heterozygous homozygous genetic test

-A high school student from Massachusetts

October 27, 2017

There is genetic testing for this sort of thing. A good company is 23andMe. Another is AncestryDNA when it is combined with an online program called Promethease.

Unfortunately, genetics isn’t as cut and dried as it is often taught in school.

This means there are three possible combinations for people:

  • RR
  • Rr
  • rr

It is pretty obvious what color hair you would predict for the homozygous combinations, the ones with two of the same type of MC1R gene. RR people should not have red hair while rr people should have red hair.

But what about the heterozygote? The people who are Rr?

Because red hair is recessive, Rr people should not have red hair. The R dominates the r keeping red hair at bay.

Here is a table of data from a few years ago from 23andMe:


As you can see, it isn’t like in the textbook! Genetics is marvelously messy.

So there are tests that can tell you about your genes and which versions (or alleles) you have but they may not be as informative as you’d like. You may be heterozygous for an important gene but that will not tell you the whole story.

You might predict brown eyes but have blue instead. Or green or a multitude of other colors.

Odds for a color blind child

-A curious adult from Maryland

October 18, 2017

Each of their sons has around a 50% chance for being color blind. Most likely none of their daughters would be (although there is a chance the sons of these daughters, the grandsons, could be).

This is because both daughters are carriers. They have the bit of DNA that can lead to color blindness but they are not color blind themselves.

The reason they aren’t color blind but their sons are at risk has to do with where that bit of DNA is and how it works.

To make things easier to follow, I have color coded the X’s. So mom has a blue one that can lead to color blindness and a black one that can’t. And dad has an orange one that can’t lead to color blindness.

Remember that when they have kids, each parent will only pass down one chromosome from their pair.

Let’s say that mom happened to pass down her blue X and dad passed down his Y. Here is the combination their son would have:

Every Genetic Combination

Here is a table with all the possible outcomes for the most common form of color blindness. It is a big table: 

(Click here for a larger version of the table.)

Scientists Repair a Gene in a Human Embryo. Or Did They?

(Max Pixel)

October 6, 2017

Scientists from Oregon recently reported that they had fixed a gene in human embryos. Click here to get the details on how they did it and why it is such a big deal.

Dangers of converting ancestry data to health data

-A curious adult from Australia

September 28, 2017

This does sound scary but if you use sunscreen and try to stay out of the sun you should be fine. This particular DNA difference (or SNP) is linked to lighter eye, hair, and skin color. So what this test basically told you was that you have a DNA change that leads to lighter skin color.

And having lighter skin color, of course, increases your risk for skin cancer which includes melanoma. So that is where your melanoma risk comes from.

Worldwide around 1.2% of people has these two A’s and, like the one you brought up, it is much more common in Europeans and much less common in Africans. Around 0.3% of Africans and 4.6% of Europeans has these two A’s that can lead to red hair.

Another thing to keep in mind is that these risks are not always a for sure thing. Plenty of people with the same DNA won’t get the disease and plenty of people without it will. Your DNA just makes it more or less likely.

This becomes clear in your case. Imagine someone who has a G, G at rs1393350. They are at a genetically lower risk for getting melanoma compared to your A, A at the same SNP.

But if you wear sunscreen and he doesn’t, you may be at a lower risk. What we do can mean more than what our DNA says.

Cancer immunotherapy

-A curious adult from California

September 19, 2017

This is a big question! Cancer immunotherapy can cover a lot of different approaches.

What I’ll focus on are those that get a patient’s immune system able to see a cancer cell for the awful thing it is. This is not easy.

Our immune system is great at dealing with bacteria and viruses. It can quickly tell that these invaders are foreign and kill them off.

Unmasking the Villain

As I said earlier, cancers have found ways to hide from the immune system. One approach is to give patients something that gets rid of whatever trick the cancer cell is using. Then the immune system can destroy the cancer.

Homing in on the Right Cells

Besides getting around cancer’s defenses, another approach is to train the immune system to go after the cancer cells. There are a couple of approaches.

One of the most exciting right now is called CAR-T or “Chimeric Antigen Receptor T” Cell Therapy. I promise to only use CAR-T from now on!

The idea here is to get the immune system to only attack the kind of cell that has gone cancerous. To do this, you need to take immune cells out of a patient, engineer them, and then put them back in.

Duchenne muscular dystrophy carrier

-A curious adult from New York

September 11, 2017

The answer depends on whether or not that someone is a brother or a sister.

Brothers usually can’t be carriers. But they are at a far higher risk of ending up with Duchenne muscular dystrophy (DMD) than are their sisters. In this case they have a 50% chance.

Sisters on the other hand are very unlikely to end up with DMD. Instead they typically have a 50% or 1 in 2 chance of being a carrier.

Mom is a Carrier

As I said, usually only women can be carriers for DMD. So in this case, since dad does not have the disease, we will say mom is the carrier. She passed it down to her daughter, the sister in your question.

Let’s draw out mom’s two X chromosomes this way:


Green eyed parents brown eyed child

-A curious adult from California

August 30, 2017

As your case shows, it can definitely happen. And it is surprisingly common too.

But while we know it can and does happen, figuring out the how has proved to be tricky.

It turns out that eye color genetics is not simple. Not by a long shot.

So HERC2 can come in either a “T” or a “C” version, OCA2 in either an “A” or a “G”, and so on. (Scientists call each version an allele.)

OK, getting tricky but still just manageable. We are only dealing with something like 64 different possibilities.

Except that we aren’t.

It turns out that like most of the rest of our genes, we have two copies of each of these eye color genes too. This means that each of us has three possibilities for each gene instead of just two.

Green + Green = Brown Example

Let’s imagine the following parent:

So he has a T copy of HERC2 and a C copy of HERC2, an A copy of OCA2 and a G copy of OCA2 and so on through all six genes.

IrisPlex predicts that this person has a pretty good chance for green eyes (24.4%). Or as they put it, intermediate colored eyes which is pretty much everything except brown and blue.

Sisters not in same ancestry genetic community

-A curious adult from Georgia

August 24, 2017

No it does not. In fact, it is not uncommon for two siblings to be in separate genetic communities.

At first this might seem weird given that two sisters are about as closely related as any two people can be—50% or so on average. How can they not be in the same genetic community?

There are a couple of reasons.

A 50-50 Chance

First off let’s talk about how only one sister can get the bit of DNA that links her to a genetic community.

To do this we will imagine that mom is a member of this genetic community. She has a bit of DNA that is linked to the genetic community.

Let’s represent mom’s DNA like this:

A little explanation is in order.

From Chromosome to Sliver

Next let’s figure out where that little bit of blue at the end of the chromosome came from. As you’ll see, it came from an all blue chromosome from that genetic community.

Let’s say this is the original ancestor you are tracing your genetic community back to:

Note all of the chromosomes are blue. Now let’s have her have children with someone outside of the community:

AB and O blood type parents having A and B children

-A curious adult from California

August 16, 2017

Believe it or not, in this case having a blood type different from either parent is by far the most common result. In most cases, an O parent and an AB parent will have only A or B kids.

It is only very rarely that they might have an AB or an O child (see the links at the end for these exceptions). Isn’t genetics fun!

Three versions

As humans, we all have the same basic set of genes. What makes you different from me is that we have different versions of some of our genes.

So there isn’t a blue and a brown eye gene for example. Instead there is a gene that comes in a brown and a blue version. (Well, that is a simplification. It actually comes in a brown and a not-brown version.)

The ABO gene comes in three versions: A, B, and O.

Four Blood Types

One Shall Pass

From the previous section we can see that you have an A and a B and that your partner has two O’s. Maybe something like this:

(Adapted from Pixabay image)

(I made the mom AB and the dad OO here but it works the other way too.)

CRISPR human embryo

-A curious adult from California

August 8, 2017

It is a big deal but probably not as big as some media outlets would have you believe (or believe themselves).

It is big because this is the first time that scientists have used the gene editing tool CRISPR/Cas9 to specifically change a gene in a human embryo here in the US. This is a very big step.

Gene Editing in an Embryo

In the past, the CRISPR gene editing system has needed three things to edit a gene:

Two Copies: More than Twice as Hard?

We have two copies of most of our genes. We get one from mom and one from dad.

In this study, they worked on a disease where only one of the copies needs to be damaged to cause a disease. This is called a “dominant” genetic disease.

In this case, Cas9 cut the damaged copy but left the working one alone. The embryo the used its second working copy to fix its broken gene. It ignored the DNA the scientists added.