If my husband contains a dominant trait such as a broad nose, is there any possibility that my child will not have a broad nose?
-A curious adult from Alabama
October 18, 2012
There are lots of ways that a child can end up not having a parent’s dominant trait. It can happen even when both parents have that same dominant trait!
For example, two brown-eyed parents can have a child with green or blue eyes. And parents with cleft chins can have a child without one. This is possible even though brown eyes and cleft chins are dominant traits.
Now having said this, I need to point out that we don’t know exactly which genes are at play for nose shape. It is almost certainly controlled by more than one gene. This usually means there isn’t any simple dominant trait which means all bets are off for your kids’ nose shapes. A wide range from broad to narrow are probably possible. (Click here and/or here for more information on these sorts of traits.)
But for the moment, let’s say that a broad nose is dominant and a single gene controls nose shape. In this case, a broad-nosed dad might have a narrow-nosed child if dad is a ‘carrier’ for a narrow nose.
To understand how this all works, we first need to step back and talk a bit about how genes get passed down. And about what it means to be dominant.
One Copy From Mom, One Copy From Dad
Like everyone else, you got one copy of most every* one of your genes from your mom and one from your dad. This means you have two copies of most of your genes.
Not all copies of a gene are the same. In other words, genes can come in different versions (or alleles as scientists like to call them). It is these different versions that cause differences in traits.
So if nose shape were due to a single gene, there might be a broad version and a narrow one. Now it is obvious what happens if you have two copies of the broad version…you have a broad nose. And the same thing with two copies of the narrow version, you end up with a narrow nose.
But what if you have one of each? That is where it matters if one version is dominant. If a broad nose is dominant, you will have a broad nose.
People who have both versions of a gene but show only the dominant trait are called carriers. It is these carriers that can have kids that lack the dominant trait. Let’s see how.
From Broad Comes Narrow
Imagine that your broad-nosed husband carries the narrow nose version of the gene. Let’s also imagine that broad- and narrow- are the only two versions of the nose shape gene. Since you don’t have a broad nose, you must have two narrow nose copies.
Now to get right at your question, what will your kids’ noses look like? Remember, everyone gets one copy of most every gene from mom and one from dad. Also, each parent has two copies of most every gene that they can pass on to their kid.
An important point here is that the copy that gets passed from a parent to a kid is passed on at random. That is, about half of a carrier’s kids should get the broad nose copy and about half should get the narrow nose copy by chance.
Now, let’s get back to thinking about what versions of the gene you and your husband have to pass on. You might already be able to see what will determine your kid’s nose shape. The picture at the right should help make it clear as well.
As you can see, you and your husband have two possible children (at least with regard to nose shape!). The two of you could have a child with a broad nose or one with a narrow nose.
Looking at the picture you can see that you passed a narrow version (n) to both your kids. This isn’t surprising since this is all you had to give!
But your husband has two different versions to pass on. He gave your daughter his dominant broad nose version (N) and she now has a broad nose like him. Your son on the other hand got your husband’s narrow nose copy (n) and now has a narrow nose like you!
Now just because he is a carrier, that doesn’t mean he has to have one child with a broad nose and one with a narrow nose. The two of you could have had two broad-nosed or two narrow-nosed kids.
This is because the particular version that gets passed down is chosen at random. So each child has a 50% chance for having a broad nose and a 50% chance for a narrow nose. (Click here for a more detailed explanation of this point.)
As I mentioned before, what determines nose shape is actually likely to be a bit more complicated than this. Multiple genes are likely involved which means a wide range of nose shapes are probably possible. But, generally, this is how you can think about how it is possible for an individual with any dominant trait to have kids that don’t have the trait.
* A notable exception to this occurs with the so-called X and Y sex chromosomes. For more information on these chromosomes, click here.
By Dan Van de Mark, Stanford University