Why the human sex ratio is only approximately 1:1 (1 Viewer)

[Chrisssss]

I have the following question:

"Briefly explain why males and females are born roughly in a ratio of 1:1. Why is it not exactly 1:1 in the human population"

Now I was considering sex linked diseases as well as the fact that some societies in certain parts of the world intentionally push the ratio to one side e.g. a particular society favouring boys due to the stereotype that they'll one day look after the family etc. However the question asks why they're born roughly in the ratio of 1:1. This, I cannot think of any reasons for.
Anyone?

 

[brenton1987]

Because the selection of the X or Y chromosome from the father is completely random. When dealing with random assignments youre likely to find equal numbers of each.

There is a statistical formula or term for what I'm thinking of but its name escapes me at the minute.

 

[Enteebee]

If you start off with less males, then a male born into that society will have greater mating prospects than a female. So then those parents who are genetically predisposed to have male children are more likely to have greater numbers of grandchildren, passing on the 'male birthing gene', until eventually it is much more common for male births to continue... as more male births happen and you get closer to 1:1 the advantage of being male is diminished.

edit: It's not exactly 1:1 because of human interference with natural birthing patterns and because it's highly unlikely you'll ever reach a perfect equilibrium whereby there is no slight advantage.

 

[Enteebee]

Because the selection of the X or Y chromosome from the father is completely random. When dealing with random assignments youre likely to find equal numbers of each.

There is a statistical formula or term for what I'm thinking of but its name escapes me at the minute.

This isn't true, some people are genetically more likely to have males than females...

 

[brenton1987]

This isn't true, some people are genetically more likely to have males than females...

Do you happen to know what genes causes this?

 

[Chrisssss]

If you start off with less males, then a male born into that society will have greater mating prospects than a female. So then those parents who are genetically predisposed to have male children are more likely to have greater numbers of grandchildren, passing on the 'male birthing gene', until eventually it is much more common for male births to continue... as more male births happen and you get closer to 1:1 the advantage of being male is diminished..

Fisher's principle!

edit: It's not exactly 1:1 because of human interference with natural birthing patterns and because it's highly unlikely you'll ever reach a perfect equilibrium whereby there is no slight advantage.

True, in the end I wrote about things such as sex selective abortion and infanticide; there's also nowadays the ability for people to actually have selective IVF however I haven't heard much about it or if it's something actually available as of yet.

Thanks for the responses though.

 

[withoutaface]

Chadd, I don't think that's correct. It'd be because females have genetic redundancy, and as such they're a lot less likely to contract diseases which are determined by the 23rd chromosomal pair (e.g. muscular dystrophy) which in turn means that the life expectancy of males is diminished.

On top of this there are a whole heap of gender specific, or gender biased, diseases which affect one sex and not the other.

 

[Enteebee]

Chadd, I don't think that's correct. It'd be because females have genetic redundancy, and as such they're a lot less likely to contract diseases which are determined by the 23rd chromosomal pair (e.g. muscular dystrophy) which in turn means that the life expectancy of males is diminished.

On top of this there are a whole heap of gender specific, or gender biased, diseases which affect one sex and not the other.

That would explain why there are more of one sex than the other, but not why more of 1 than the other are born.

 

[The_highwayman]

i dont really think that this question required consideration of social factors and disease.
it's basically the same as asking why is the ratio of heads:tails 1:1 in coin tosses.
It's simply a question about chance and probablity based on the male supplying an X or Y chromosome.

 

[Chrisssss]

i dont really think that this question required consideration of social factors and disease.
it's basically the same as asking why is the ratio of heads:tails 1:1 in coin tosses.
It's simply a question about chance and probablity based on the male supplying an X or Y chromosome.

If you meant it's basically the same as asking why the ratio of heads:tails in only approximately 1:1 (chance, probability etc), with a massive sample space you would get an extremely close result compared to the expected. The world's population is ~6.65 billion and to my knowledge it is something along the lines of 1.05:1 (boys:girls) and though it might seem small it's arguably significant when considering the huge sample space.

 

[fretsonfire74]

The woman always has an Z chromosome for her egg, so the male gives the X or a Y. (X for female baby, Y for a male). It's 1:1 because it's totally random. To model it, try flipping a coin 20 times. you'll get almost 10 each. It probably won't be exact, but it'll be close.

 

[axlenatore]

Its only a guidelines of the propable outcome, from a punnet square you get a 1:1 ratio, but thats not definite, mendal ratio is only the predicted outcome its possible to get more of a organism expressing the ressive gene than the dominate because of that fact its ONLY A PROBABLITY

 

[syriangabsta]

Blame China.

LMAO

 

[always.dreaming]

Blame China.

LMAO +1

 

[Trebla]

The obvious answer would be probability and statistics. They would only equal 1:1 at infinite population.

However, an article I read before does come to mind. It said something about a world without males as there is a slightly higher female birth rate at the moment. It pinpoints to something about the Y chromosome being unable to repair itself or something like that resulting in slightly higher likelihood of female births than males. Couple that with the lower life expectancy of males, it predicts there will be very limited males in the world population in the future...

 

[Chrisssss]

The obvious answer would be probability and statistics. They would only equal 1:1 at infinite population.

However, an article I read before does come to mind. It said something about a world without males as there is a slightly higher female birth rate at the moment. It pinpoints to something about the Y chromosome being unable to repair itself or something like that resulting in slightly higher likelihood of female births than males. Couple that with the lower life expectancy of males, it predicts there will be very limited males in the world population in the future...

That's interesting and seems logical, do you remember where you read that article?

By the way this question is from a while ago, I have the suggested answer for it (it was from an assessment). I'll post it up when I get a chance.

 

[Sassed]

Couple that with the lower life expectancy of males, it predicts there will be very limited males in the world population in the future...

Not sure how the lower life expectancy implicates (significantly at least)??? Is it implied that there are less males present for the 'counting'? It is very unlikely that this significantly alters the ratio as (not going into it indepth) but there are so many modern causes of premature death that those that reach the natural life expectancy are not comparable to those that don't?
If the life cycle is in relavence to the rate of reproduction- it again does not affect as a mans fertility is not affected greatly?

Sorry if I sound too critical but would be interested if anyone is able to provide further information?
To reply to the OP with all factors, it somewhat comes down to probability you can flip a coin countless times and although approximate, results will not be equal 100% of the time.

 

[syriangabsta]

Could we includethe fact that nowadays people can choose what gender they wish for thier offspring to be? due to like advancement in science? lol?

 

[jannali rulz]

Is that in a test how many marks is it?

If it's like 2 or 3 i reckon all you have to say is:
draw a punnet square
X Y
X XX XY
X XX XY
if genotype = XX then phenotype = male
if genotype = XY then phenotype = female
therefore probability 1:1 ratio of female to males born

This is a probability that applies to each individual being born and given the large number of humans on the earth the chance of having exactly 1:1 over the whole population is incredibly unlikely. Therefore the population is roughly 1:1 but not exactly 1:1

anything besides that would be outside the course

 

[eddy11]

Its all to do with penis length and the epic race between the sperm.

'male sperm' with a Y chromosome (therefore carrying less wieght as y chromosome is smaller then x) win the 'sprint' to the egg. 'female' sperm are a bit slower, BUT as they say, slow and steady wins the race. Consequently men with a larger penis tend to sire more boys as they deliver the sperm closer to the egg, setting the male sperm up for the sprint. Overall however, female sperm tend to win the race to the finish a bit more often, represented as more females in the world population. Therefore the reason there are more females is because there are more males with penis size less then the length which would set the male and female sperm up for a even race, an even race being where the male sperm get a quick start but the female sperm catch them just as they reach the finish line (the egg).

this is why some people are genetically more likely to have a certain sex

Do you happen to know what genes causes this?

yes, the gene for penis length in males.