Conjoined twins cannot have the same father

-A high school student from China

September 23, 2015

First off, yes, it is possible for twins to have different fathers. This goes by the exciting name heteropaternal superfecundation. It is much more common in other animals but it can and does happen in people too.

And of course twins can be conjoined. But these twins are identical which means by definition they have the same mom and dad. In fact, they have the same DNA!

Duncle or Mont

Imagine you go to a fertility clinic and your sperm and your wife’s eggs are combined and then the fertilized eggs are implanted into your wife. Nine months later she gives birth to a beautiful baby boy.

A routine blood test on the baby shows he is AB and a red flag goes up. There isn’t any easy way to explain how you and your wife produced the baby’s blood type since you are both A. The clinic must have implanted the wrong embryos!

Who’s Your Daddy?

As you tell, things can get pretty confusing pretty quickly. While there may be a natural tendency to identify one set of DNA as mine and the other as my twin, this isn’t really how it works.

A chimera is a wonderful mix of both twins. Neither is the true self even if the cells that first got tested happened to have a certain DNA. A chimera is really a fusion of two sets of DNA that make one unique person.

This also means that each child is equally related to the chimera. One child is not more or less related.

Y-Adam and mtEve are not Biblical

-A curious adult from California

September 15, 2015

No, it is not true. Scientists can trace our maternal and paternal lines back to a woman and man who lived a long time ago, but they are not the Biblical Adam and Eve. 

Fossil evidence shows us that these two individuals were part of a whole population of our ancestors-- they were definitely not alone on Earth. And instead of living 6000 years ago as some would have us believe, our DNA suggests that these two lived more than 100,000 years ago.

What this means is we can count the changes in a piece of DNA to figure out how old it is. If we get 10 new changes (or mutations) in a generation, and there have been 100 mutations, then it has been ten generations. This is basically the idea behind the “molecular clock.”

This is only the first step though. We now have to figure out how long it took to get from Y-Adam’s DNA to the DNA we see today. For this we need to know how many new changes a Y chromosome gets per generation.  Once we have the mutation rate, we can calculate the years it took to get from Y-Adam’s sequence to those we see today.

Men and women genetic differences

-A high school student from Kansas

September 10, 2015

It isn’t quite right to say that men and women have exactly the same genes. Men in fact have several dozen genes that women don’t.

It is these few genes that cause someone to become biologically male. But they don’t work by themselves.

These genes can do so much by essentially changing how many of the rest of the genes get used. This changed “gene expression pattern” is one of the big reasons men and women have different traits.

Think about a cell like a construction team tasked with building a house. In the central office is a manager with the blueprints to make every structure in the city. The manager copies out instructions, sends them to her workers outside, and the workers build the house.

SRY: the Master Switch

Our 20,000-25,000 genes are grouped together on 23 pairs of chromosomes. 22 of these pairs are the same for men and women. We call these autosomes.

However, it’s the 23rd pair, the sex chromosomes, where we see the difference between men and women. Generally, women have two X chromosomes while men have an X and a Y. It is the Y chromosome that has the instructions that cause men and women to have different traits.

Same parents different ancestry

-A curious adult from California

September 1, 2015

It seems like brothers and sisters should have the same ancestry background. After all, they both got half their DNA from mom and half from dad.

But because of how DNA is passed on, it is possible for two siblings to have some big differences in their ancestry at the DNA level. Culturally they may each say they are “1/8th Cherokee” but at the DNA level, one may have no Cherokee DNA at all.

One way to think about this is to imagine DNA as a bunch of colored beads. Since we are interested in ancestry here, we will say that different colors mean different ancestries.

Imagine that a man from Japan marries a woman from Europe. Her DNA happens to be 100% European and his 100% Asian.

Let’s say that the European beads are red and the Asian beads are blue. Here is what this might look like:

Beads on a String

Of course, real DNA isn’t beads. But you can think of it as sort of like beads on a string. Each string of beads is a chromosome, a long stretch of connected DNA.

People typically have 23 pairs of chromosome. One from each pair comes from mom and one from dad.

Before mom passes down her chromosome to her child, her two chromosomes in a pair swap some DNA. Something like this:

Preimplantation genetic diagnosis PGD X-linked

-A graduate student from the United Arab Emirates

August 19, 2015

In most cases yes. And the way it is often done is to select only girl embryos.

To do PGD, scientists take eggs from the mother and sperm from the father and do something called in vitro fertilization, or IVF (click here to learn about IVF). During IVF, the father’s sperm will fertilize the mother’s eggs outside of the body and make lots of embryos.

Sex-Linked Diseases

One of the 23 pairs of chromosomes, the X/Y, determines a person’s sex. These are called sex chromosomes.

Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). Mutations in the X (and to a lesser extent the Y) can lead to X-linked or sex-linked diseases.

Sex-linked diseases are one of the most common kinds of genetic disorders. They are special because they can be much more common in males than females, or vice versa, depending on the disease.

Diseases Caused by Extra or Missing Chromosomes

While we usually have two copies of 23 chromosomes, this is not an absolute rule. Sometimes, embryos have just one copy or three copies of a chromosome instead of two.

Having one copy is called monosomy and having three is called a trisomy (click here to learn more about trisomies like Down’s Syndrome). Some scientists estimate that a few percent of pregnancies are like this but most of the embryos don’t survive to birth because extra or missing chromosomes are usually fatal.

Friedreichs ataxia

-A high school student from Denmark

August 12, 2015

Friedreich’s ataxia is one of those diseases that can seem to pop up out of nowhere. Everyone is fine for as long as anyone can remember and then, suddenly, there it is. Someone in the family starts to have movement problems because of nerve damage.

Two Broken Copies of the FXN Gene Causes Friedreich’s Ataxia

Our DNA has a collection of genes that each plays a part in making us who we are. Each gene has the instructions for making/running one small part of us.

For example, we have a gene that lets us digest the sugar in milk, lactose, as an adult. Another gene plays a part in deciding whether or not we’ll have brown eyes. And so on for all 20,000 or 25,000 of them.

Two Carriers Have a 1 in 4 Chance to Pass on Their Nonworking Genes

When someone has only one working copy of the FXN gene, they are called carriers. These carriers give one copy to each of their children, but they don’t get to pick which copy. This means that each child of a carrier has a 1 in 2 chance of getting a parent’s nonworking copy.

Why dogs and people can't have babies

-A curious adult from New Jersey

July 29, 2015

Good question! You’re right, humans and dogs can’t reproduce.

So you won’t get anything like Barf the Mog from Spaceballs. A half man/half dog wouldn’t get very far past a single cell (assuming the egg and sperm could even come together!).

Genomes are Passed Down From Generation to Generation

Every living thing has a genome. This genome is like an instruction manual made up of individual instructions that scientists call genes. These instructions tell us what to have and make to be humans.

Each species has a genome that is unique, which is why people are people and dogs are dogs. For example, the human genome tells us to have two arms and two legs. Dog genomes tell dogs to have four legs, fur and tails.

To make sure this order happens the right way, genes get turned on and off in different amounts and in a very specific order by things called transcription factors. Transcription factors attach to genes to turn them on and off, a little or a lot.

When genes are turned on and off has a lot to do with why we can’t make Mogs. Even though human and dog genes are similar, they are not regulated the same. In other words, they’re turned on and off to different levels and at different times. This would be a really bad thing for a half human/half dog.

Shared dad or mom with half siblings

-A curious adult from California

July 22, 2015

Yes definitely. With a 23andMe test there are at least a couple of ways to tell.

The first is to look at those famous X and Y chromosomes. If the two of you are half brother and sister and share DNA on your X chromosome, then you have a common mom. And if you don’t share DNA there, odds are you have a common dad.

When parents have kids, they each pass one of these chromosomes to their child. So moms always pass an X and dads pass either an X or a Y. If dad passes an X, the child will be a girl and if he passes a Y the child will be a boy.

You are a boy, so you got your Y chromosome from your dad and your X from your mom. Your half-sister got one X from her mom and the other from her dad.

Here is a diagram of what this would look like if you have a common dad:

Mitochondria only from Mom

Another way to check if you share a mom is to look at your mitochondrial DNA (mtDNA). This works because mtDNA is passed only from mother to child. All of dad’s mtDNA is destroyed right after fertilization.

This type of DNA is found in an ancient structure in our cells, called the mitochondria, also referred to as the “powerhouse of the cell.” It has its own DNA that helps it to function, and all humans ONLY get mitochondria from moms! (Click here to learn why it probably has its own DNA.) 

Skin color and natural selection

-A graduate student from Papua New Guinea

July 8, 2015

Even though there are lots of skin colors around now, all of our earliest ancestors had dark skin. There are a couple of reasons for the diversity we see today.

First, the instructions for making us, the genes in our DNA, aren’t written in stone. In each generation, a small number of changes happen in everyone’s DNA. So at some point someone somewhere must have had a change (or mutation) that caused his or her skin to lighten.

Darwin’s Slam-Dunk Theory

Over 150 years ago Charles Darwin spent many years studying creatures all over the world. He was spellbound by how many types of animals there were. And was puzzled by how they came to be.

In 1859 he published his now famous book called “On the Origin of Species.” In it he introduced the idea of natural selection. Natural selection explains how all the types of animals came to be.

Now let’s think about natural selection and human skin color. About 40,000 years ago, when some people moved from Africa to Northern Europe, their new home had a lot less sun. And perhaps people didn’t make enough vitamin D.

After some time, someone had a random DNA mutation that made his or her skin tone lighter. This person now had an advantage because it was easier to make more vitamin D. This led to better survival and more children for this lighter skinned person.

Crossing over Recombination within genes

-A graduate student from Arizona

June 16, 2015

The quick answer is that yes, it can. And it is one of the ways that the seemingly impossible in genetics does sometimes happen.

Let’s use hair color as an example. You may remember the Weasleys from Harry Potter. Both parents and all their kids had red hair which makes perfect sense genetically.

Now imagine they have another child but he doesn’t have red hair. Time to interrogate the mailman? Not necessarily.

Most people have 46 chromosomes that are arranged in 23 pairs.  They’re arranged in pairs because most people will get one from each pair from their mother and one from their father. 

Here is what this might look like for one of the 23 pairs:

In this picture each chromosome is a rectangle.  The child would have gotten one chromosome from his father (the dark green) and one from his mother (the dark purple).

Crossing Over Within Genes

As I said, we have two copies of each of our chromosomes (except men who have an X and a Y for their 23rd pair). What this also means is that we each have two copies of most of our genes too. One comes from mom and one from dad.

What this also means is that DNA swapping can sometimes happen within genes. Usually this isn’t a big deal but sometimes it can lead to some very interesting results.