Age from DNA

-A curious adult from California

February 25, 2015

Until recently, scientists could not look at someone's DNA and figure out how old they were. But that all changed in 2013.

Genes Can be On or Off

Our DNA has the instructions to make us who we are. These instructions come in the form of genes. Each gene has the instructions for one small part of us.

Every cell has the same full set of instructions for building and running a person. But of course, not every cell looks and acts the same. A nerve cell is very different from a muscle cell!

As you age, your body does not need all of its genes to be on all the time. As genes are no longer needed, epigenetic marks appear to turn these genes off. Because these genes are turned off at the same time for most people, these marks tend to appear at around the same time for everyone.

So, for example, when you turn 20 you might get certain marks and when you turn 30, you might get certain other marks. Because of this, most  people in their early 20’s will have the same marks as each other just as most people in their late 50’s will have all the same marks as each other.

Chimera DNA paternity test

-A curious adult in California

February 13, 2015

We at Ask a Geneticist get hundreds of questions from people from all over the world each month. Occasionally we get one where the story of finding the right answer will be helpful for other people to know. This is one of those questions.

Test Results

For the test we went with a company called 23andMe. It isn’t the only test out there (ancestryDNA is another one) but it is the one I know best.

The 23andMe test looks at hundreds of thousands or even a million different markers instead of the usual tens of them. This means the company can match whole segments of DNA instead of just looking at isolated bits of it. In essence they can compare huge regions of each chromosome and look for matches. (Click here to learn more about these tests.)


Chimeras are really fraternal twins who fused together very early in development. The fusion happens early enough that there aren't any issues with being conjoined or having extra limbs or anything. A chimera is simply a single person made up of cells from two siblings.

Cancer cells without telomerase

As you can see here, chromosomes do get shorter every time a cell divides. This is why cells can only divide so many times. Too many and they lose key genes and die.

This shortening is a real problem once we get to sperm and egg cells. These cells need to have long telomeres so that the eventual baby can have telomeres of a usable length. Otherwise the human race would die out pretty quickly!

Telomerase Adds DNA to the Ends of Chromosomes

As I said earlier, chromosomes lose a bit at each end each time they are copied.   A cell prevents this slow decay of its DNA ends with telomerase.

Telomerase is an enzyme that adds short DNA sequences to the end of DNA. Unlike most enzymes, telomerase is more than just protein. It also has a bit of RNA too. 

Telomerase and Cancer

Many kinds of cancers have mutations that turn on telomerase all the time. Unlike in a normal cell, once cancer cells get telomerase on, they never turn it off. Instead the enzyme just keeps adding more and more repeats to the telomeres.

Now the cancer cell can keep dividing without losing DNA and genes at the ends of the chromosomes. Turning on telomerase is critical for keeping these cells going. Good for the cell, bad for the patient.   

genetics of mosquito attraction

-A middle school student from California

January 23, 2015

Since you decided to Ask A Geneticist about this itchy question, you probably think that genetics plays a role in determining which people mosquitoes like best. And you’re totally right!

Scientists have discovered lots of reasons why some people are particularly tasty to mosquitoes. Many of them are influenced by genetics.

There are a few different ways that your personal, genetically-encoded smell, attracts mosquitoes. But believe it or not, they aren’t all due to your particular set of genes.

One of those ways is your HLA genes. HLA, or Human Leukocyte Antigen, genes are involved in the immune system.  They can also influence how you smell.  It turns out that some HLA genes make scents that mosquitoes like more than others. 

Preventing Mosquito Bites

Other than spraying some DEET, which repels mosquitoes really well, there are a few ways to get fewer mosquito bites. Of course, changing your genes isn’t one of them.

One good thing to know about mosquito attraction is that it’s relative. So you can have some genes that make mosquitoes like you a lot, but if you stand next to an exercising, pregnant women with Type O blood and a black shirt, you’ll probably be fine. Seek these people out as friends.

Double cousins share one quarter of their DNA

-A high school student from the U.K.

January 13, 2015

Double cousin is the usual term that is used. But you are also as related as half-sisters.

Instead of the usual 12.5% of DNA that first cousins share, the two of you share around 25% of your DNA. This is the same amount that you would share with a grandparent, a half sibling or an aunt or uncle.

Let’s get into a discussion of why the two of you share 25% of your DNA. 

Double Cousin

Now let’s expand things out a bit to include grandparents:

As you can see, a grandchild gets around 25% of her DNA from each grandparent. So she has ¼ dark pink, ¼ light pink, ¼ dark blue and ¼ light blue.

And her parents got 50% of their DNA from each of their parents. So mom has half light blue and half light pink and dad has half dark blue and half dark pink.

DNA is Stored as Chromosomes

The first thing we need to do is to give each person two rectangles of different colors. These are supposed to represent that our DNA is organized into chromosomes.

This is still too simple as we each have 23 pairs of chromosomes but it is a start. You can just take what we learn from the one pair I will be showing and expand it to the other 22 pairs. (Well, to be a stickler, 21 of the pairs. The X and Y pair in males acts differently.)

OK so here is what someone will look like now:

Relationship cousin equals great uncle

-A curious adult from Oklahoma

December 16, 2014

At their simplest, these genetic tests look at how much DNA two people share. On average, for reasons discussed here, first cousins share 12.5% of their DNA. This is usually (although not always) too little DNA for an uncle to share with a niece or nephew. On average they would share around 25% of their DNA.

We can take this information and use it to rule in or out certain possibilities. Imagine this family tree: 

Men are represented by squares and women with circles. Since we are only following the Y chromosome, that is the only DNA I am showing.

In this case if you and the tested man share a Y chromosome, then he may be a great uncle on your dad’s side of the family. If not, then he is not that great uncle.

Grandma’s Brother

In the previous examples I pointed out how you couldn’t use the Y to trace your mother’s brother back to you. But you might be able to use a different bit of DNA, the mitochondrial DNA (mtDNA).

This type of DNA is passed from mother to children. What this means is that you have none of your dad’s side of the family in terms of mtDNA.

Here is an example where you could check to see if the tested man was your mom’s brother:

Brown eyed parents blue eyed kids

-A high school teacher from California

December 3, 2014

A lot of different colors can lurk behind someone’s brown eyes. This is true even if an eye color like blue hasn’t been seen in a family for generations. The blue eyes could still be hiding there, waiting for the right time to appear.

Three Eye Colors, Two Genes

For something seemingly so simple, eye color is surprisingly complex. Even the simplest models of eye color that try to explain just brown, green, and blue need two genes.

The first gene determines whether or not you will have brown eyes. If it says have brown, then the other gene doesn’t matter. You will have brown eyes.

If it says not to have brown eyes, then the second gene kicks in. This gene will determine whether you will have green or blue eyes.

Two Genes, Four Copies

Getting back to our card example, let’s say that instead of one of each type of card, we have two. So for the first gene you could have two browns, a brown and a clear or two clears. And for the second you could have two green, a green and a blue, or two blues.

Again the brown and clear genes are always on top of the green and blue. But one difference is that now we say that green is always on top of blue.

Bringing It Back to Genes

Up until now I have been talking about cards but of course they are just a representation of genes. We have two eye color genes.

Gene 1 comes in two versions, brown and not brown. And gene 2 comes in two versions, green and blue. Geneticists call different versions “alleles.”

Here are our examples again with genes added:

African vs European albinism

-An undergraduate from the Netherlands

November 12, 2014

When many people think “albino,” they think of an animal or a person with perfectly white skin and hair and bright red eyes*. In this form of albinism, the pigments that lead to darker hair, eyes, and skin do not get made. No pigment means no pigmentation, which means no color.

If this were the only form of albinism out there, then what you describe would be very weird. But it turns out there are many other kinds, some of which can make some pigment.

Some of these genes work one after the other to build melanin up from the amino acid tyrosine. Still other genes work to create the right environment in the melanosome, the special place inside cells where melanin is made.

You can think of making melanin like making something in a factory. Say, donuts.

Some of the genes are like the assembly line; cutting, cooking, and glazing the donuts. Others provide the lights, air conditioning, power, and so on, so the conditions are right for the assembly line to work well and make lots of donuts.

Albinism is Rare

People from all over the world can have albinism. Overall, about 1 in 17,000 people have one type or another.

About 1 in 40,000 people have OCA1. However, people of African descent are less likely to have this form.

OCA2 is the most common type of albinism. About 1 in 36,000 Americans have OCA2. It’s much more prevalent in African Americans: about 1 in 10,000. In some parts of Africa it can be as high as 1 in 3,900 people.

The Person You Describe Probably has OCA2

Triallelic diseases and inheritance

-A curious adult from Australia

November 6, 2014

Tri-allelic inheritance is really a complicated way to say that more than one gene is involved in a condition.  It is a bit more complicated than a simple, single gene, recessive trait but it isn’t too bad.

In a classic recessive condition, you need to get a bad version (or allele) of a particular gene from each of your parents. So to end up with cystic fibrosis, you usually need to get a broken CFTR gene from mom and a broken one from dad.

Let’s look at sickle cell anemia as an example. There are two versions of the hemoglobin gene—one, S, does not lead to sickle cell and the other, s, can. People with sickle cell anemia have two copies of the sickle cell allele (s) of the hemoglobin gene.

Let’s imagine two parents who are carriers for sickle cell anemia. This means they have one copy of the hemoglobin gene that can cause sickle cell anemia and one that does not. They are both Ss.

Bardet-Biedl Syndrome

To make things a bit simpler, we will focus on just two of the key genes in Bardet-Biedl syndrome, BBS2 and BBS6. Just like with sickle cell, these genes come in two alleles. One allele, B, does not lead to the disease while the other, b, sometimes does.

Codominant traits in people

-A middle school student from Colorado

October 30,2014

There are definitely codominant traits in people.  But having two different colored eyes is not one of them.  This heterochromia happens for different reasons (click here to learn more).

Genes Make Proteins

When we look at a cookbook to make a dish, we don’t eat the paper recipe. We take the instructions, gather the ingredients and create that tasty burrito.

Like recipes, genes can’t really do anything as a piece of DNA. A cell needs to read the gene and make proteins that can go out and do something in a cell. So it is the proteins (whose instructions are in genes) that cause the traits we see.

Sickle-Cell Anemia: Dominant, Recessive, and Codominant!

One thing to keep in mind is that dominance is only important in how it affects the trait. Terms like recessive, dominant, codominant, and incomplete dominance all refer to the trait (phenotype), not the set of genes we have (genotype).

The sickle cell version of the hemoglobin gene is a great example of this idea. As you can see below, depending on what trait we look at, the same allele can be dominant, recessive, or codominant: