Brothers and Sisters

[GMH]

I have noticed people are more likely to have siblings (brothers or sisters) of their own gender than of the opposite sex. For example, men are more likely to have brothers, and are more likely to refer to their sibling as "my brother" and women are more likely to have sisters. It seems that a family pattern that if the mother gives birth to a girl, they are more likely to give birth to a girl again if she becomes pregnant again.

What is bizarre that despite the fact that I am male, I have two older sisters, rather than two older brothers. It almost makes me feel that the probability would have been that I would have been born female when I was born, but I was born male instead at the last moment. Talking to my male friends, they say that all have at least one brother and some have no sisters.

It makes me think whether it is something to do with the XX or the XY factor, but in my case it's YY, as in why would I be born male if my sisters were not?

[Drisgirl]

Its the luck of the draw.Your dad determines the sex so it probably just comes down to the sperm which swims the fastest.I dont think there is a scientific reason - unless some males have an excess of either the x or the y.Just like its the female who detemines if its to be twins or not because she produces the egg/s.Even if men have twins in their family it cant run in the family.

[neathgirl]

ok firstly GMH congratulations on one of the most bizarre questions - it put a smile on my face thank you!

in answer to ur question - u better have a word with ur dads genes - as scientific research has concluded that males determine the sex of the foetus. xx

[joko]

your gender is not decided 'at the last minute'

you cannot say that this is the case based on the circumstances of a few of your mates.

your 'reasoning' makes no sense.

[Buenchico]

I studied statistics (as a specific option within my mathematics degree course) at college and I remember that the probability of a child being born as a boy or as a girl was one of the more complicated areas simply because the normal assumptions of probability don't apply. I can't remember exact probabilities but the following is a rough summary:

When a couple have a first child, the probability of it being a boy is approximately 50% (and, self-evidently, the same probability relates to that child being a girl).

Unlike 'toss of a coin' probabilities, however, (where the first 'outcome' has no bearing upon the second), the probability of a second child being the same sex as the first child is higher than 50%. (I can't remember the exact figure but I think it was in the region of 55%-60%).

By the time you come to examine the situation of a couple who already have three children, all of the same sex, the probability that a fourth child will also share that sex is surprisingly high. (I seem to remember a figure somewhere in the region of 75%-80%).

These figures, remember, were cited in mathematics texts which didn't need to find a reason for these events. They were, however, stated as scientific FACTS based upon extensive research. So, while Joko might be correct in saying that GMH's 'reasoning' makes no sense, I feel confident in stating that GMH's 'observations' make perfect sense and fit in with established scientific data.

Chris

[Dreaming]

Hi you boffins, well I have 4 sisters (am second oldest) Do think though my parents were hoping for a boy, but gave up after child 4. Anyway, I have 4 children. In the following order! Boy, girl, boy, girl.

[Dreaming]

Get my maths straight! They gave up after child 5!

[gessoo]

GMH I think your observations are correct. I have no scientific knowledge except I do know that some women are not able to carry babies of one sex or the other and often miscarry, so perhaps this also causes their body to reject either male or female sperm. I am sorry this sounds totally silly, but I know what I mean. Perhaps someone with more biological knowledge than I could explain it more accurately.

[lb24]

A question was posted a while ago (i don't think i could find it again) about what determines whether a baby is a girl or boy. I can't remember which way round it is, but one type is small fast swimming sperm which doesn't live very long (i think they're the boys) and the other is large slow swimming sperm which lives longer (the girls). It depends on the time of the cycle as to which sperm reaches the egg,

i.e. if the boy sperm swim fast and find the egg first it's a boy but if the egg isn't far enough down and the boy sperm die before they reach it or before the egg reaches the right place, the longer living girl sperm catch it.

I'm pretty sure that's right. As for whether you have brothers or sisters, i'm sure it's just pot luck (and perhaps a bit of statistics, etc. as posted above!

[Dougie69]

I may be way off the mark here, but I heard that the sex of a baby isn't determined until a few weeks into the pregnancy, and that's why the male scrotum has a joing down it, because it could have been either or. In saying that, it might have been from an episode of Friends, so I might not be totally accurate there !

[joko]

I have read that foetus are the same up until a point when they 'decide' which sex to be - the same 3 important things exist - for a boy they turn into the penis and testicles and for a girl the clitoris and the ovaries.