Chimeras are not more likely to have chimeric children

-A high school student from Denmark

June 27, 2016

No, the children of chimeras are not automatically chimeras themselves. In fact, they aren’t any more likely than anyone else to have kids with chimerism.

This is because each sperm or egg will have DNA from only one of the “twins” that makes up a chimera. The DNA from both twins does not mix in a single sperm or egg cell.

There are two sets of cells in a chimera, each set with their own DNA. And only one sperm fuses with one egg.

Here I’ve colored each of the fraternal twins a different color (blue & red). So during the same cycle, two of mom’s eggs are fertilized by two different sperm.

Things go a little differently with chimeras. Instead of growing up as two separate people, you end up with one person with both sets of cells. Here is what that looks like:

So what is essentially happening is two siblings are fusing together to become one person. Or to put in another way, you have a person who has some cells from one sibling and the rest from the second.  

Whereas mixing blue and red paint makes purple paint, this is not the case with a chimera’s cells.  There are some blue cells and some red ones. And fusing the two does not make a clump of purple—each cell has DNA from its original fertilized egg (blue or red).

hypertrophic cardiomyopathy genetics

-An undergraduate from Australia

June 22, 2016

Hypertrophic cardiomyopathy or HCM is a common heart condition (about 1 in 500 people has it) that is often genetic. But how it is passed down is not so simple.

The Nuts and Bolts of HCM

One such problem is the condition you asked about, hypertrophic cardiomyopathy (HCM). Let's break down those words so we can understand what that means:

Passing Down HCM

Alright, enough ancient Greek and physiology, now back to those genes!

There have been a handful of genes found to cause HCM. They are all instructions for the different pieces that form cardiac muscle and help to keep it organized and working right. If there's a change in one (or more) of these genes then the heart muscle can get disturbed so that over time it grows thicker and thicker.

Range of shared DNA between relatives

-A curious adult from Illinois

June 15, 2016

You could definitely share less than 25% of your DNA. In fact, most nephews share a bit more or less than 25% of their DNA with their aunts or uncles.

Having said this, 16% is starting to get pretty far from 25%. This doesn’t make it impossible, just less likely than say 24% or even 20%.

The Farther Away, the Wider the Range

You got half your DNA from your mom and half from your dad. This is pretty much the only relationship that is almost always an exact number like this. (Click here for one of the rare exceptions.)

Other relationships will share an amount of DNA around an average amount. And the further you get away from a relative, the wider that range can be.

komodo dragon parthenogenesis not clone

-A curious adult from California

June 7, 2016

If humans had babies like Komodo dragons sometimes do, there'd be no need for sperm banks. Well, not unless mom wanted a little extra genetic variety from dad.  

Normally, babies get half their DNA from mom and the other half from dad. But this isn’t always the case in nature.

When 2 x ½ is Not the Same as 1

To see why this is, let’s imagine that a woman had babies by doubling the DNA in her egg like a Komodo dragon sometimes does. And that she has AB blood type.

Turns out that none of her kids would be AB like her—they’d all be A or B. Let’s see how this works.

Scattered throughout your DNA are the genes that make up the instructions for making you. Each gene has the instructions for one small part of you.

A Virgin Birth Gives Males (For Komodo Dragons)

For most animals, whether you are genetically a boy or a girl is determined by which sex chromosomes you have.

In people this is decided by the X and the Y chromosome. Genetic males have an X and a Y and genetic females have two X’s.

This is why in people, dad determines the sex of the baby. Mom can only give an X, but dad can give an X or a Y.  If baby gets X from mom and X from dad, then genetically the baby is a girl.  If baby gets X from mom and Y from dad, then the baby is a boy.

Pectus Carinatum genetics

-A graduate student from Belgium

May 31, 2016

Pectus carinatum (PC), as you probably know, is when someone has a difference in the way his or her chest wall is formed. It often looks like the chest sticks out father than is typical.

How Does Pectus Carinatum Run In Families?

There are different ways that PC can be passed on.

If someone has a genetic condition like Marfan syndrome, then we know exactly how it gets passed down. In this case, if one parent has Marfan syndrome, then each child has a 50% chance of having it too.

Scientists have even been able to figure out which gene is involved. People with Marfan syndrome often have mutations in a gene called FBN1.

So What’re The Chances My Children Will Have Pectus Carinatum?

It sounds like from your description that you have isolated PC. If this is the case then there’s a low chance that your kids would have PC too. Because it is isolated PC and not associated with a condition like Marfan syndrome, we can’t put an exact number on this chance.

Human inbreeding Neanderthal

-A curious adult from Mexico

May 24, 2016

First let’s make sure we’re on the same page when it comes to inbreeding. It basically happens when two close relatives, like an uncle and a niece, have kids together.

There has been inbreeding ever since modern humans burst onto the scene about 200,000 years ago. And inbreeding still happens today in many parts of the world.

Measuring Inbreeding

So how do people measure inbreeding? For example, how do we know if our parents (let’s call them John and Jane) are closely related?

If we have John and Jane’s family trees, we can answer this question. We look at how far we must go back in their family trees before we get to a common ancestor.

Does Inbreeding Matter?

Finally, why should we care about inbreeding? Because the children can end up with serious problems. To understand why, we need to step back and talk about genes.

Genes are stretches of DNA that each have the instructions for one small part of us.

They are like individual recipes in the cookbook that is our DNA. We have genes for eye color, blood type, and many other important traits.

Remember we have two sets of DNA, one from mom and one from dad. This means we actually have two copies of our genes too.

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: