Commonly Asked Questions

Over the years we have been asked many questions. Here are some of the most common we receive.

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DNA Basics
Basic genetic principles that may help you understand more complex questions

What are genes?
Genes are small units of DNA, which contain instructions for how to make proteins. A chromosome contains many genes. 

What is "junk DNA"?
Only 1% of DNA is made up of genes. The rest -- previously called "junk DNA" -- is noncoding DNA that is very important for turning genes on and off.

How much information is in our DNA?
A lot! Your DNA would fill up around 100 encyclopedia volumes.

What are mutations?
Mutations are permanent changes to the DNA. While some mutations cause disease, many other mutations do not impact health.

What is recombination?
Recombination is an important process that can help repair broken DNA, and help shuffle the DNA when making eggs and sperm.

What does dominant vs recessive mean?
dominant trait can overpower and hide another (the "recessive" trait). The dominant trait is not always the most common one -- how common a trait is has to do with how many copies of that DNA are present in a population. There are many reasons why a particular trait is dominant.

How do scientists figure out what gene controls a trait?
Scientists compare DNA from people (or animals!) with different versions of the trait, to figure out what piece of DNA is correlated with the trait. For an example, read about how scientists figured out a key gene in eye color!

How can tests like 23andMe link genetic markers to a disease?
Scientists can compare DNA from people who have a disease and people who don't. With enough people, it is possible to see what pieces of the DNA are associated with the disease. This is called an "association study."

Why can't genetic tests predict certain diseases, like type 2 diabetes?
Some disease are hard to predict because they are caused by a lot of different genes along with environmental influence. These are known as "complex diseases."

How does radiation damage DNA?
UV radiation in sunlight can cause mutations.

Why don't identical twins look the same?
Identical twins look slightly different, even though they have identical DNA. Small differences in their environment also make a difference. This includes things like diet, exercise, sleep, sun exposure, and more!



Blood Types
The genetics of blood type and how it is inherited

What are the most common blood types of a child from any two parents?

Can two positive parents have a negative-type child? 
Yes, absolutely! Rh factor determines positive/negative. Negative blood type is recessive, so it is common for positive-type parents to have negative-type children.

How does blood type work?
There is one gene that determines the ABO part of blood type. This gene comes in three versions -- A, B, and O. Each of our two copies of this gene can be different versions. Click here to learn more.

Why do Rh- mothers have pregnancy risks?
"Rh incompatibility" is when Rh- mothers carry an Rh+ baby. If untreated, it can lead to severe risks, but it is preventable with the medicine RhoGAM. If you are pregnant, or planning to have a child, talk to your doctor about blood type incompatibility.

What about other blood type pregnancy risks?
In general, incompatibility of ABO blood type does not cause major pregnancy problems. Most commonly, an incompatible blood type will result in mild jaundice. Rarely, it will lead to more severe complications. If you are pregnant, or planning to have a child, talk to your doctor about blood type incompatibility. Incompatible blood types include:

  • O mothers carrying an A, B, or AB baby
  • B mothers carrying an A or AB baby 
  • A mothers carrying a B or AB baby

What does it mean to be a "secretor"?
If you’re a secretor, it means that your ABO blood type (A, B, AB, or O) is not only in your blood, but also in other body fluids like saliva and mucus. Around 80% of people are secretors. Click here to learn more. 


While blood type inheritance almost always follows the rules, there can be exceptions.
If the table says you can't have the blood type that you have, don't jump to any conclusions! There are documented examples where these rules are broken.

Can two O parents have a non-O child?
This is possible with rare mutationschimerism, or the Bombay blood group.

Can an AB parent have an O baby?
This is possible if one parent is a chimera or has the cis-AB blood type.

Can an O parent have an AB child?
This is possible with rare mutationschimerism, the Bombay blood group, or if the non-O parent has the cis-AB blood type.

Can two A parents have an AB child? 
This is possible if one parent is a chimera.

Can two B parents have an AB child?
This is possible if one parent is a chimera.

Can two B parents have an A child?
This is possible if one parent is a chimera.

Can two A parents have a B child?
This is possible if one parent is a chimera.

Can an A and an O parent have a B child?
This is possible if one parent is a chimera, or if the O parent has the Bombay blood group.

Can a B and an O parent have an A child?
This is possible with rare mutationschimerism, or if the O parent has the Bombay blood group.

Can two Rh negative parents have an Rh positive child?
Yes, this can happen if the Rh test was inaccurate. Sometimes people who are Rh positive show up as negative in the test.



Eye Color
The genetics of eye color and how it is inherited

(Image from Pixabay)

What eye color will my children have?
It's actually hard to predict! While this simple model of eye color genetics can be helpful, eye color is actually influenced by many different genes.

Can brown-eyed parents have a blue-eyed baby?
Absolutely! Blue eyes are generally recessive, and can pop up in unexpected places. Learn more about a simple model of eye color genetics. 

Can blue-eyed parents have a brown-eyed baby?
Yes! It's more complicated than the "simple model" mentioned above. Eye color is actually influenced by many different genes.

What causes heterochromia (two different color eyes)?
Heterochromia is usually caused by quirks during development, eye injuries, or disease, not by genetics. Since it isn't genetic, this trait is usually not passed down to a child.

I have an unusual eye color. What causes that?
Eye color is complicated! There are lots of genes that influence it, as well as environmental effects. Learn more about the complexity of eye color here.

Why are blue eyes uncommon?
Blue eyes only occurred in the ancestors of a few populations, including Europeans. In the past, blue-eyed people usually married other people with blue eyes ... and then had blue-eyed babies! But as more people travel and immigrate to other countries, people with blue eyes are more likely to marry people with other eye colors. Because of this, blue eyes are becoming less common.

Why do blonde hair and blue eyes tend to go together?
Hair and eye color are both a result of how much pigmentation a person has. Since they are influenced by the same genes, hair and eye color are often linked. However, since many genes affect hair color and eye color, it is possible to get all sorts of different combinations!




Hair Color
The genetics of hair and how it is inherited

Can two brunette parents have a blond child?
Yes! Blond hair can hide in your DNA and pop up unexpectedly.

Can two blond parents have a brunette child?
Yes! Even though we often talk about brown hair being dominant to blond, hair color genetics are complicated.

What are my chances of having a red haired child?
Click here to learn what the chances are if one parent has red hair and the other has a red haired parent, here to learn the chances if one parent is a redhead and the other has a red haired grandparent, and here to learn how it is possible to have a red haired child even if neither parent has red hair!

Is it possible to end up with red hair by getting the red hair gene from just one parent?
Yes! Even though we talk about recessive MC1R mutations causing red hair, as with many things in genetics it is sometimes more comlicated than this. Click here to learn more.

Why do blonde hair and blue eyes tend to go together?
Hair and eye color are both a result of how much pigmentation a person has. Since they are influenced by the same genes, hair and eye color are often linked. However, since many genes affect hair color and eye color, it is possible to get all sorts of different combinations!

Why is it uncommon to have a red hair and brown eyes?
Hair and eye color are often linked. More than that, mutations for lighter coloration happened most often in the ancestors of Europeans. So the redhead mutation originally occurred in a group that had a high rate of light eyes (read more here). As more mixing between populations occurs, redheads with brown eyes may become more common!

Why does hair color sometimes change with age?
Hair color can change as genes turn on and off! 

Could my hair get curlier as I grow up?
Hair texture can change with age, as genes turn on or off. 

Will red hair eventually go extinct?
No, red hair is here to stay!



Other Traits
The genetics of human traits

Why don't I look like my parent?
Physical appearance is complex. Most children aren't mini versions of their parents ... and sometimes they look quite different! 

Is vision genetic? Can you inherit nearsightedness or farsightedness?
Vision is complicated! Genetics do play a role, but environmental effects matter a lot too. 

Is a person born gay, or is it a learned behaviour?
Orientation not a choice for most people, and it is not possible to "unlearn" your sexual orientation.

Is bisexuality something that is taught, or it is something that is natural in a person's genetics such as being gay or straight? 
We don't know much about the genetics of bisexuality, but it is also almost certainly not a choice.

Is intelligence more of a genetic or environmental trait?
The IQ of a child is very unpredictable. Intelligence is a complex trait, influenced by many different genes and the environment.

Why am I the only colorblind person in my family? 
Colorblindness is an X-linked recessive trait, which causes it to be more common in men than women. Since it is recessive, it can skip generations.

Everyone else in my family has attached earlobes. Why are mine hanging?
Earlobe attachment is a more complicated trait than we were taught in school! 

Do redheads need more anasthesia?
Maybe, but the jury is still out on this question.

Twins run in my family. Am I more likely to have twins?
Maybe! The "risk" for twins comes from the mom, as a woman who is a fraternal twin is more likely to have twins herself. If dad is a fraternal twin, it does not seem to influence the chances ... though his daughter may be more likely to have twins!




Questions about DNA tests and predictions of how related you are to someone else

How do paternity tests work?
Click here to learn what the markers mean, how they are inherited, and why it's easy to detect parent/child relationships.

What is the best test to tell if two people are related?
Ancestry tests will give you the clearest answer. These tests are very sensitive, and able to detect even distant relatives! While paternity tests are very accurate for parent/child relationships, they do not give you as much information. Sibling tests work in the same way as paternity tests, and are not as good as AncestryDNA or 23andMe at detecting different relationship types. 

Why don't I look like my parent?
Physical appearance is complex. Most children aren't mini versions of their parents ... and sometimes they look quite different! 

What's the difference between shared centimorgans and shared DNA segments?
A shared segment is a piece of DNA that is shared between two individuals. The length of a segment is reported in centimorgans, which is a unit of measurement. Read more about centimorgans vs segments.

How can I tell if my sibling and I share both parents, or just one?
While both full and half siblings share DNA, full siblings will have stretches of completely identical DNA that is easily detected by ancestry tests. The DNA of full vs half siblings looks different in 23andMe. You will also get an easily interpreted result from AncestryDNA, though it is easist to load your data into GEDmatch to compare sibling types.

Why do the results from my genetic test give me an inexact percentage?
While you would share on average 50% of your DNA with a full sibling (or 25% with an aunt, 12.5% with a cousin, etc), the actual amount of shared DNA will vary. Check out this interactive tool for shared DNA to understand what the range of shared DNA may be: Shared cM Project

Why do I share 50% of my DNA with a full-sibling, but only 25% with a half sibling?
Click here to learn about heredity, how much DNA you inherit from each parent, and how that relates to % relatedness.

How is it possible that siblings share 50% DNA, while humans and chimpanzees are 98% alike?
There are many ways to compare two sets of DNA! All humans are actually very, very similar at the DNA level. There are only a handful of differences between any two humans -- so sometimes it's easiest to compare what percent of those spots are different between two people.

Can you calculate shared DNA in more complicated families? Like ones that intermarry, or with incest?
While the actual amount of shared DNA will vary, you can calculate the theoretical amount of shared DNA for any relative type. This article walks through an example of how to do that.

Is it possible to detect incest in a genetic test?
It may be possible, but you probably won't be able to say conclusively unless you look at the parents' DNA as well. 

What genetic information does a father pass on? 
You get half of all your DNA from your dad! Dads pass on much more than just a Y chromosome.

What are the risks in marrying your cousin?
The risk in marrying a relative is lower than many people think. Children of first cousins are are only 2x more likely to be born with a birth defect than children of non-related individuals. And the risk for children of second cousins is similar to that of unrelated individuals.



Are you confused by your 23andMe or results?

How do ancestry tests work?
Ancestry tests compare your DNA to a set of people with known ancestries. Your ancestry is then assigned based on how similar you are to those people. But this means they are limited by who is in the database! If no one from Croatia is in the reference data, ancestry from there will not be detected. Read more on how ancestry tests work.

My ancestry percentage doesn’t make sense! Why did I get this number?
The ancestry DNA percentages aren't completely solid. There are a few things to keep in mind:

  • Instead of viewing it as "I'm 25% Italian", you should think of it as just saying that you have Italian ancestry. 
  • It can be difficult for these test to distinguish between closely related groups, so your "Italian" ancestry might just show up as "Southern European."
  • Even though you may have an ancestor from Italy, they might have had Greek, African, or Spanish ancestry themselves.
  • A very small percent value may not indicate true ancestry.
  • Your DNA ancestry does not need to define your cultural ancestry. 

Why don’t my sibling and I have the same ancestry results?
Siblings often have slightly different results. Even though you both have 50% of your DNA from mom and 50% from dad, it may not be the same 50%.

Why don’t identical twins have the same ancestry results?
Even people with the same DNA can get different results! This is due to how the algorithms in the genetic tests define ancestry.

Why are mitochondrial DNA and the Y chromosome good for detecting distant ancestry?
Mitochondial and Y DNA are inherited differently than other types of DNA, and do not change very much between generations. This makes them particularly useful for detecting ancient ancestry.



Other cool stuff
Things that don't fit neatly into the other categories.

Can I cure my disease with gene therapy?
Unfortunately, probably not. Gene therapy is an experimental technique that is still being studied. While there are promising results coming out of clinical trials, they are not widely available yet. If you are interested in participating in a clinical trial, talk to your doctor or genetic counselor about how to participate. You can also search for clinical trials online at, a service of the National Institutes of Health.

Summer is coming and I would like to get rid of my red hair and fair skin. Is there any way to fix my broken MC1R gene?
Gene therapy for appearance would be very cool, but we can't do this (yet).

What is CRISPR and why is it so great?
CRISPR/Cas9 is a revolutionary genome editing tool that is changing modern biology. Click here to read an older article on how hard it used to be!

Are genetically modified foods bad for me?
Genetically modified foods are generally safe, and labeling foods as GMOs might not be a great idea.

Are all humans the same subspecies?
Yes! All modern humans are very, very similar. Our DNA is 99.9% identical!

Would it be possible to bring back Neanderthals?
It might be possible to bring back Neanderthals, though it would be quite challenging. "De-extinction" is a hot topic right now in genetics, and we may be able to bring back wooly mammoths soon!

Could we grow an organ in a lab?
Not yet, but scientists are working on this.

Could we use gene editing to regrow a limb?
Maybe in the distant future, but not right now.

Could we bioengineer immortality?
Possibly, but aging is a hard problem to solve.

How can chimps and humans have different chromosome numbers?
Click herehere, or here to learn more about chromosome number can change in the course of evolution. And click here for some information on chromosome number in our distant cousins, the Neanderthals and Denisovans.

Why are zebras black and white?
We're not totally sure how the biology works, but stripes are hard to make.

What kinds of careers can you have with a biology degree?
You can have many different types of jobs, that need different degrees and education levels. Click here for a flowchart of what careers you can pursue in genetics, organized by minimum required education level.


Want to learn all sorts of other cool facts? Click here to order our book, When Will Broccoli Taste Like Chocolate?: Your Questions on Genetic Traits Answered by Stanford University Scientists.


Still have questions?

Browse through our archive of previously answered questions, or you can submit a question to The Tech Geneticist. While we can't answer every question we receive, we do our best to respond.











Genes are pieces of DNA that contain information to make proteins.
(Image adapted from Wikimedia Commons)





A person who has one copy of a recessive genetic disease is called a "carrier." They can pass the recessive DNA on to their children.
(Image from Wikimedia Commons)










Parents who are types O and AB will have children with either blood type A or blood type B.
(Image adapted from Pixabay image)
























Named after this mythical Greek creature that was a part goat, part lion, and part snake, a human chimera is made up of cells from two siblings. 
(Image from Wikimedia Commons)


















A lot of eye color can be explained by two genes. But while this explains most eye color, there are actually many more eye-color genes that can lead to unexpected results!























Hair gets its color from pigment, which is made in special cells called melanocytes.

















Whether your ear lobe is attached or not is surprisingly complicated genetically. 
(Image from Flickr)










There are great genetic tests out there that can tell if these two are aunt/nephew. They are ancestry tests, not "avuncular" tests. 
(Image from Flickr)




















The ancestry percentages are part of a range. The true amount may be higher or lower.
















Golden rice is a GM food that could save hundreds of thousands of people from blindness and millions from an early death each year.
(Image from Wikimedia Commons)