First cousin parents not as risky as previously thought

-A curious adult from California

May 14, 2015

Turns out it is at least partly because our DNA isn’t as messed up as we thought. There are fewer deadly diseases lurking in our genome.

It was always thought that each of us has 5-10 potentially fatal diseases in our DNA. Even if this were true, it isn’t as bad as it sounds.

Mom + Dad = Genetic Disease

A gene is a piece of DNA that has the instructions for one small part of us. Sometimes the gene ends up with a mistake (or mutation) that can affect those instructions. If the gene is an important one, then that mutation might lead to a disease.

Game Changers

One way to increase the odds of both parents sharing the same gene version that leads to a disease is if they have a similar background. For example, around 1 in 29 people of European descent (“whites”) are carriers for cystic fibrosis while only 1 in 65 African Americans are.

Repairing a broken MC1R gene

-A curious adult from Poland

May 7, 2015

Hi curious adult from Poland! I’m sorry to say the short answer to your question is no we can’t (at least not right now). Sunscreen and the right clothing are still your best bet this summer.

We can’t really fix the broken MC1R gene responsible for your red hair (and fair skin) for a couple of reasons. First off, we are not very good at fixing broken genes yet.

Strategies in Gene Therapy

There are really three distinct ways gene therapy could work. First, we could completely remove the broken gene. This works well if the broken gene is doing something new that it shouldn’t.

This doesn’t seem to be the case for red hair and fair skin. Here the MC1R gene is simply broken, and failing to do something it should.

Removing a gene like this would be like removing a flat tire from a car. Yes the flat is gone, but the car still won’t move!

Cutting-and-Pasting With Your Genes

The third strategy in gene therapy would be to replace the broken gene with a fixed version. Essentially changing the flat tire on the car.

Very recently, there has been a lot of excitement in genetics about a new technology called CRISPR/Cas9. A group in China recently tried to use this system to fix a gene in nonviable human embryos. (Nonviable means they don’t grow past the very earliest part of development).

Four color blind sons

-A curious adult from Texas

April 29, 2015

The information that you found is correct. There is almost certainly a 50% chance that any one of your sons will be affected by red-green colorblindness.

But this doesn’t mean that if you have four sons, two will be colorblind. This is just the most likely result. All or none of your sons being colorblind is possible too.

Odds of a Child Being Colorblind

Let’s refer to an X chromosome with a broken gene as Xc and an unaffected X chromosome as just X. Since you’re an unaffected carrier we know your chromosomes are X/Xc, and since the father is unaffected, he is X/Y. This description of your genetic makeup is called a genotype.

When you have children, it is completely random which genes you pass on. Gregor Mendel figured this out in the earliest description of genes way back in the mid-18th century.

A Matter of Statistics

So let’s imagine flipping four coins. The most likely scenario is that you get two heads and two tails. However, some of the time you’ll get heads three times and tails one time, and once in a while you’ll happen to get all four heads or all four tails.

For each flip, the chance of getting heads is 1 in 2 or 1/2. To find the odds of several events happening together, you multiply the odds of the individual events. So for four heads in a row it’s:

1/2 x 1/2 x 1/2 x 1/2 = 1/16 = 6.25%

Children of cloned parents not identical twins

-An undergraduate from Bangladesh

April 23, 2015

Whoa, that sounds like a pretty complicated family! Or maybe it isn’t…

Let’s break this down to make it a little easier to understand: There’s a husband and a wife, named A and B. They have a child named C (I know, not a very creative family).

But then you clone A and B to make A’ and B’, who have exactly the same DNA as A and B. They then go on to have their own baby, C’. Here’s a little family tree to (hopefully) make it a bit clearer:

DNA Gets “Mixed and Matched” Before You’re Even Born

Each person is created from a unique combination of DNA from their mother’s egg and their father’s sperm. Each sperm and egg donates half of the parent’s DNA to the child. The child then ends up with two different copies of a complete set of DNA (or a genome) – one from their mother, and one from their father.

The parents also each have 2 copies of the genome – one from each of their parents. But how does the body decide which copy to pass on to the next generation?

DNA is Not Everything

Let’s go back to the cloning part for a bit. Earlier, I said that A and B are exactly like their clones, A’ and B’. But is that strictly true? Yes, they have the same DNA, but it turns out that genetics is not as straightforward as that.

Which, of course, if you’ve ever met identical twins, you already kind of know. They do look an awful lot alike, but not exactly. There are differences that you can see once you look for them.

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.

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