Why is it a 50-50 chance that a baby will be a boy/girl rather than something else, like 55-45, etc.?
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Anonymous2007-11-26 4:19
Because there are two sex chromosomes.
Not the whole answer, but close enough.
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Anonymous2007-11-26 4:37
Take biology class. Lets just say there are two chromosomes, X and Y, there are two of each. Now place 2 of those four together:
XX
XY
YY
YX
XX and XY is male, YY and YX is female. This is coming from memory though so I might have completely fucked it up.
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Anonymous2007-11-26 4:48
But that doesn't explain why both are donated at equal rates. One could have some magical advantage.
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Anonymous2007-11-26 4:51
I either did it wrong and X is dominant or they are both equal. This shit has been tested at the microscopic level. It is 50/50 if everything is normal but some shit could make it different. Ask your local biology teacher or something.
YY does not exist, the chromosome given by the female is ALWAYS X, then the male gives either X or Y, XX is male and XY is female, there is no YX or YY.
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Anonymous2007-11-26 5:38
>>4
The 50:50 sex ratio is evolutionarily stable.
Quoting from Wikipedia, because it's mostly accurate and I can't be bothered to phrase it myself:
If reproductive success is measured by number of children produced, then the total reproductive success of all living males must equal that of all living females. If the sex ratio remains at 50:50, then males and females enjoy equal average reproductive success per individual. However, in a population of animals, the sex ratio could theoretically change over time, resulting in differences in average reproductive success between males and females. For instance, a sex ratio of 25:50 would make an average male twice as reproductively successful as an average female. Such inequality in reproductive success would give parents (in evolutionary speak) an incentive to produce more boys, assuming that parental expenditure is the same for boys and girls, and that both of them will have equal opportunities at mating in the future. Over time, such negative feedback by parents will bring the sex ratio towards the 50:50 equilibrium.
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Anonymous2007-11-26 8:38
You get X from the mum and either X or Y from the dad.
XX = girl
XY = boy
Women don't have the Y chromosome.
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Anonymous2007-11-26 12:06
ANOTHER PROOF THAT EVOLUTION EXISTS!
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Anonymous2007-11-26 12:20
Saw on the Discovery channel that temperature determines the sex of crocodile eggs.
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Anonymous2007-11-26 15:01
>>10
But that'd mean that it'd be much less likely to be a 50:50 outcome
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Anonymous2007-11-26 15:35
In mammals, the egg carries an X chromosome and the sperm has a 50-50 chance of carrying an X or Y chromosome. The sex of the offspring is dependent on which chromosome the fertilizing sperm carries (X or Y).
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Anonymous2007-11-26 21:11
>>12
That's not an explanation, that's a rephrasing of the question.
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Anonymous2007-11-27 0:40
theres subtle differences in sperm carrying either chromosome, i think X chromosome carriers can swim for longer and survive longer after ejaculation, whereas Y chromosome carriers can swim faster but die much more quickly
Actually, there exists a probably of either 0 or 1 that a given baby is a boy (or girl). As one cannot assign an intermediary probability (between 0 and 1) to a single event. You can, however, assign an intermediary probability to a series of events; for example, what is the probability that a baby will be born in a given population. Then you would use a proportion, and if you wanted to test the level of significance do a hypothesis test and so on.
TLDR
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Anonymous2007-11-27 5:53
>>14
Sperm cells carrying the Y chromosome are lighter, yes, so yes, they're a bit faster.
Male fetuses are also spontaneously aborted more often than female ones. I forget why. Possibly the mother's immune system rejects it.
>>16
You don't understand anything about probability.
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Anonymous2007-11-27 9:28
>>18
seconded. >>16 doesn't have a clue what probability means
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Anonymous2007-11-27 18:43
>>18
anyone got a source for his first point? seriously weird
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Anonymous2007-11-27 22:30
The Y chromosome is smaller. Therefore the sperm cells swim faster.
Source: Common sense.
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Anonymous2007-11-27 22:30
The Y chromosome is smaller. Therefore the sperm cells swim faster.
Source: Common sense.
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Anonymous2007-11-27 22:39
>>prevailing posts:
It would do you all well to take a STATS101 night class. The final probability of the observed sex is either male or female. And if we arbitrarily define success as male, it can be defined to be binomial. And if you're trying to assign some sort of confidence interval, a population parameter cannot be within the interval X1 and X2 with a probability of anything between 0 or 1, because the OBSERVED value either is or is not a male, meaning it can only take on a probability of 0 or 1.
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Anonymous2007-11-27 23:12
>>23
You're a dipshit. Lurk moar and read a goddamn book. Maybe it'll help you keep very basic concepts in statistics straight and keep you from shitting up threads with elementary-school-level misconceptions.
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Anonymous2007-11-28 1:37
>>24
Care to explain your reasoning with more than an overly emotional array of ad hominem attacks --perhaps something substantive?
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Cell Biologist2007-11-28 2:48
Mathfags, you're missing the point:
You don't need advanced statistics to answer the problem. It's just 2 possibilities, X or Y.
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Anonymous2007-11-28 3:33
>>1
its not 50/50. there are more female births than male.
There re in fct more mle births thn femle. The mle sperm is slightly stronger nd fster... However, the femle humn lives longer on verge... so it works out.
A baby gets one chromosome from the XY/XX pair from each parent.
As such, there are four possibilities:
1. Father X \ Mother X = Girl
2. Father X \ Mother X = Girl
3. Father Y \ Mother X = Boy
4. Father Y \ Mother X = Boy
1 and 2 are equivalent.
3 and 4 are equivalent.
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Anonymous2007-11-29 2:03
>>32
Again, not answering the question, just restating it. There's no inherent reason for all outcomes to have the same probability.
The question has already been answered in this thread, but holy fuck, that's a very, very basic error.
No wonder mathematicians look down on biologists.
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Anonymous2007-11-29 2:38
hurr... every cell in a guy has equal number of X and Y, and after meiosis you end up with equal number of sperm cells of each kind. If you randomly pick one you get a 50% of getting either one.
>>34 every cell in a guy has equal number of X and Y, and after meiosis you end up with equal number of sperm cells of each kind.
And the question reduces to why this should be the case, when it's possible to produce an unbalanced number as well.
If you try to explain the gender distribution in function of sperm production, you aren't explaining anything, you're going around in circles.
And it's no wonder statisticians look down upon uncreative mathematicians.
(same fag)
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Anonymous2007-11-30 2:48
>>35
The question is why it is the case that after meiosis you have 50% X and 50% Y? Definition of meiosis is that a sex cell splits so the 26 chromosomes in a sex cell are 25 and then the X or Y. A man has 100% XYs. Thus, after meiosis, the XYs become Xs and Ys. If there are 100 pre-meiosis cells, there are 100 Xs and 100 Ys. After meiosis, there are 100 Xs and 100 Ys in separate sex cells. Thus, 50% of sperm are X and 50% are Y.
Holy fucking shit, I learned that in 7th grade science class. God ddammit and I went to Texas public school. And I've never had biology since then. Holy fucking shit no wonder scientists look down on everyone.
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Anonymous2007-11-30 3:46
right. we get the point that by cell production, it's a 50-50 chance. applause all around.
the problem is, the birth ratio of men to women, globally, is 1.07 male/female. for those stellar statisticians who were posting above, that means 107 dudes are born for every 100 chicks. in the u.s. the ratio is 1.05 m/f, e.u. is about 1.06 m/f, china is 1.11 m/f.
soooooo biology be damned, it's not actually 50/50. O shit!
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Anonymous2007-11-30 4:19
In the world of statistics--and to those who do not understand fucking basic probability, >>16 and >>25 was in fact correct--one cannot interpret cold data without further analysis, as it can be misleading. That ratio alludes to the outside factors, as if it is established (I am terrible with biology, so I will assume you're correct) that the cell-production probability for let's say male is .5 as observed in the population of births, there must be an external force--that is just "common sense." Statistics is not "ruining" /sci/ the corrosion of logic is.
>>37
So how do you explain the fact that there are a fair number of species that have unbalanced sex ratios?
The fact that if you split XY cells fairly you get equal amounts of X and Y has no bearing on the matter, since the cells don't have to split fairly.
It's pretty obvious you went to a Texas public school.
>>16,25,39
Samefag.
You aren't lurking moar. Rectify this.
>>38
notice BIRTH ratio, have you considered that many people might abort a girl and not a boy?
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Anonymous2007-12-04 15:11
China doesn't help with the 'Lol girls suck, abort abort' thought.
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Anonymous2007-12-04 17:31
There are two levels at which to look at this problem.
Purely statistically, the mother donates one of two X chromosomes while the father donates either a X or Y chromosome. This second one determines the gender. It should be obvious why there is a 50/50 chance here.
Biologically, I'm pretty sure that chromosomes have no effect on the sperm cell carrying them, but it's quite possible that those carrying Y chromosomes are lighter, especially since Y chromosomes are much smaller than X chromosomes (http://home.planet.nl/~gkorthof/images/XYchrom.jpg). I have no idea what other mechanisms affect the chance of selection at this level.
Purely statistically, the mother donates one of two X chromosomes while the father donates either a X or Y chromosome. This second one determines the gender. It should be obvious why there is a 50/50 chance here.
If you go about it naively, sure. However, it's been pointed out already that this is a retarded way of looking at things.