• Question: Is it true that all babies in the mother's tummy start of as girls?

    Asked by popthebottle to Stephen on 17 Jun 2010 in Categories: .
    • Photo: Stephen Curry

      Stephen Curry answered on 17 Jun 2010:

      Hi popthebottle – No I don’t think so. To be a girl you need two X chromosomes; boys have an X and a Y. The sex is determined at the moment of fertilisation when the egg and the sperm, each of which carries a single chromosome, come together.

      Now the egg will always have an X (since it comes from the Mum – maybe that is what you meant?) but the sperm could be carrying X or Y from the Dad. So it is the chromosome in the sperm that determines the sex of the baby. If it is X-Y, it will be a boy from that first moment.

Comments

  • Photo: graceblack

    graceblack commented on :

    I dont think so… A fetus starts of as neither a girl or a boy. It is then the x or y chromosomes that then decides whether it will be a boy or a girl.

  • andrewleitch commented on :

    (sticking neb in) Popthe bottle makes a good point here. Sex is about more tha chromosomes. Look at the problems caster semenya the 800m runner has had. Jamie Lee Curtis has an X and a Y chromosome and i suspect most people would happily say she’s a woman. Popthebottle is right. The hormonal default position for a human is female (not developing a penis+ descended testes and growing breasts). If you can’t respond to testosterone normally you will end up looking like a girl (the condition is called androgen insensitivity) even if you have X and Y chromosomes. Hope you don’t mind me interrupting. Slow day in my zone and I’m stuck at home with a toddler who has chicken pox.

  • Photo: Martin

    Martin commented on :

    Hah! I can out-interfere Andrew any day. I am not even in this edition of I’m a Scientist I am just lurking from the May round and my password still works!

    We all have to be careful here. The confusions can pile up thick and fast if we are not clear about what we mean. Andrew and Stephens answers both have very good points to make but from different perspectives.

    People have a chromasomal composition. People have a physical appearance. People have hormonal balances and receptiveness which change throughout there lives and are difficult to measure. People have behavioural attributes. And people have a self-image. These are not independant of one-another but neither are they as closely linked as you might think.

    For most of us, most of the time, most of these things point broadly in the same direction. This makes the idea that there are two types of people (Male and Female) an understandable simplification for everyday use.

    As scientists we need to recognize that if we could measure and plot all of these things on a big, mad, graph (it is not clear that this would be possible -but stick with me) with one dot for every adult member of the human race we might see two distinct clusters but it would be a bit of a mess. You couldn’t draw a circle and say ‘all the males are in this circle’. It would be arbitrary.

    But most of these measurements would be unavailable in the first few days of pregnancy when people are just a ball of cells!

    Different babies have different chromasomes – they are not all the same. If that is the definition of sex you are using then the answer to popthebottle’s question is ‘no’ (Stephen’s point).

    As a definition of what sex is, just looking at chromasomes is flawed – as are all other definitions (Andrew’s point).