Dad gives more than Y

-An undergraduate from Oklahoma

April 15, 2015

Yes, there is genetic information passed down in ways other than through a “Y” chromosome. In fact, the Y actually contributes only a small amount.

As I’ll explain below, you and your paternal grandpa share on average just a bit less DNA than you do with your maternal grandpa. You share around 25% of your genetic information with each.

We Get One Chromosome from each Parent

As I mentioned, chromosomes come in pairs. We get one chromosome in each pair from mom and the other from dad. That’s why we are half our dad and half our mom!

Our sex chromosomes are the same. We get one from our mom and one from our dad. Girls have two X chromosomes and boys have an X and a Y.

Your dad got 50% of his DNA from his dad. Here is what that looks like:

Now when your dad has you, he passes half of his DNA to you like this:

As you can see, you share around 25% of your DNA with your dad’s dad. Half of half is one quarter.

Developmental plasticity in evolution

-A graduate student from Australia

April 1, 2015

Developmental plasticity is just another way of saying that the environment plays a big role in our traits too. We are more than just our genes.

One way the environment can affect our traits is by changing how our cells use our DNA. So in one condition a gene may be turned way up and in another it might even be off.

A Butterfly that can Change its Wing Color

Another cool example of developmental plasticity is the “squinting bush brown butterfly” (Bicyclus anynana), from eastern Africa. It is actually able to vary its wing color depending on the season!

In the warmer dry season, the butterfly is entirely brown. But in the cooler wet season, the butterfly has large eyespots on its wings.

Why does it do this? Why is this beneficial?

At this point you might be asking, OK who needs evolution anyway? Plasticity is doing just fine.

The key is that there may be a better way. Imagine that by random chance a mutation happens in one of the butterflies that gives it bigger spots. These bigger spots are even better at fooling predators and so, within a few generations, all of the butterflies have these large eyespots.

DNA shared between children of identical twins

-A curious adult from Ireland

March 25, 2015

Because you and your sister are identical twins, you pretty much have the same DNA. Not exactly but pretty darned close.

This is because you both started from the same fertilized egg. Which is why identical twins are also called monozygotic.

In this picture, each chromosome is represented by a rectangle. Dad has a dark blue chromosome and a light blue one while mom has a pink and a red one. In this case the child got a light blue one from dad and a red one from mom. The child could also have instead ended up with the dark blue and/or the pink.

If only it was that simple! Chromosomes are almost never passed down whole. Instead, parent chromosomes in each pair swap DNA with each other while forming sperm and egg cells. This process is called recombination.

Here is something closer to reality:

First Cousins vs. Half Siblings

Now let’s see what happens if you and your sister were not identical twins.  In that case you would share only half your DNA instead of all of it.  The DNA for each of you from one chromosome pair might look like this:

As you can see, the two of you no longer have matching chromosomes. Here is what happens when we add you and your sister’s partner:

Risk for second cousins having a child with a disability

-An elementary school teacher from Malta

March 11, 2015

Most likely not. In most cases second cousins are far enough apart that there is only a little increased risk.

Of course this doesn’t mean there is no risk. Everyone who has children has the risk of having a child with a disability.

The chance that a baby is born with a birth defect or disability is between 2-3%. So, if a hundred people have babies, we would expect that 2 or 3 to be born with some sort of problem. Which of course means that 97 or 98 are fine.

Our genes can tell us what color eyes to have, what color hair to have or even how tall to be. They are a big part of what makes each of us unique!

We have two copies of most every gene in our body and we inherit these genes from our parents. In fact, we actually get one copy of most of our genes from our mom and one copy from our dad. This is why we tend to look like both of our parents

As I mentioned before, genes are important for making each of us who we are. Typically, we need most of our genes to be working in our body for us to be healthy.

Relatives Share Genes

As I said above, genes are passed down to you from your parents. And your parents got their genes from their parents (your grandparents) and so on.

Because genes are passed down from person to person, everyone in a family shares some percentage of their genes with everyone else in their family. That is why you are linked to everybody in your family tree!

Risk for huntington's disease (HD)

-A curious adult from Iowa

March 4, 2015

This is a trickier question than you might think. With dominant diseases like Huntington’s Disease (HD), it is usually pretty easy to figure out risks. Generally if one parent has it then each child has a 50% chance of having it too.

Where the 50% Number Comes From

You get HD if you have a broken version of a specific gene. Since you get your genes from your parents, a common way to get the disease is to inherit a broken copy of the gene from them.

As I mentioned, HD is a dominantly inherited disease. This means that if one of your parents has the disease, you have a 50% chance of getting it from them. And if your parents don’t have the disease, you probably won’t get it. Let’s look more closely at why.

A Tangled Problem

Not all diseases are dominant like HD. Many genetic diseases only happen when both copies of a gene are broken or lost.

But HD is different. One broken copy is enough to cause the disease.

Each gene has the instructions for making a specific protein. And each protein has a specific job to do in the cell. In this case, the huntingtin gene has the instructions for making the huntingtin protein.

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: