• Question: when your in your mums tummy what decides whether your a boy or girl?

    Asked by freddieh to Amy, Sarah, Will on 21 Jun 2012.
    • Photo: Amy Birch

      Amy Birch answered on 21 Jun 2012:

      Hi freddieh,
      This decided by something called chromosomes – these are made from our DNA and come in pairs. If you are a boy you have a pair of X Y chromosomes and if you are a girl then you have a pair of X X chromosomes.
      These chromosomes come from your parents, so you will get one half from your mum and one half from your dad. If your dad gives you an X chromosome then you will be a girl (because you mum can only give you an X) and if your dad gives you a Y chromosome then you will be a boy.
      Hope this answers your question 🙂

    • Photo: Sarah Martin

      Sarah Martin answered on 21 Jun 2012:

      Hi freddieh!

      Humans have 44 normal chromosomes, and two more that decide whether they’re a boy or a girl. Your dad’s two extra chromosomes are an X and a Y chromosome, your mum’s are two X chromosomes.
      You get one of each of theirs – so you get an X from your mum, and either an X or a Y from your dad. That determines whether you turn into a boy (XY) or a girl (XX) in your mummy’s tummy!
      What happens is that male embryos start using the DNA in the Y chromosome to make proteins that switch off some of the processes that would lead to the development of ovaries for example. About nineteen different genes are probably involved! In the mean time the cells turn to producing testes and sperm tubules.
      In a female embryo, from about the sixth week, cells develop to build up the fallopian tubes, uterus and vagina. It used to be thought that this was an automatic process that happened – unless the boy genes were activated, but now it’s known that several genes on the X chromosomes are switched on to form a baby girl.

      Great question!
      🙂 Sarah

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