• Question: if both your parents have brown eyes and you have blue eyes how is that possible

    Asked by anon-217197 to Savannah, Philippe, Lucy, Joanna, Harrison, Edoardo on 12 Jun 2019.
    • Photo: Joanna Huang

      Joanna Huang answered on 12 Jun 2019:

      From what I know of biology, every human carries two copies of every gene, one from mum and one from dad. So in the eye colour gene, there are also two copies. Only one copy of the gene is “expressed”, so this is the actual eye colour of that person. But the other gene is there! In the case of blue and brown eyes, the brown-eyed gene is called the “dominant” gene. So it will always be the one that is expressed. If the baby has blue eyes and the parents both have brown eyes, it means that both parents had a blue-eyed and brown-eyed gene. Thus both parents will express the brown-eye gene because it is dominant. But the baby got BOTH blue-eye genes from the parents, so the baby has TWO blue-eye genes, and therefore has blue eyes!

    • Photo: Savannah Clawson

      Savannah Clawson answered on 12 Jun 2019:

      Like Joanna said, it’s all to do with which genes get passed on from your parents. Blue eyes are rarer because the blue gene is what we call “recessive” and brown is “dominant”. So if you have one of each gene, the brown gene will take charge and voila, you have brown eyes! But if you manage to get two blue genes passed on, there is no dominant gene to take control and your eyes will be blue. If you have two brown-eyed biological parents and you have blue eyes, there was a 25% (1 in 4) chance of this happening so you should feel at least slightly special 😉
      .
      In real life, the story is a little bit more complicated as there are more than just brown and blue eyes in the world – you can also have green eyes, grey eyes, etc! There are also certain conditions that exist which can make you have two eyes that are different colours or even a mix of colours in each eye – this is called heterochromia. The science of genes (genetics) is really complicated and we still don’t fully understand how all the different genes work together to make a person.

    • Photo: Edoardo Vescovi

      Edoardo Vescovi answered on 14 Jun 2019: last edited 16 Jun 2019 4:04 pm

      Short intro and then answer. This is valid to understand blood groups (A,B,AB and 0) too after some tweaks.
      — The child’s colour is not the blending of the parents’ colours. Not an intermediate shade from mixing two paints in a bucket. Why? Because that information — as well as on the whole body — is carried by a large molecule (DNA) sitting in all cells. The parents combine their DNA to give birth to the child. As explained above, the child’s DNA builds a colour according to some rules.
      — These rules make what you say possible. The general rules are difficult (see above), so I stick to your example. Parents with brown eyes have DNA that carries info on blue and brown. The child with blue eyes has DNA that carries info on blue only.

    • Photo: Harrison Prosper

      Harrison Prosper answered on 17 Jun 2019:

      Well my fellow scientists have beat me to it, which is just as well because their answers are better than what I could muster!

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