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.

BRCA distant relatives inheritance

-A graduate student from California

June 3, 2015

Great question! This is a bit complex to answer, but based off of the information you have given, there is a 1 in 8 or 12.5% chance that you carry the same BRCA mutation.

But really, you have either a 50% chance of getting it from your father or a 0% chance. The 1 in 8 chance essentially comes from the odds your dad carries the gene plus the odds he will pass it down to you if he has it.

One of its main jobs is to keep mistakes out of our DNA. When too many of the wrong mistakes build up, a cell becomes cancerous. Working BRCA genes can make this less likely.

Typically, we get two working copies of the BRCA gene from our parents. These genes are working and preventing cancer.

Sometimes though, we can inherit a copy from either mom or dad that has a change (or mutation) in it that causes it to stop working. Now mistakes build up faster in our DNA increasing our chances for developing cancer.

In a pedigree, squares are for boys and circles are for girls. A diamond means we do not know if you are a boy or a girl.

The arrow represents the person we are evaluating. And each row represents generations.

The top generation represents your grandparents and the middle row is for your parents, aunts, and uncles. The bottom row is for you, your siblings and your cousins.

Codominant dominant incomplete dominant traits

-A middle school student from Illinois

May 27, 2015

Great question! The answer has to do with how different genes work to give traits. And how differences in genes affect those traits.

Before we go into the why, let’s back up and go over different kinds of traits. Then, we’ll review how genes cause traits. And then finally, we’ll tackle the why of your question!

There Are Different Kinds of Traits

Now let’s say that one parent plant has white flowers and another parent plant has red flowers. This means that the parent plant with white flowers is rr. And for reasons we’ll discuss below, the parent plant with red flower is RR. When these parents are paired, the Rr seeds make plants with pink flowers!

This is an example of incomplete dominance, when the inherited alleles of a trait blend. This happens when the dominant trait is not totally dominant, and the recessive trait is not totally recessive. It’s like mixing paint: red paint + white paint = pink paint.

Codominant Traits

Now let’s tackle codominant traits. For this situation we will use the example of blood type.

There are three alleles for blood type: A, B and O. For codominant traits, we will focus on blood types A and B. (If you want to learn about how type O fits into the mix, click here).

Imagine we have one parent with type A blood and the other with type B blood. To keep things simple, we will say that the parent with type A blood is AA and the other parent with type B blood is BB.

Some people are NOT at a higher risk for autism from vaccines

-A curious adult from California

May 21, 2015

A recent study shows that this is almost certainly not the case. There doesn’t seem to be a set of genes that make some kids extra susceptible to ending up with autism from a vaccine. At least not from the one that is always implicated in autism—the MMR or measles-mumps-rubella vaccine.

Shared Genetics, Shared Risk (or Lack Thereof)

To show that vaccines predispose no one towards autism, the researchers compared families who had either one or two kids with autism. Basically they wanted to see if MMR vaccination rates had any impact on the chances for a second child to end up with autism. They didn’t.

In families with two kids with autism, it didn’t matter whether the second child was vaccinated or not. The risk for autism was the same in both cases.

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.

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