• Question: If someone's mum and dad both have brown eyes then how does the child have blue eyes?

    Asked by gekogeorgie to Ben, Dave, Ed, Sam, Susana on 21 Jun 2013.
    • Photo: Sam Horrell

      Sam Horrell answered on 21 Jun 2013:

      Eye colour is a very complex genetic trait, they don’t actually know how many genes go into determining eye colour. It all comes down to dominant and recessive genes. If a gene is dominant it can override another gene, if it is recessive it can’t.

      If we simplify it down to only 2 genes determining eye colour, Br for brown and Bl for blue. Brown eyes are dominant over blue eyes so both the parents could have the genes BrBl.

      We can put this into a genetics cross.
      mother: BrBL father: BrBl
      A child will get one gene from the mother and the other from the father. this means there are 4 different combinations:
      BrBr, BrBl (Bl from mother), BrBl (Bl from father) and BlBl

      This measn there is a 1 in 4 chance the child could get the 2 Bl genes from their parents making them have blue eyes.

    • Photo: Benjamin Hall

      Benjamin Hall answered on 22 Jun 2013:

      Sam has explained this pretty comprehensively.

      If a child inherits two sets of recessive blue eye gene varients from their parents, who were brown eyed but carried the blue eyed gene, they’d have blue eyes.

    • Photo: David Briggs

      David Briggs answered on 23 Jun 2013:

      Eye colour inheritance is not straight-forwards, and Sam has explained the complexities nicely.

      There is also an online tool which can help predict probabilities of a child’s eye colour here:

      http://www.babymed.com/tools/eye-color-predictor

    • Photo: Susana Teixeira

      Susana Teixeira answered on 26 Jun 2013:

      It’s already explained above, but in simple terms it is because we only manifest, or show, some of the characteristics that we have stored in our genes.

      So even if a mum and dad have brown eyes (that is the characteristic they show), they can have the blue eye code in them (that is the characteristic they are hiding 🙂 the recessive gene that Sam mentions), and both transmit it to their child.

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