I am coming here for a lack of better place to answer an argument I had this evening.

If someone has a child, the probability that they have a certain sex will be somewhere around 50% as I understand girls are actullay slightly more common. With a second child, does the overall probabilty diminish that the child will be the same sex as the first.

This is where the question gets tricky in my mind. It is just like flipping a coin, each time you have a child you are still in the 50-50 probability of having a certain sex. But is there an equation for that coin flip over a long period of coin flips, or in this case, child birth?

A child would be 0% probable if you're as gay as you sound.

It is exactly like a coin flip. Each event (birth) is independent of the one before it.

Girls are slightly more common because of the amount of x-linked genetic diseases that show up in boys.

Yes, each birth is independent, but over the long run is it just as probable to have 100 boys as 50 boys and 50 girls

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Yes, each birth is independent, but over the long run is it just as probable to have 100 boys as 50 boys and 50 girls

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Actually, the latter is 10^29 times more probable.

I really don't think this is something you will have to worry about.

Each "flip" will have a 50% probability of success. However, there is an equation that you need to figure the probability of having, say five girls out of five children. I'm not going to post the equation, though, because it's too long.

If you had five children, the chance to you had 5 girls would be about 3%, the chance of having 4 girls is about 16%, 3 girls 31%, 2 girls 31%, 1 girl 16%, and 0 girls 3%.

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Girls are slightly more common because of the amount of x-linked genetic diseases that show up in boys.

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This isn't true. There are more boys born than girls.

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If you had five children, the chance to you had 5 girls would be about 3%, the chance of having 4 girls is about 16%, 3 girls 31%, 2 girls 31%, 1 girl 16%, and 0 girls 3%.

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To see how this works, note that there are 2^5=32 equally likely birth orders (G=girl, B=boy):

• GGGGG
  
• GGGGB
  
• GGGBG
  
• GGGBB
  
• GGBGG
  
• GGBGB
  
• GGBBG
  
• GGBBB
  
• GBGGG
  
• GBGGB
  
• GBGBG
  
• GBGBB
  
• GBBGG
  
• GBBGB
  
• GBBBG
  
• GBBBB
  
• BGGGG
  
• BGGGB
  
• BGGBG
  
• BGGBB
  
• BGBGG
  
• BGBGB
  
• BGBBG
  
• BGBBB
  
• BBGGG
  
• BBGGB
  
• BBGBG
  
• BBGBB
  
• BBBGG
  
• BBBGB
  
• BBBBG
  
• BBBBB

Grouping them by the number of girls gives:

5 girls: 1 outcome

• GGGGG

4 girls: 5 outcomes

• GGGGB
  
• GGGBG
  
• GGBGG
  
• GBGGG
  
• BGGGG

3 girls: 10 outcomes

• GGGBB
  
• GGBGB
  
• GGBBG
  
• GBGGB
  
• GBGBG
  
• GBBGG
  
• BGGGB
  
• BGGBG
  
• BGBGG
  
• BBGGG

2 girls: 10 outcomes

• GGBBB
  
• GBGBB
  
• GBBGB
  
• GBBBG
  
• BGGBB
  
• BGBGB
  
• BGBBG
  
• BBGGB
  
• BBGBG
  
• BBBGG

1 girl: 5 outcomes

• GBBBB
  
• BGBBB
  
• BBGBB
  
• BBBGB
  
• BBBBG

0 girls: 1 outcome

• BBBBB

To get the probabilities, divide by the number of outcomes (32):

• P(5G) = 1/32
  
• P(4G+1B) = 5/32
  
• P(3G+2B) = 10/32 = 5/16
  
• P(2G+3B) = 10/32 = 5/16
  
• P(1G+4B) = 5/32
  
• P(5B) = 1/32

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I'm not going to post the equation, though, because it's too long.

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The probability that there are g girls and b boys, given that there are b+g children, is

(b+g)! / (b! × g! × 2^(b+g))

I don't think that's too long.

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With a second child, does the overall probabilty diminish that the child will be the same sex as the first.

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The answer to this question is no. The probability is the same for each child, 50/50, no matter what the previous children are. However, if you are going to predict the overall number of boys/girls, then the above works. But if you have a boy, and you are pregnant with another child, the odds are still 50/50 that it will be a boy.

nerds

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A child would be 0% probable if you're as gay as you sound.

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I know it's a 50/50 deal, but we all know people that seem to only be able to have one or another. We have family friends that just had the first girl in 3 generations, out of all of the cousins and everything... dozens of kids.

It's just weird how sometimes stats don't fall the way they "should."

Wow, I have never seen a more appropriate nickname on here than leckNERD2004...

That is pretty basic math, not really nerd-defining material.

hth

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Big nerd

You should keep in mind that the probability of a child being male or female is 50% within the entire population. In other words, if you chose 100 random pregnant women, approximately 50 of them will have a boy and 50 of them will have a girl.

The odds change dramatically within a single couple because there are complex genetics predisposing you to favor having children of one sex over the other.

Granted, these genetic predisopostions are relatively minor and this may not matter at all in terms of your bet, but I just wanted to clear up some things...

From what i learned in genetics classes


females have XX
males have XY

Each parent gives a chromosone..females can only can give an X

SO its a 50/50 chance of a girl (if the male gives an X) and a 50/50 chance of a boy (if the male gives a Y)

The Sperm with the Y chromosome do weight a lil less than ones that carry the X (the Y chrom is much smaller) but really its a 50/50 split

Jerzzy, the genetics are far more complex than that as you have to take into account differences in sperm viability and production.

In essence, the male does "decide" whether or not the child is male or female, depending on whether or not a "male" sperm or a "female" sperm fertilizes the oocyte. However, some men tend to produce more of one sperm type than the other for reasons both genetic and environmental. For example, due the high g-forces that fighter pilots are often exposed to, fighter pilots tend to have more female children than males.

All of these variables tend to alter the 50/50 probability within any given couple.

My major in undergrad was genetics


I work in teh medical field

but i have no idea who is reading this thread so i just spelled out the simplest concept there is.......i did take all teh genetics classes at A&M....molecular genetics, advances in human genetics, mamalian genetics etc

Yeah, I probably went into way too much detail.

its cool, its fun to talk to ppl who knows details about stuff like this


i just didnt want to talk about something ppl had no idea about

TexAgs: We Know Sperm!

gay people can have kids too.


a gay woman can still get pregnant.

and you wonder why people make the gay jokes about you...