Why can't I conceive a boy?

Kat Arney put this to fellow Naked Scientist Chris Smith...

Chris - Well, it's actually her husband's job, isn't it? Because as geneticists will tell you, there are sperms that have X and there are sperms that have Y. if you are having a girl then your egg is being fertilised by a sperm that has an X chromosome in it. if you are having a boy then your egg is being fertilised by a sperm that's got a Y chromosome in it.

Kat - Because women's eggs just have one X chromosome. All eggs have an X chromosome...

Chris - That's right. Women are just XX. That's their genetic makeup; whereas men are XY. Therefore, the only genetic material a woman can contribute to her egg is an X chromosome, whereas men can make sperm that have either an X or a Y. Now, why should you only have girls? Well, what's the chance or having a girl? You've got roughly a 50/50 chance because there's equal numbers, roughly, of X and Y sperms in a healthy person. So, what's the chance of having five in a row? Well, that's 1/2 - a half - to the power of 5. So that's a half times a half, times a half, times a half times a half. And that means you've got one in two times one and two is one in four, times one in eight times two again, one and 16, times two again, one in 32 - about a 3 per cent chance - of that happening.

But that's not zero! And so, therefore, just because it's low odds doesn't mean it is abnormal. 

So I would say that somebody has to be that 3 per cent, because we believe that the population is what we call a "Normal distribution". So it's probably perfectly natural.

There's probably nothing that this lady can do to shift the odds apart from to go and see a clinic who can put the sperm through a system to sort out the X and the Ys, which you can do. Some people don't agree with it. They don't think it's ethical. It may also have health implications for the future. Who knows what the long term consequence of doing this is, and thwarting nature in that way?

But, the bottom line is, you should be - as one person put it to us - delighted you've got five girls, because they're much easier to bring up than the male equivalent. Because they said the conversation extends beyond 'yeah'! Unless, of course, you happen to like all the mates around from the pub for beery parties and shirts and stuff strewn all over the dining room...

Kat - I think girls smell nicer as well!