DNA Basics Chapter 8: Genotypes and Phenotypes

DNA Basics Chapter 8: Genotypes and Phenotypes

We’ve gotten a lot of questions recently about the relationship between DNA test results, and traits like blood type and eye color. The answer lies in the relationship between genotypes and phenotypes. Genotypes are the exact DNA — the A’s, T’s, G’s, and C’s — that you inherited from your parents. Phenotypes are the resulting traits that we can see. Phenotypes are more straightforward to explain, so let’s start there.

Phenotype

Generally speaking, a phenotype is an inherited characteristic that we perceive. Eye color, hair color, and blood type are all phenotypes. You may have a brown-eye phenotype, and your eyes will, therefore, be brown; a brown-hair phenotype and your hair will be brown, or an A blood type phenotype and your blood type will be A.

Genotype

Genotypes are the DNA that code for the phenotype. Have you ever thought about how a person might have a blue-eyed parent and a brown-eyed parent and yet have only brown eyes themselves? DNA is inherited from both parents, so how can a person have eyes that only match one?

Parents don’t pass down eye color — they pass down an allele. Together, the allele from your mother and the allele from your father are your genotype. Your genotype codes for your phenotype — the trait we can actually see. For example, if the blue-eyed parent passed down a blue allele, and the brown-eyed parent passed down a brown allele, the child’s genotype will be blue-brown, and his or her phenotype will be brown. How do we know that the child’s eyes will be brown?

Alleles

Some alleles are recessive, and others are dominant. In this case, blue is a recessive allele while brown is dominant. When you get different alleles from each parent, you are heterozygous for that trait. When you get the same allele from both parents, you are homozygous for that trait. (For a review of terminology, see DNA Basics Chapter 4: A Glossary of Terms.) Because blue is recessive and brown is dominant, any blue-brown heterozygote will almost definitely have a brown-eye phenotype, and the person’s eyes will be brown.

To map the possible genotypes and phenotypes you can inherit from your parents, you can use a tool called a Punnett square. Conventionally, upper case letters are used to indicate dominant alleles and lower case letters are used to indicate recessive alleles. Below is a Punnett square mapping the possible genotypes for a person with a homozygous blue-eyed parent and a heterozygous brown-eyed parent.

Punnett square showing the possible genotypes and phenotypes a person can inherit from a blue-eyed parent and a heterozygous brown-eyed parent.

Punnett square showing the possible genotypes and phenotypes a person can inherit from a homozygous blue-eyed parent and a heterozygous brown-eyed parent.

The homozygous blue-eyed parent can only pass down a blue allele. The heterozygous brown-eyed parent can pass down a blue allele or a brown allele. The children who inherit a blue allele and a brown allele, i.e., a blue-brown genotype, will have a brown-eye phenotype and have brown eyes. Similarly, the children who inherit two blue alleles, i.e., a blue-blue genotype, will have a blue-eye phenotype and have blue eyes.

Co-dominance

Blood type is a little trickier because A and B are co-dominant. O is recessive. Just like a blue-eyed parent can only pass on a blue allele, a parent with type O blood can only pass on an O allele. An A heterozygote (AO) can pass on an A or an O; an a homozygote (AA) can only pass on an A.

Here is a list of blood type inheritance possibilities:

Allele from parent 1 Allele from parent 2 Genotype Heterozygote or homozygote Phenotype
A A AA homo A
A O AO hetero A
A B AB hetero AB
B B BB homo B
B O BO hetero B
B A AB hetero AB
O O OO homo O
O A OA hetero A
O B OB hetero B

A table of blood type inheritance patterns. Note that there is no significance to the order of the alleles within a genotype, e.g., AB and BA are exactly the same genotype.

Ethnicities

In the eye color example, you don’t need a DNA test to tell you your eye color! You can see in the mirror whether you have brown eyes. What you can’t see in the mirror is your genotype. Similarly, while you might have some characteristics that are associated with certain ethnicities, the only way to find out all of the ethnicities in your DNA that you inherited from your parents is to take a DNA test.

Just like in the Punnett square above, you can see that two people with the same parents can end up with different eye colors, and two people with the same parents can end up with different ethnicities.

Order a MyHeritage DNA kit to find out the hidden ethnicities passed down to you by your ancestors — you may be surprised by what you uncover!

Comments

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  • Helen Baker

    September 14, 2018

    my DNA results are not in line with any known ancestry (going back to 1600’s) or any of my relatives who have been tested (elsewhere). 48% Greek and no other member has any!! all have Italian (at least 40%) and I have none! Have you got my results wrong? (and I am definitely my parents child!)

  • Amanda Cook

    September 14, 2018

    my daughter is hetero for brown/blue eyes, but her eyes are not pure brown. she had blue eyes for a long time. they ended up changing to a blue-green, then settled into a hazel brown/green color. her eye color changes depending on the lighting and color of her clothing.

  • Adrian

    October 28, 2018

    Good article! It provides such interesting information.

  • tyrone patmore

    December 7, 2018

    How far back in time does my dna take me in years. 100s or 1000s?

    • E

      Esther

      December 18, 2018

      Hi Tyrone,

      We provide you with a breakdown of your ethnic percentages and connect you with relatives descended from any of your ancestral lines within approximately the last 6 generations.

      Best,
      Esther / MyHeritage Team

  • Mike

    December 7, 2018

    I’ve got different coloured eyes – one brown one blue. Explanation?

  • Magdalena Lambropoulos

    December 8, 2018

    How fascinating. I have just learned why my husband and I produced all brown eyed children even though he has brown eyes and I have blue eyes. Our older grandchildren have brown eyes as their parents are brown eyed and2 of the younger ones have blue.
    Thank you

  • Charles Aby

    December 8, 2018

    So what happens with hazel eye color? I have green hazel eyes and my wife has blue hazel eyes. It appears that hazel would be the dominant allele as my mother had blue eyes and father had green hazel eyes. Both of my wife’s parents had blue eyes but her uncle who is a fraternal twin to her mother had brown hazel eyes. This would be a good question to research.

  • Kristine

    December 8, 2018

    I would like to know where hazel eyes come into it. Brown & blue don’t make hazel? Loved the article.

  • Kelly

    December 9, 2018

    My husband has light brown eyes and I have hazel eyes, our daughter has blue eyes. My husband is half Native American and our daughter is quarter. Did she get her blue eyes from a grandparent?

  • Walter Petrovics

    December 10, 2018

    How can you get green eyes from one parent with blue eyes and the other parent has brown eyes???

  • Susan

    December 10, 2018

    My father had blue eyes and my mum has hazel eyes but I have green with a dark blue ring on the outside as do all my children my father was AB positive and my mums roh negative but my father has a Jewish father my mums father was Irish with black hair but I have dark hair as every second child in the family seems to get the very dark hair so I’d like to no what I would come under in eye colour please

  • Katherine Lynn

    December 10, 2018

    What about green eyes? They aren’t as common. How does it happen?

  • PB

    Paulette Baker

    December 10, 2018

    I just want to ask one question because no one can help me with finding my great grandfather (paternal). What i need is: how does a blue eyed set of grandparents (maternal) have a blue eyed daughter have a daughter with blonde hair, green eyes and a daughter with jet black hair and dark brown eyes?

  • Y Hall

    December 11, 2018

    Hi, I read your explanation of eye color. But I have a cousin who has brown eyes, while both his parents have blue eyes. How did that happen? Thank you.

  • Marshall Alan,

    December 15, 2018

    My mother had brown eyes because she was part indigenous, however my Father had blue eyes and yet I have green eyes. I have read your article above yet I’m left feeling more confused and suspicious of my background.

  • Penny Mindham

    December 15, 2018

    how would a person like myself get grey color eyes when my my one parent has brown eyes and my other parent has hazel eyes?

  • Jennifer Lucas

    January 12, 2019

    My husband has hazel eyes and I have brown eyes. We have one hazel eyed child and three with brown eyes.
    One of our sons with brown eyes has two children. One has blue eyes and the other has hazel eyes, so no brown eyed grandchildren.