you’ll have red-headed grandkids.

Another possibility has to do with simple statistics. In this case you are a carrier but you just happened not to pass the red hair gene down to any of your kids. Each child of a carrier has a 50 percent chance of getting the “red hair” gene from that parent. This does not mean, however, that 50 percent of the carrier’s kids will get it. All the kids might get it or none of them.

Note that red hair is also not a perfectly recessive trait. It is way more predictable than tongue rolling, ear lobe attachment, widow’s peak, hitchhiker thumb and so on, but there are still plenty of exceptions. A study from 2007 showed that a surprising number of people with two red hair genes do not have red hair. So it might be the case that your children did indeed get a red hair gene from you but still didn’t end up with red hair.  More on red hair genetics here.