In my last blog entry, I wrote a quiz that tested some basic knowledge about genetics that experts have found the public struggles with. What I found from the responses I received is that the QUEST public doesn’t struggle with them or, more likely, people only answer quizzes like this if they are pretty confident in the first place. The ten respondents did very well indeed!
I also learned that I need to work on my question writing skills. In my efforts to be brief and to the point, I ended up writing the occasional question that had more than one correct answer. Of course once I realized that I scored either of the correct answers as correct.
What I’ll do below is go over each question in a bit more detail. I’ll talk about why I asked it and what the answer is.
1) Should you necessarily be scared if you find out you are 10 times more likely to get a rare cancer?
The answer to this one is no, you should not be scared. Everyone got this one right.
If 1 in a million people get a certain cancer, the increase to 1 in 100,000 is pretty minimal. It is equivalent to the difference between getting struck by lightning and dating a supermodel. Neither is at all likely!
The reason I included this question is that one of the studies suggested that this is a pretty common problem for people getting genetic counseling. They struggle with the idea of absolute vs. relative risk. In other words, the difference between how much more likely I am to get a disease compared to you versus how likely I am to get a disease period.
2) If you look like your mom, are you more likely to get the diseases that run in her family or your dad’s family?
The answer to this one is no as well. Nine out of ten respondents got this one right.
The few genes associated with looks have very little to do with disease risk. Just because you and your mom have blue eyes, this has nothing to do with whether you have your mom’s increased risk for type 2 diabetes. There are different genes associated with each and they are independent of one another.
Here is another example. Let’s say Huntington’s disease runs in your dad’s family and your dad has the version of the huntingtin gene that will lead to him getting the disease. Your chances of getting the disease are 50% whether or not you have mom’s blue eyes and blonde hair or dad’s brown eyes and hair. The disease and the traits are caused by different genes...they do not come as a package deal.
One of the exceptions would be red hair and fair skin. These traits, which are caused by certain versions of the MC1R gene, do increase a person’s risk for skin cancer.
3) If you have the recessive disease sickle cell anemia, should your kids be tested to see if they are carriers?
In the way I was thinking about the question, the answer would be no. Only one out of ten people got this right but I suspect that may have to do with how I wrote the question.
What I was trying to get at is if you have a recessive disease, then all of your kids will almost certainly be carriers of that disease. So there isn’t really any need for genetic testing.
Remember, we have two copies of each of our genes and for a recessive trait to be seen, both copies have to be the recessive version. Also remember that people pass one of their two copies of each gene to their child.
What this means is people with sickle cell anemia have to pass a version of the hemoglobin gene that can lead to sickle cell anemia down to each of their kids. All their kids will be carriers.
So a genetic test probably isn’t necessary for the kids because they are undoubtedly carriers. Now that doesn’t mean the other partner shouldn’t be tested. He or she definitely should be tested to determine his or her carrier status. This will affect their chances for having a child with sickle cell anemia.
I included this question because both the genetic counseling and the Australian study showed that the idea of dominant and recessive traits was a confusing one. This also bears out in our list of Ask a Geneticist questions.
4) If you look like your dad, did you get more of his DNA?
The answer here is a definite no. People almost always get half their DNA from their mom and half from their dad no matter who they look like. (Click here, here and here for some exceptions to this rule.) Eight out of ten people got this question right.
Again we have the issue where what we look like comes from only a few genes. Looking like your dad might mean you happened to get 20 or 50 of his genes related to physical appearance. This is out of the 25,000 or so genes we all have.
In fact, there are even some traits where you look like one parent because both parents contributed the right DNA. Let’s say you and your dad are a redhead. It is very unlikely you have red hair only because of him because red hair is a recessive trait and usually has to come from both sides of the family. In other words, mom had to pass you a red hair version of MC1R so you could look like your dad!
5) If you flip a coin and get heads ten times in a row, what are the chances that the next flip will be a head?
The answer is 50%. Each coin flip is independent so what came before cannot affect the next flip. The coin doesn’t remember on which side it landed before. Nine out of ten got this one right.
It is a very common misconception that if you flip a coin and get ten heads in a row, the next flip is more likely to be a tail (it is due!). Heck, my dad plays the craps tables in Las Vegas by looking for tables where a seven hasn’t come up in a while and places his bets at that table. This is no way to win.
In genetics this mostly becomes an issue in predicting a child’s traits based on the children that came before. If two parents have four girls, they might think that a boy is more likely next time around. And if both parents are carriers for cystic fibrosis (CF), they might feel that if they’ve already had a child with CF, the risk is decreased for the next child. After all, that Punnett square showed that 1 in 4 of their kids would have CF, right? Well, no.
The Punnett square was used to calculate the risk that each child has for getting CF. If your first child has CF, your second still has a 1 in 4 chance for getting CF. Just because your first got it, that does not affect your second child’s chances.
6) True or false: Each cell in your body has the same DNA.
OK, this is the question I messed up on big time. By trying to keep it short and sweet, I didn’t ask the right question. What I should have asked was:
A muscle and a brain cell have the same DNA. True or False
The answer to that question is “True” and we could have a nice discussion about how most of the cells in our body have the exact same DNA even though they look different. And then I’d go on to talk about how the differences come from the fact that each cell uses a different subset of genes. Sort of like following a single recipe in a cookbook to make cookies instead of following every recipe.
But alas, I asked the wrong question and most of the respondents correctly noted the exceptions to the “every cell has the same DNA” idea. They noted that sperm and eggs have half the usual amount of DNA. And that because of mutations, various cells in our bodies have small differences from one another. They also mentioned that we are actually composed of our cells plus the trillions of bacteria that we all carry. (No one mentioned mature red blood cells which have no DNA.)
I hope you enjoyed the quiz and that you’ll enjoy the book, When Will Broccoli Taste Like Chocolate?. I am happy to do more quizzes like this in the future if people are interested. Any suggestions on genetics topics?
More scary news:
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