Balanced translocation Down syndrome

-An undergraduate from India

August 31, 2016

In something like 96% of cases, Down syndrome is a random accident. Something happens while an egg is being made and that egg gets fertilized by a sperm. The end result is an extra chromosome 21 and a child with Down syndrome. 

Or, less often, the reverse happens. There is a mix-up when a sperm gets made and this sperm then goes on to fertilize the egg.

DNA Is Packaged in Chromosomes

The instructions for making and running your body are found in long stretches of DNA called chromosomes. Most people have 23 pairs for a total of 46.

We each get one set of 23 chromosomes from mom and the other set of 23 from dad. Here is what this looks like:

Passing Down Chromosomes

When we have a child we pass down one chromosome from each of our pairs. So mom passes down one of her chromosome 1’s one of her 2’s and so on. Dad does the same. In the end the child has two pairs of 23 chromosomes for a total of 46.

Let’s look at this process in a little more detail. I’ll focus on just two pairs:  

Imagine this is mom’s two pairs of chromosomes:

How a mutation can cause a disease

-A high school student from New Jersey

August 16, 2016

To many people the word “mutation” sounds scary or magical. They might think of giant ants attacking Tokyo or the X-men.

It turns out that a mutation isn’t really any of these. It is simply a change in DNA. That’s it.

And they aren’t even all that rare. Mutations happen all the time and you have lots of them.

While not all mutations cause problems, some do. One of these is the DNA change that can lead to sickle cell anemia.

A Mutation in the Hemoglobin Gene Causes Sickle Cell Anemia

As I said, a mutation in the hemoglobin gene causes sickle cell anemia. And what a tiny mutation!

People with sickle cell anemia have a mutation in the hemoglobin that changes a GAG to a GTG.  Sounds harmless enough, but single letter changes can have a big effect.  

Look at this recipe for instance:

“Add the HAM to the soup”

Now imagine we change the “H” in ham to “Y” like so:

“Add the YAM to the soup”

Mutations Make Genetic Diversity

What’s even more amazing is that mutations are absolutely essential for life. They are where all the wonderful differences between you and me and between you and a tuna or a daisy come from. Without them life could not have started and it would not have survived.

You see, mutations are the reason why plants and animals (including people!) can adapt to a changing environment. Let’s look to moths as an example.

Mitochondrial DNA (mtDNA) in fingerprints

-A high school student from the US

August 10, 2016

DNA is often found in human fingerprints. But because there is so little of it there, scientists often need to turn to a certain kind of DNA—mitochondrial DNA (mtDNA).

While mtDNA can’t uniquely identify a human being, it can still help. For example, police can use this DNA to rule out suspects.

Fingerprint Patterns

Before going into why fingerprints have DNA, let’s go over what fingerprints are.

DNA Profiling

After the police get DNA from a crime scene, they use a process called DNA profiling to identify people. 

First, they use a method called PCR to make more copies of the DNA. Next, they can look at spots in the DNA that tend to be different between people. If someone shares a lot of the same spots with the DNA at a crime scene, then chances are good that person was there.

Nuclear DNA

Each cell has a single nucleus, which has the nuclear DNA (nuDNA). NuDNA is organized in structures called chromosomes.

In total, we have 23 pairs of chromosomes, for a total of 46. One from each pair comes from mom and the other from dad. This is why, for the most part, we have two copies of each bit of our nuDNA. 

Henry VIII Kell blood type

-A high school student from Spain

August 4, 2016

Henry VIII is famous for killing off his wives for failing to give him a son. But his wives weren’t just failing to have sons. Instead, hardly any of their children survived after the first one

By chance most of his wives and mistresses had daughters for their first child. And then, thereafter, all of the sons and daughters either were miscarried or died soon after birth.

In today’s world these babies have a better chance at survival. Doctors can often give the babies transfusions soon after birth or even before birth.

And luckily for everyone, being Kell positive is not very common. Over 90% of people are Kell negative.

Still, some kids do suffer from Kell-based HDN today. But this does not have to be forever.  

One day there may be a way to keep this form of HDN from happening. We just need to look at the Rh blood system to see how this might work.

How Kell and Rh Are Similar

Right now there is no equivalent to RhoGAM for moms who are Kell negative and are carrying a Kell positive child. In fact, it isn’t even routinely tested for when people donate blood.

This last point matters because it means that some Kell negative women are given Kell positive blood during a transfusion. Now their immune system reacts to this transfused blood making antibodies that will attack any Kell positive blood they encounter. This is how a woman’s first child might be affected (not a problem in Henry VIII’s time as there were no transfusions).

Red Hair Gene Increases Risk for Skin Cancer

The DNA in her skin will age faster than someone without red hair. (Pixabay)

July 15, 2016

Genes and obesity

-A curious adult from India

Genes don’t usually cause obesity. But they can sometimes make it harder for some people to keep a healthy weight.

Some genes can affect obesity by influencing what decisions we make. If you have a set of genes that makes you crave sugar more than your friend, you may struggle with your weight more than he does. Or if your genes make you hate exercise, you might be more likely to end up heavier than you’d like to be. 

Why Does Obesity Happen?

If being obese is unhealthy, why are our bodies able to get overweight in the first place? Why doesn’t our body choose to not take in more food once it has enough? One of the leading theories called the “thrifty gene hypothesis” tries to answer this.

Long ago, before we were farmers, food was hard to come by during some parts of the year. Long winters and/or droughts meant that people had to be ready to starve some of the time.

Are There “Obesity Genes” in Other Animals?

In lab animals such as mice, each gene can be modified in an experiment to study what they do. A gene called ob is a famous example.

Mice with mutations of the ob gene cannot make a hormone called leptin. Leptin signals to the mouse’s brain that it is full and should stop eating. Mice with copies of ob that don’t work properly can weigh up to three times as much as a healthy mouse! 

Third generation clone worse than original

-A middle school student from Italy

July 5, 2016

No, it almost certainly won’t! Depending on your “starting” cell, a clone from the clone of a clone might just be a bag of mutations on the brink of dying from cancer.

This might all seem weird since the word “clone” usually means a creature that is completely identical to the original. But it turns out that a clone isn’t really identical.

DNA and Mutations

DNA is the instructions for making a living thing. We have one head, two arms, and all the things that make us human because of these instructions. There are also instructions that keep our cells from turning cancerous, give us our eye color, and lots of other things that makes each one of us unique.

When a mutation happens, it might change these instructions. This could be a really big problem!

Shortened Ends Means Shortened Life?

Inside the cell, DNA is packed into bundles that we call chromosomes. Telomeres are pieces of DNA at the ends of chromosomes that protect the DNA from getting damaged there.

Think of telomeres as “caps” for DNA. Every time DNA gets copied, the telomere “caps” get a little bit smaller.

As you grow older, the telomeres on your DNA keep getting shorter. Once telomeres become too short, they can’t protect your DNA anymore.

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.