Making viruses that attack other viruses

-A high school student from Ghana

May 17, 2016

We can’t make a virus that works like our immune system. But we just might be able to make one that works a bit like the immune system in some bacteria. This system is called CRISPR.

Bacteria basically use a very simple immune system that finds the DNA of viruses and cuts it into pieces. When the viral DNA is cut, the virus can’t make new viruses. It is dead.

The guide RNA travels through the cell looking for a DNA that has those 18 bases. To get the CRISPR/Cas system working, the DNA and RNA must match at all 18 bases (click here to see how this matching works). If they don’t match, the RNA will keep looking. 

This is important because if the RNA matches any DNA in the bacteria or patient, then CRISPR/Cas will cut that DNA too. This can mess up the bacterial or patient’s DNA causing problems.

Using Viruses to Cure Diseases

In some ways, a form of killer viruses are being used in real life!

For some diseases, such as sickle cell anemia or cystic fibrosis, we know that there is just one gene that causes all of the problems. It has a mistake in it that we can’t fix (at least not right now).

What if we could get a fixed copy of that gene into the cells that need it?  Scientists are working on this right now and are using viruses to deliver the fixed gene.

Mom and daughter do not share same relatives

-A curious adult from Colorado

May 12, 2016

You can be related on a family tree but see no shared DNA. (BroderickFlickr)

In this image, each pair (except for the X and the Y) are represented by a long rectangle with a pinch in the middle. So each rectangle is actually a representation of a pair of chromosomes.   

The DNA I share with this relative is that little blue box on chromosome 7. That is all of the shared DNA that this test can see.

Since this is a light blue box, this means that the DNA is only on one of the chromosome 7’s in this pair. This is what they are trying to tell you with the term “half-identical.”

We Get Half Our DNA from Mom (and Half from Dad)

As I said, for the most part, our chromosomes come in pairs. One from each pair comes from mom and the other comes from dad.

This means that when we have kids, we pass only one from each pair to our child. And the one the child gets is chosen at random.

Let’s imagine that your mom has the same pair of chromosomes that I showed earlier:

Why CRISPR is revolutionary and how it works

May 4, 2016

-A curious adult from California

CRISPR is a game changing DNA editing tool that has already revolutionized biology and may be set to do the same for medicine. It is a game changer.

What makes it such a breakthrough is how easy it is to program it to go to a specific spot in a cell’s DNA (its genome). This is quite a feat given that the right spot is somewhere in the six feet of 6 billion microscopic DNA letters scrunched together inside of a microscopic cell.

Getting There

DNA is made up of four bases: A, G, C, and T. The instructions for you are written with a long string of 6 billion or so of these four bases.

CRISPR relies on something Watson and Crick figured out back in 1953—A always pairs with T and G with C. This underpins why a cell can easily copy its DNA when it gets ready to divide, how a cell reads its genes and lots of other aspects of how a cell works.

Getting More Efficient

If CRISPR could do this in most cells, we’d be able to cure a whole lot of genetic diseases. But it can’t.

It takes real effort to get 5% of cells to have their DNA changed in the right way. And when that happens, often many more cells have damaged DNA instead of edited DNA.

So the next steps are really to get more cells with changed DNA and fewer cells with damaged DNA.

half siblings with common dad DNA sharing

-A curious adult from Illinois

April 22, 2016

You are more likely to have more DNA in common with your half-sisters than with your half-brother. But this is not a for sure thing. It is definitely possible that you happen to share more DNA with your half-brother.

The usual satisfying genetics answer right?

It turns out that half-siblings share 25% of their DNA on average. But this is only an average.

X Really Does Mark the Spot

First let’s go over the easy part—why half-sisters with a shared dad have more DNA in common on average than does a half-sister and half-brother.

For this we are going to just focus on the X and Y chromosomes. Here is an image that tries to show what each parent’s X and Y chromosome pairs look like:

Mixing and Matching

In the last image, each parent passed down a whole chromosome. So mom 1 passed her green chromosome down to her son and so on.

What this means, of course, is that the brother and sister, your half-siblings, end up sharing no DNA on their X chromosomes because she got the black X and he got the green one.  Reality is a bit more complicated and is more like this:

Typos in the Genetic Code: How Much Should They Matter?

Unlike this engine, there are some seemingly important parts in our DNA that some of us are fine without. (Wikimedia Commons)

Mosaicism with X and Y

- A curious adult from Florida


April 6, 2016


Your chromosome arrangement or karyotype is indeed rare enough that it doesn’t have a name. One study said that there have only been 10 or so reported cases. This is really rare!

Greenwood Genetic Center

Chromosomes come in pairs because we get one of each from our mom, and one of each from our dad. Sometimes, though, someone can have three copies of a chromosome instead of two.

Here's an example of what one of these kinds of karyotypes might look like:


Sometimes people have cells with different sets of chromosomes. Some of their cells have one set and the rest have a second set. This is called mosaicism.

This is what you are describing. Some of the cells are 47,XXY (Klinefelter syndrome) while some others are 46XX, the karyotype of a typical female.

Genetics does not match eye color

-A curious adult from California

March 30, 2016

You have hit on a big problem in this new genetic age. While we can easily take a peek at our DNA, we don’t always understand what it means.

See, the two SNPs they use are a big deal in deciding between green and blue in people with non-brown eyes. But they are not the whole story. Many, many SNPs play a role in determining your eye color.

How SNPs Affect How a Gene Works

So how do we know that certain SNPs relate to different color eyes? Scientists can look for relationships between the letters someone has and the traits they have.

For instance, maybe they see that people with a G at that position usually have blue eyes. And maybe they see that people with a T at that position usually have green eyes.

Very often we don’t know why a SNP causes a trait. We just know that people with that SNP for some reason have that trait.

Genes Don’t Work in a Vacuum

As you know from perusing the 23andMe site, there is more than one SNP that affects eye color. And these SNPs can be in different genes.

All the genes that affect eye color aren’t just working by themselves. They can all affect each other too.

This makes it hard to make predictions even with known SNPs/genes. Throw in a bunch of unknown SNPs that can affect eye color too and you begin to see what we are up against with predicting even something as seemingly simple as eye color.

Which X chromosome from mom

- A curious adult from Oklahoma

March 23, 2016

Believe it or not, a woman actually passes some of each of her X’s to her kids. And each child gets different parts of each X.

So whether you are a boy or a girl, you got some of your maternal grandma’s X and some of your maternal grandpa’s X. You have a unique combination of the two of them.

DNA is made up of units of information called genes. We have genes for hair color, eye color, and so on. If DNA is the recipe book for life, you can think of chromosomes as the pages, and genes as the individual instructions.

Although the two chromosomes in a pair both code for the same genes, they are also slightly different. After all, not everybody makes, say, brownies the same way – someone may use nuts, while another person might not.

Also, the daughter’s and son’s X’s don’t have to be mirror images of each other. I just drew them this way for simplicity. There are many different combinations of the mom’s X chromosomes they can receive!

That’s because there are almost an infinite number of ways that mom’s two X’s can swap DNA. And this is true for the other 22 chromosome pairs in both mom and dad.

A Weakened Gene Pool

In the Last Mimzy, a scientist uses time travel to get good, undamaged DNA from the past. In this film the DNA damage is caused by the environment. Something similar might actually happen in the future but ironically, it may be caused by, among other things, good healthcare.  

Risk for parents who are first cousins once removed

-A graduate student from India

March 15, 2016

The more closely related two people are, the more likely their kids will end up with some sort of genetic problem. Since the two of you are first cousins, once removed, that means your chances of having a child with a genetic problem are higher.

But as discussed here, this higher chance for cousins isn’t that bad. Yes, there is a higher chance but it is still relatively small. And for you, the chance is even a bit less since you are actually first cousins, once removed.

Different Diseases, Different Gene Combinations

So, genetic diseases often happen when someone has a version of a gene that doesn’t work. For example, people with cystic fibrosis have a CFTR gene that can’t do its job.

Remember how we have two copies of each gene?

Sometimes it takes just one nonworking gene to cause a problem. These are called dominant traits or conditions.

Autosomal Recessive Inheritance

As you may remember, recessive means both copies of a gene have to not work to have a disease. If this was the case here, then your cousin would have two nonworking copies.

But since he just contributes one of his copies, his child would also need to get a copy from the mom. If mom doesn’t have the condition, then the only way this could happen would be if she was a carrier. Carriers have one nonworking copy, but since they also have one working copy, they don’t have the disease.