I was wondering how it is possible that I ended up with one ear lobe clearly attached, and the other clearly unattached. I have read all about the dominant gene process etc., but it never seems to address how you end up with one of each.
This was first noticed when I got my ears pierced at age 5, in 1978. The lady who pierced my ears was one of the only people in the area who did ear piercing, and therefore had seen many ear lobes before mine, but she had never seen one attached, and one loose or unattached. She was very surprised, and I have always wondered about it, since to this day I have never met anyone else with two different ear lobes, at least not that I noticed.
-A curious adult from Washington
I have a friend who has an attached and an unattached earlobe too. Very cool. But how in the world can something like this happen?
As you say, unattached earlobes are thought to be dominant over attached ones. So if one ear is unattached, they should both be unattached. And vice versa.
But what may have happened for you is one ear followed the dominant instructions and the other followed the recessive ones. In other words, each ear had a different set of instructions.
One way this could happen is if the DNA is different for each ear. There are a couple of ways this can happen.
One is if you are a chimera. A chimera is someone who has two groups of cells in their bodies, each with their own DNA.
The differences between the two DNAs are huge. As different as two siblings.
And this makes sense because a chimera is the result of a fusion of two siblings. What happens is that two fraternal twins fuse very early on in development*.
Now, this new combined embryo grows into a single person. Some of their cells are the DNA of one sibling and some are of another sibling.
Imagine one twin was going to have attached earlobes. And the other was going to have unattached earlobes. Fuse the two and you might end up with someone with an attached and an unattached earlobe.
This isn't the most likely explanation since chimeras are thought to be relatively rare. As of 2003, there were only 30 documented cases. But this number may be an underestimate because no comprehensive study has been done to figure out how many there actually are.
Another much more common way to end up with two different DNAs is something called mosaicism. This isn't as bad as it sounds. As we talk about at /ask/ask172, we are all most likely mosaics.
Mosaics happen when a single cell gets a DNA change very early in a fetus' development. That change is then passed down to some cells and not others. The end result is someone with two different sets of slightly different DNAs.
Imagine that as an embryo, you are growing and dividing. Suddenly, a change happens in one of your cell's DNA. That change could have come from the environment or simply been a mistake that the cell made copying its DNA.
Wherever it came from, let's say the DNA change happens in the earlobe attachment gene. And the hit changes the instructions from unattached to attached.
You now continue to grow and divide. Each of the cells divides many times to get to the 100 trillion or so cells that make up you.
Now some of your cells have instructions for attaching an earlobe and some cells have instructions for leaving it unattached. If the change happened after the embryo has started making ears but before the earlobe has attached or remained loose, then you might end up with an attached and an unattached earlobe.
But how likely is such a precise change to a certain gene? More likely than you might think if you are converting a dominant version into a recessive one.
This is because recessive genes are often versions of a gene that don't work. So all our hit has to do is garble the instructions, not change them in a precise way.
A final way to end up with two sets of instructions is similar to the mosaic. Remember, a mosaic has two sets of DNA with a small number of changes between them. These changes are at the level of the bases.
What does this mean? The instructions in our DNA are written in a language made up of 4 bases, A, G, C, and T. These 4 bases form three letter words that make up a large part of the instructions for making you.
The type of mosaic we talked about earlier changes one of these letters. For example, imagine that the ear lobe instructions are this sentence: "The old man had one new hat."
The mosaic has this intact sentence in some of his or her cells. In the other cells, a letter goes missing and you end up with: "Tho ldm anh ado nen ewh am." A bunch of gibberish that leads to an attached earlobe.
The second kind of mosaic would be one that leaves the letters intact but gets rid of the punctuation. So our sentence goes from, "The old man had one new hat" to "the old man had one new hat." In other words, it loses its capital letter.
What happens in this case is the body doesn't know a new sentence has begun so it doesn't know where to start. This is the same as the gene not being there. In our case, it would lead to attached earlobes.
Of course our genes don't really have punctuation. But what they do have is something called methylation.
Methylation hides the start of genes by adding a methyl group to the DNA. The bases are the same, but the DNA is used differently.
For methylation to work in your case, we would need it to happen in one set of cells and not in others. The capitalization would be lost only in some cells.
So there you have it. A way to end up with a dominant and a recessive trait is if you have two different sets of instructions.
Of course, we can be pretty sure there are other possible explanations as well. Perhaps some sort of trauma in the womb that causes only one earlobe to unattach. This would be similar to how some people end up with two different colored eyes (click here to learn more). But since we are a genetics site, I thought we'd focus on genetic explanations.
* This happens when the embryo is made up entirely of embryonic stem cells which is why they don't end up with four arms, two heads, etc.